F3 Chemistry Notes
1. What is chemistry ?
What is chemistry ?
Chemistry is the science of substances.
What is chemistry concerned with ?
• Chemistry is concerned with the composition and properties of substances,
• how and when substances change,
• and what new substances are made in the changes.
What are physical properties ?
Physical properties are properties that do not change after the observation.
properties include colour, density, physical state, smell, hardness, etc.,
Examples of physical
What are chemical properties ?
Chemical properties are properties that can only be observed during a change. Examples of chemical
properties include reactivity (for e.g. how fast a metal reacts with water), reaction towards air, acids,
water and alkalis, etc.,
What is a physical change ?
A physical change is one that does not cause any change to the nature of the substance. During a physical
change, no new substance is (new substances are) formed. Most physical changes are reversible, and no
new substances are formed during a physical change. For example, freezing water into ice does not
change the nature of water (reversible), and tearing a piece of paper into two does not change the nature
of the piece of paper, despite that in both cases the appearance of the object have changed (reversible if
we recycle the paper).
What is a chemical change ?
A chemical change is one that changes the nature of the substance. During a chemical change, a new
substance is (new substances are) formed. Most chemical changes are irreversible. For example,
burning a piece of paper (carbon dioxide is formed) or dropping water onto a piece of sodium metal
(hydrogen and sodium hydroxide are formed).
Why do we have to study chemistry ?
Chemistry can help us to
• identify things already known to us (chemical tests, chemical analysis),
• make new things that are useful to us (plastics, alloys, etc.),
• make things change faster (increase the speed of making medicines, etc.) and
• make things change slower (delay the turning bad of food in food preservation).
• What is more, proper use of chemical techniques can help us to solve pollution problems.
In conclusion, Chemistry is essential for the good living of people in the modern world.
F3 Chemistry Notes – Page 1
2. Mixtures and their separation
One of the things that chemists must be able to do is to make pure things. For example, we need our
food, water, medicine, etc. to be clean or pure. The fuel for cars and factories need to be free of
impurities like sulphur for reducing air pollution (as a result of sulphur oxides). To make things pure we
have to be able of separating things from one another.
What is a pure substance ?
A pure substance contains only one kind of matter.
substance, sand is another pure substance.
For example, iron (in the form of filings) is a pure
What is a mixture ?
A mixture contains more than one pure substance mixed together. For example, a mixture of solids is
formed when iron filings and sand are mixed together. Air is a mixture of gases (nitrogen, oxygen, etc).
Common Methods of Purification
1. Filtration can be used to separate an insoluble substance from a liquid or a solution. For example,
sand can be separated from water by filtration. After filtration, the clear liquid collected is called the
filtrate, and the solid left on the filter paper is called the residue. However, filtration cannot separate
a soluble substance from a solution. For example, when water is added to a mixture of sand and salt
and then the mixture filtered, the residue is sand while the filtrate is salt solution (i.e., salt and water).
Filtration does not remove the salt (which is soluble) from the mixture. To obtain dry, pure salt,
either crystallisation or evaporation has to be employed.
2. Crystallisation can automatically exclude impurities when it crystallises, so it is a method of purifying
things because the impurities are usually not soluble in a hot solvent. For example, pure sugar can be
purified from a crude sugar solution (containing impurities and pure sugar) by crystallisation.
3. Evaporation can be used to remove water from a dissolved solid. For example, when sea water is
evaporated, salt is left behind. The things to be separated must differ widely in their boiling points.
4. Distillation is evaporation followed by condensation. This can be used to purify a liquid that is made
dirty with soluble impurities. For example, when sea water is distilled, pure water (distilled water)
will be obtained as the distillate. Salts with high boiling point are not easy to evaporate and remain
as residue. The things to be separated must have differ widely in their boiling points.
5. Fractional distillation can be used to separate two liquids with more or less the same boiling points.
It is very similar to distillation, but is actually a series of repeated distillations. To simplify the
F3 Chemistry Notes – Page 2
procedure, a fractionating column is usually used. For example, wine containing more water but less
alcohol can be fractionally distilled to make it richer in alcohol (i.e., more concentrated in alcohol).
The method of purifying a mixture depends on the properties of the mixture to be separated. For
example, iron filings can be separated from sand by using a magnet. However sawdust (wood powder)
cannot be separated from sand by using a magnet. Sand can be separated from sawdust by first adding
water and then filtering. (can you explain why?)
3. Elements and Compounds
What is an element ?
An element is a substance that cannot be further broken down to two or more simpler substances by
chemical methods.
• An element is one of the simplest substances on Earth, e.g., oxygen, nitrogen, carbon, iron, copper,
silver, gold, iodine, chlorine, neon, argon, etc., are elements.
• Scientists have discovered about 100 elements (some books list 103, some may list up to 108 elements)
on Earth. Of these elements, about 90 are found in nature. The rest are made by scientists under
high-energy conditions. It is possible that in the future scientists can discover more elements.
• Some elements are more abundant (more plentiful) on Earth. The most abundant elements in the Earth
crust are oxygen (49.5%), silicon (25.7%), aluminium (7.5%) and iron (4.7%).
English Name
Symbol
Carbon
Latin Name
English Name
Symbol
C
Cobalt
Co
Calcium
Ca
Copper
Cu
Sulphur
S
Iodine
I
Silicon
Si
Iridium
Ir
Silver
Ag
Argentum
Iron
Fe
Sodium
Na
Natrium
Phosphorus
P
Lead
Pb
Plumbum
Potassium
K
Latin Name
Cuprum
Ferrum
Kalium
F3 Chemistry Notes – Page 3
F3 Chemistry Notes – Page 4
• Each element has a name and a symbol. The symbol is usually made up of a capital letter, or a capital
letter followed by a small letter. The symbol of an element usually resembles its English name. In
some cases it resembles its Latin name.
• Most elements can be classified as metals and non-metals. Many known elements are metals.
physical properties of metals and non-metals are summarised in the following table :
The
Metals
Non-metals
1. Have bright shiny surfaces when freshly cut or
polished.
2. Are good conductors of heat and electricity.
3. Are malleable (can be hammered into sheets)
and ductile (can be drawn into wires).
4. Are usually solids (except mercury) with high
melting points (except sodium, potassium).
1. Usually have dull surfaces.
2. Are poor conductors of heat and electricity
(except graphite)
3. Are brittle, i.e., break to pieces when
hammered or bent.
4. Usually have low melting and boiling points
(except diamond and graphite). Many are
liquids (e.g., bromine) or gases (e.g., chlorine)
at room temperatures.
• A few elements have properties that are in between those of metals and non-metals, and are called
semi-metals or metalloids, e.g., silicon and germanium.
What is a compound ?
A compound is a pure substance made up of two or more elements chemically combined together.
• A pure compound has constant composition. This means that no matter where and how a compound
is obtained or made, it is always made up of the same elements combined in the same proportion by
mass.
• A pure compound always has the same properties or characteristics (e.g., colour, smell, reactions, etc.).
• The properties of a compound is different from those of the elements from which it is made. e.g.,
water can be used to put out a fire, but oxygen can be used to relight a glowing splint, while hydrogen
can produce an explosion when it is lit.
• Each compound has a name and a formula. For example, the formula of water is H2O, which means
that there are two hydrogen atoms and one oxygen atom in each molecule of water. The formula of
carbon dioxide is CO2, and this means that there is one carbon atom and two oxygen atoms in each
molecule of carbon dioxide. We can give a fixed formula to each compound because no matter where
and how you obtain this compound it always has the same composition.
• Usually an energy change will accompany the formation of a compound. If heat energy is given out
during the change, the change is described as exothermic (Greek : exo = outside, theroms = hot). If
heat energy is absorbed during the change, the change is described as endothermic (Greek : endo =
within).
F3 Chemistry Notes – Page 5
4. Atomic Structure and the Periodic Table
What is an atom ?
An atom is the smallest particle of an element that can still have the properties of that element.
If a piece of metal is cut into a smaller one, the smaller piece still has the same properties as the original
piece of metal.
Is an atom the smallest particle in the world ?
No. Inside the atom, there are smaller particles called the sub-atomic particles (sub = below, under, or
subordinate), namely the protons, the neutrons and the electrons.
Properties of the sub-atomic particles are listed in the table below :
Sub-atomic particle
Relative charge
Relative mass
proton
+1
1
neutron
0
1
electron
-1
1
1840
,
(negligible)
• In an atom, the protons and the neutrons make up a small (diameter of about 10-15 m ) but heavy central
particle for the atom, and this particle is called the nucleus (Latin : nucula = little nut). The nucleus
carries positive charge.
• At a relatively long distance away, electrons move in orbits (Middle English : orbite = eye socket)
around the nucleus. (The diameter of an atom is about 10-10m, 100,000 time the diameter of the
nucleus.
Imagine a football field : it is about 100m long. 100m × 10-5 = 1mm : if the atom is as big
as a football field, the nucleus would be about the size of the head of a pin) The electrons carry negative
charges. Between the nucleus and the electrons is an empty space. The electrons are extremely
small in size.
Structure of a hydrogen atom :
What is the atomic number of an element ?
The atomic number of an element is the number of protons in an atom of the element.
• Charges on each proton and on each electron are equal in amount but opposite in sign.
• An atom always has an equal number of protons and electrons, so an atom is always electrically neutral.
• The atomic number of an element is also the number of electrons in a neutral atom of the element.
• The atomic number is a characteristic of an element.
F3 Chemistry Notes – Page 6
Element
atomic number
Element
atomic number
Element
atomic number
hydrogen
1
oxygen
8
phosphorus
15
helium
2
fluorine
9
sulphur
16
lithium
3
neon
10
chlorine
17
beryllium
4
sodium
11
argon
18
boron
5
magnesium
12
potassium
19
carbon
6
aluminium
13
calcium
20
nitrogen
7
silicon
14
gold
79
• The atomic number of an element is given the symbol Z. Thus for hydrogen, Z = 1, and for helium, Z
= 2.
What is the mass number of an atom ?
The mass number of an atom is the sum of the number of protons and the number of neutrons in the
atom.
• Since the electrons have negligible mass, the mass of an atom is approximately the total mass of the
protons and neutrons. So the relative mass of an atom can be represented by the mass number of the
atom.
• The mass number of an atom is given the symbol A. Thus for a helium atom with 2 protons and 2
neutrons, A = 4.
• Since the neutrons are electrically neutral, number of neutrons of an atom bears no direct relationship to
the atomic number. For example, most hydrogen atoms have 1 proton but no neutron at all, and in
most sodium atoms, there are 11 protons but 12 neutrons. However, many atoms have the same
number of protons and neutrons in each atom, e.g., most carbon atoms have 6 protons and 6 neutrons in
the nucleus, and most oxygen atoms have 8 protons and 8 neutrons in the nucleus.
• The number of neutrons in the nucleus of an atom can be calculated by using the formula :
number of neutrons = A – Z
• The following notation (method of representation) is used to show the structure of the nucleus of
atoms :
Mass number → A
atomic number →
X
Z
e.g., the structure of the helium nucleus can be represented as :
4
2
He
What are isotopes ?
Isotopes are atoms of the same element but with different number of neutrons in the nucleus of the atoms.
In other words, isotopes have same atomic number, but different mass numbers.
Isotopes are atoms.
F3 Chemistry Notes – Page 7
Similarities
Differences
of the same element
with same atomic number
with different mass number
with same number of protons
with different number of neutrons
• Most elements are made up of a mixture of many isotopes.
abundance of the isotopes are not equal. For example,
Element
hydrogen
lithium
isotope
H
99.985
2
H
0.015
6
7
carbon
% abundance
1
12
13
C
C
Element
nitrogen
isotope
% abundance
14
N
99.635
15
N
0.365
24
Mg
78.60
92.58
25
Mg
10.11
98.892
26
Mg
11.29
Li
Li
For each element with isotopes, the
7.42
magnesium
1.108
chlorine
35
Cl
75.53
37
Cl
24.47
What is the ‘carbon-12’ scale ?
• Atoms are so small and light that it is not useful to show their masses in gram (g) or kilogram (kg).
The mass of an atom is better compared with the mass of another atom.
• The mass of an atom of the isotope of carbon, 12C is chosen as the standard for comparing the masses of
different atoms. The mass of a 12C atom is given a relative mass of 12.0000. This is called the
carbon-12 scale.
• On the carbon-12 scale, the mass of the 12C atom is the same as its mass number.
• On the carbon-12 scale, the mass of
1
12
the mass of a
12
C atom has a mass of 1.0000.
This is the
same as the mass number of the lightest atom, 1H (the most abundant isotope of hydrogen).
• Loosely speaking, the mass of an atom on the carbon-12 scale is the number of times that this atom is
as heavy as the hydrogen atom. This is a ratio, so it is a pure number. It has no unit.
What is the relative atomic mass of an element ?
The relative atomic mass of an element is the average mass of all the naturally occurring isotopes on the
carbon-12 scale.
• The relative atomic mass is given the symbol of Ar .
• The ‘average’ taken is a ‘weighted average’. This is illustrated in the example below :
Example : In chlorine, 75.53% of the atoms have the mass of 35 and 24.47% of the atoms have the
mass of 37 on the carbon-12 scale.
The average mass of the chlorine atoms
35 × 75.53 + 37 × 24.47
=
= 35 × 75.53% + 37 × 24.47% = 35.49
100
• Notice that a relative atomic mass has no unit.
• Notice that the average mass of the isotopes of chlorine, 35Cl and 37Cl, is not 36.
F3 Chemistry Notes – Page 8
• The relative atomic masses of some elements are not whole numbers because of the existence of
isotopes.
Can you calculate the relative atomic masses of (1) magnesium, and (2) hydrogen ?
• The relative atomic masses of some elements (like hydrogen) are whole numbers not because there are
no isotopes, but because the abundance of the less common isotopes is negligible. In such cases, the
relative atomic mass of the element is (approximately) the same as the mass number of the most
abundant isotope.
Electronic Structure of Atoms
• The atomic number of an atom is the number of protons in the nucleus of the atom. It is the same as
the number of electrons the atom (for the atom to maintain electrical neutrality).
• For example, a hydrogen atom has only one electron. A carbon atom has 6 electrons. A potassium
atom has 19 electrons.
• The electrons are placed inside spherical orbits, which we call shells (or electronic shells, energy levels).
• The shell nearest to the nucleus has the lowest energy. Shells farther away have higher energy.
• The building-up principle states that electrons must first be filled into the shells of lowest energy
before the shells of higher energy are filled. Electrons are filled into another shell of higher energy when
a shell of lower energy if full, until all the electrons are placed into the shells.
• Each shell can only hold a maximum of a certain number of electrons:
1. The 1st shell can hold a maximum of 2 electrons.
2. The 2nd shell can hold a maximum of 8 electrons.
• The next electrons will have to be filled to a higher energy shell when the following number of electrons
are already present in the shells :
1. 2 electrons in the 1st shell.
2. 8 electrons in the 2nd shell.
3. 8 electrons in the 3rd shell.
You may be interested to know :
The 3rd shell can hold a maximum of 18 electrons.
The 4th shell can hold a maximum of 32 electrons.
Representing the Electronic Structure of an Atom
(1) By energy level diagrams (electronic diagrams)
1. The nucleus is represented by the atomic symbol.
2. The atomic number (= number of protons = number of electrons) of the element is first found.
F3 Chemistry Notes – Page 9
3. Electrons are filled into the lowest energy shell first, and then to the higher energy shells one by one.
4. Starting from the 2nd shell, the electrons in a shell are shown singly if there are only 4 electrons or less
in the shell. From the fifth electron onwards, the newly added electrons have to be paired with
another one of the first 4 electrons already present. Compare the electronic diagrams of carbon,
fluorine and sodium below.
The electronic structures of the first 20 elements can be represented as below :
number of electrons in shell
Element
Z
1st
2nd
3rd
4th
H
1
1
He
2
2
Li
3
2
1
Be
4
2
2
B
5
2
3
C
6
2
4
N
7
2
5
O
8
2
6
F
9
2
7
Ne
10
2
8
Na
11
2
8
1
Mg
12
2
8
2
Al
13
2
8
3
Si
14
2
8
4
P
15
2
8
5
S
16
2
8
6
Cl
17
2
8
7
Ar
18
2
8
8
K
19
2
8
8
1
Ca
20
2
8
8
2
(2) By notation
1. The atomic number of the element is first found.
2. The symbol of the element is written on the left.
3. The number of electrons in each shell is listed, starting from the first shell, next to the symbol.
Examples :
O 2,6
P 2,8,5
Ca 2,8,8,2
F3 Chemistry Notes – Page 10
5. Fossil Fuels
Coal, Petroleum and Natural gas as fossil fuels
Fossil Fuels
Energy is very important to all living things. An important way to obtain energy is by burning fuels.
The commonest and most useful fuels are fossil fuels.
We have three main fossil fuels :
(1) Petroleum (Oil)
(2) Coal
(3) Natural Gas
Coal, petroleum and natural gas are called fossil fuels (化石燃料) because they were formed from the
remains of plants and animals that lived millions of years ago.
Coal
1. Coal is formed from the remains of plants that lived hundreds of millions of years ago.
2. The plant remains were buried deep inside the earth crust.
3. Under high temperature and pressure the plant remains gradually turned to coal, which consists
mainly of carbon.
Petroleum (Oil) and Natural Gas
1. Oil and natural gas come from the remains of plants and animals that lived in the sea hundreds of
millions ago.
2. These remains were buried deep beneath sand and mud.
3. Under high temperature and pressure, and assisted by bacterial action, the remains gradually turned
into oil and natural gas.
How long will world fuel resources last
Fossil fuels are non-renewable resources. Once burnt they are lost. Fossil fuels have taken millions of
years to form and cannot be regenerated in a short period of time.
There is a limit within which our fuel resources will be used up. Hence the concept of conservation of
fuel is important.
List some of the ways to conserve fuels
Petroleum as a mixture of hydrocarbons and its separation into useful fractions by fractional
distillation
Crude oil is a mixture of hydrocarbons. Useful fractions of crude oil can be separated by fractional
distillation. In the school laboratory, a simple distillation set-up could be used to separate crude oil. In
the petroleum refinery industry, a fractionating tower is used.
F3 Chemistry Notes – Page 11
Oil fractions and their uses
Name of fractions
boiling point (°C)
carbon number
uses
refinery gas
below 40
C1-C4
LPG as fuel
petrol
40-170
C5-C10
motor car fuel
naphtha
raw material to make
town gas
kerosene
170-250
C10-C16
fuel for jet aircraft
gas oil
250-350
C16-C25
raw material to make
more gasoline
fuel oil
>350
> C25
fuel for buses, trucks
and ships
lubricating oils
and waxes
to make candles and
polishes
bitumen
road surfacing
Fractions usually differ in terms of boiling points, colour, volatility, viscosity and burning characteristics.
The following table compares the properties of the light and heavy petroleum fractions.
Boiling points
Colour
Volatility
Viscosity
Burning characteristics
Light Fraction
lower
colourless
more volatile
watery
burns without residue
Heavy Fraction
higher
yellowish
less volatile
more viscous
burns with a smoky flame, leaving
a residue
F3 Chemistry Notes – Page 12
6. The Mole
Avogadro Number
H
O
An oxygen atom is bigger in size, and is heavier, than a hydrogen atom.
Actually an O atom is approx. 16 times more heavy than an H atom.
It is difficult to weigh one atom because we could not have such an accurate balance.
Chemists have introduced a quantity called mole so that if we weigh one mole of atoms, it would be
6.02 × 1023 atoms.
Avogadro number, L is equal to 6.02 × 1023 mol-1.
How heavy is one atom of hydrogen ?
The approximate mass for 1 mole, i.e., 6.02 × 1023 atoms of hydrogen is 1 g.
atom of hydrogen.
1
Mass of 1 atom of hydrogen =
= ________ g
6.02 × 1023
Calculate the mass of 1
Similarly calculate the mass for 1 atom of oxygen. The approximate mass for 6.02 × 1023 atoms of
oxygen is 16 g.
Mass of 1 atom of oxygen = __________ = ________ g
Work out the ratio of
mO
=
mH
where mO is the mass for one atom of oxygen and mH is mass for one atom of hydrogen
Complete the following table
Isotope
12
C
32
Cl
37
Cl
Pb
37
Note :
Mass of 6.02 x 1023 particles
12
12 g
S
35
207
Relative mass of isotope
35 g
37
2+
Cl-
Pb2+ and Cl- are called ions. An ion is a charged particle made from an atom or a group of
atoms. In these examples, Pb2+ is a lead atom which has lost 2 electrons, and Cl- is a chlorine
atom that has gained 1 electron.
F3 Chemistry Notes – Page 13
Relative Masses of Molecules
A molecule is made up of two or more than two atom chemically combined together.
We can work out the relative masses for molecules by simple addition.
For HCl, the relative molecular mass = 1 + 35.5 = 36.5.
For H2O, the relative molecular mass = 2(1) + 16 = 18.
Complete the following table :
(Given : H = 1, O = 16, Cl = 35.5, C = 12)
Molecule
Relative Molecular Mass
H2 (hydrogen molecule)
2(1) = 2
O2 (oxygen molecule)
Cl2 (chlorine molecule)
O3 (ozone, found in upper layers of the atmosphere)
CO2 (carbon dioxide gas)
CHCl3 (trichloromethane, or chloroform)
Molar Mass
The relative molecular mass of H2 = 2.
For one mole of H2, we simply add the unit grams to get the mass of H2 = 2 g
Hence, the molar mass of H2 = 2 g (or : 2 g mol-1)
What would be the molar mass for O2 ? ____ g ( ____ g mol-1)
Relationship between Molar Mass and Mass
The molar mass of H2 is 2 g. That means if we had 2 g of H2, we have one mole of H2.
How many moles of H2 do we have if we have got only 1 g of H2 ? _____
A simple relationship between mass, molar mass and number of moles is as follows :
mass
Amount in moles =
molar mass
Example
(1) How many moles of molecules are there in 64 g of oxygen molecules, O2 ?
Given : relative atomic mass of O = 16
Molar mass of O2 = 2 (16) = 32 g
Amount in moles
=
mass
molar mass
64
= 32
=
2
(2) Calculate the number of moles of H2O molecules in 72 g of water
Given : relative atomic masses O = 16, H = 1
Molar mass of H2O = 2 (1) + 16 = 18 g
Amount in moles
=
mass
molar mass
=
72
18
= 4
(3) Calculate the mass of 1 atom of O2, given that the relative atomic mass of O = 16.
F3 Chemistry Notes – Page 14
Chemical Formulae
A chemical formula is an expression (or a symbol) to show the composition of a compound (or a
substance). E.g.,
He
H2O
NaCl
a compound, the
formula shows that
there are two hydrogen
atoms and one oxygen
atom per particle
(molecule)
a compound, the
formula shows that the
ratio of sodium atom to
chlorine atom in the
compound is
1:1
H2
an element, the formula an element, the formula
shows that there is only
shows that there are
one helium atom per
two atoms of hydrogen
particle
per particle (molecule)
The chemical formula of a compound can be obtained by working out the ratio in moles for each element
in the compound.
The following diagram shows the apparatus used in an experiment to determine the formula of black
copper oxide :
Town gas (containing mainly hydrogen by volume) was passed over the oxide before heating began.
During heating, the copper oxide changed colour from black to light brown. After some time, heating
was stopped, but town gas was allowed to pass over the copper until it was cold.
The results of a typical run of the above experiment were as follows :
Mass of tube
= 20.10 g
Mass of tube and copper oxide
= 22.68 g
Mass of tube and copper
= 22.16 g
Town gas was burnt at the jet after it has passed over the copper oxide, otherwise the posionous
component (carbon monoxide) in town gas can kill, and the main component (hydrogen) may
explode.
(2) The experimental results are interpreted as follows :
(a) Mass of copper = 22.16 - 20.10 = 2.06 g
(b) Mass of oxygen = 22.68 - 22.16 = 0.52 g
(c) The moles of copper and oxygen can be worked out as follows :
(1)
Mass (in g)
Number of moles
Mole ratio
(d)
Cu
O
2.06
0.52
2.06/63.5 = 0.0324
0.52/16 = 0.0325
1
1
The mole ratio of Cu to O is 1 : 1, hence the formula of the black copper oxide is CuO
F3 Chemistry Notes – Page 15
Empirical Formula
The empirical formula of a compound is the formula that shows the simplest ratio of atoms of the
elements present in the compound.
Formulae that show the simplest ratio are called empirical formulae because they can be easily obtained
from experiments. (empirical = relying on or derived from observation or experiment.)
The chemical formula of a compound could be worked out if the mass of each element contained in the
compound is worked out. The masses of each element are then converted into number of moles. The
ratio in moles of each element gives the chemical formula of the compound.
The chemical formula obtained from this method (from an experiment) is called the empirical formula.
Example :
Mass analysis of a compound containing C, H and O only gives the following information.
Total mass of compound = 4.4 g
Mass of C = 2.4 g
Mass of H = 0.4 g
Work out the empirical formula of the compound
(Relative atomic masses : C = 12, H = 1, O = 16)
Solution
Mass of O = Total mass - Mass of C - Mass of H = 1.6 g
Mass (g)
Number of moles
Mole Ratio
C
H
O
2.4
0.4
1.6
2.4/12 = 0.2
0.4/1 = 0.4
1.6/16 = 0.1
2
4
1
The Empirical Formula of the compound is C2H4O
Molecular Formula
The empirical formula only shows the ratio of atoms in a compound.
To find out the exact number of atoms in a compound, the molar mass of the compound has to be stated.
Example
A compound P has the empirical formula C2H4O. P has a molar mass of 88 g.
Determine the molecular formula of P.
Let the molecular formula of P be (C2H4O)n
Molar mass of C2H4O = 2(12) + 4(1) + 16 = 44
44n = 88 ⇒ n = 2
Hence the molecular formula of P is (C2H4O)2.
The formula simplifies to C4H8O2.
Structural Formula
Structural formulae give further information of compounds.
The arrangement of atoms in space is given by the structural formula.
F3 Chemistry Notes – Page 16
7. The Periodic Table and Periodicity
How are the elements arranged in the Periodic Table ?
•
•
•
•
In the Periodic Table, the elements are arranged in order of increasing atomic numbers.
The horizontal rows in the Periodic Table are periods. There are 7 periods.
The vertical column in the Periodic Table are groups. There are 8 main groups.
Starting from period 4, more elements appear between Group II and Group III. These elements are the
transition metals, e.g., iron (Fe), copper (Cu), mercury (Hg), silver (Ag) and gold (Au).
What is the meaning of ‘periodicity’ ?
• The Periodic Law states that when the elements are arranged in order of increasing atomic numbers,
the elements having similar properties reappear at regular intervals. Similar properties reappear at
intervals of eight elements for the first twenty elements.
• The phenomenon of properties of elements recurring at regular intervals is called periodicity.
• A characteristic or property that reappears at regular intervals is said to show periodic variation. E.g.
Li, Na and K, all react towards water similarly.
What are the characteristics of a period ?
• Atoms of elements in the same period have the same number of electronic shells.
• The period number is the number of electronic shells. e.g., sodium (Na), which is in period 3, has 3
shells (2, 8, 1).
• Metallic character decreases and non-metallic character increases across a period from left to right.
e.g., in period 3, sodium (Na, on the left) is a metal; chlorine (Cl, on the right) is a non-metal.
• A diagonal line in the Periodic Table passing through boron and silicon roughly divides the metals
from the non-metals. Elements to the left of this diagonal line are metals and those to the right are
non-metals. Elements lying just besides the diagonal line are semi-metals, e.g. silicon and germanium.
• Elements on the far left of the Periodic Table are very reactive metals while those on the far right are
very reactive non-metals.
• The group 0 elements on the extreme right of the Periodic Table are very unreactive. They are called
the noble gases. e.g., helium (He), neon (Ne), argon (Ar), krypton (Kr) and xenon (Xe).
What are the characteristics of a group ?
•
•
•
•
Elements in the same group have the same number of electrons in their outermost shells.
The group number of an atom is equal to the number of outermost shell electrons in the atom.
The chemical properties of an element depend on the number of electrons in its outermost shell.
As elements in the same group have the same number of outermost electrons, they have similar
properties.
F3 Chemistry Notes – Page 17
Group similarities and trends in chemical properties
What are noble gases and why are they unreactive ?
• All noble gases (i.e. group 0 elements) have eight electrons in the outermost shell (an octet) except
for helium, which has 2 electrons in its only one shell (a duplet).
• The special electronic structures of noble gases (duplet or octet) are very stable and that is why noble
gases are unreactive. i.e., noble gases do not want to have any change to their stable electronic
structures.
What is metal character ?
• Metals are elements on the left side of the Periodic Table.
• The outermost electrons of the metals are not so firmly held. Metals tend to lose electrons from its
outermost shell to form positive ion (or cation, i.e., positively charged particle) which has the same
electronic structure as the nearest noble gas. (Cation is the positive ion, it contains the letter “t”
that looks like a “+” sign.)
• The atoms that tend to lose electrons from their outermost shells are said to show metallic character.
e.g., the Na atom tends to lose one electron to form a Na+ ion :
Na →
Na+ + e2,8,1
2,8
electron
ion with stable
noble gas structure
e.g., the Mg atom tends to lose two electrons to form a Mg2+ ion :
+ 2eMg →
Mg2+
2,8,2
2,8
electrons
ion with stable
noble gas structure
What is the meaning of electropositive ?
• Metals tend to form positively charged ions by losing electrons. They are said to be electropositive.
(Question : Which is a more electropositive element, sodium or magnesium ?)
....................................................................................................................................................................
• On going across a period, the electropositive (or metallic) character of the elements decreases.
• On going down a group, the electropositive character of the elements increases. The outermost shell
electrons gets further away from the nucleus and thus the electrons are easier to be lost.
(Question : Sodium and potassium are both group I metals, which is more electropositive ?)
....................................................................................................................................................................
What is non-metal character ?
• A non-metal atom tends to attract or gain electrons to its outermost electron shell to form a
negative ion (or an anion) which has the same electronic structure of the nearest noble gas.
F3 Chemistry Notes – Page 18
• Non- metals are elements on the right side of the Periodic Table.
e.g., the Cl atom tends to gain one electron to form a Cl- ion :
Cl + e→ Cl2,8,7
electron
2,8,8
ion with stable
noble gas structure
e.g., the S atom tends to gain two electrons to form a S2- ion.
S + 2e→ S22,8,6
electrons 2,8,8
ion with stable
noble gas structure
• The atoms that tend to gain electrons to form negative ions are said to show non-metallic character.
(Question : Which is a more electronegative element, chlorine or sulphur ?)
....................................................................................................................................................................
• On going across a period, the non-metallic character (or electronegativity) increases.
• On going down a group, the non-metallic character (or electronegativity) decreases. The attraction
between the nucleus and the electrons in the outermost electron shell is weaker because the number of
electronic shells increases. Thus the atoms have weaker tendencies to gain electrons into their
outermost shell.
(Question : Fluorine and chlorine both are group VII elements, which is more electronegative ?)
....................................................................................................................................................................
Electropositivity and Electronegativity
In general, metals are electropositive and non-metals are electronegative. (Why ?)
F3 Chemistry Notes – Page 19
8. Conductors, Non-conductors and Electrolytes
In Topic 3, we have classified elements into conductors and non-conductors (or insulators). This
classification cannot be applied to compounds.
None of the compounds are conductors of electricity,
whereas some compounds can conduct electricity under certain specific conditions, which we call them
electrolytes. Compounds that cannot conduct electricity under all conditions are called non-electrolytes.
What is an electrolyte ?
A compound that can conduct electricity when molten or in aqueous solution is called an electrolyte.
What are conductors ?
• They are elements.
• They conduct electricity but are not chemically changed after the conduction of electricity.
• Metals are examples of conductors. They conduct electricity in both solid and liquid states because
they have free electrons in both physical states.
What are insulators ?
• They include both elements and compounds.
• They do not conduct electricity in any state.
• They include all non-metal elements (except graphite) and compounds that have no free electrons.
Characteristics of electrolytes :
• Electrolytes include ionic compounds (compounds made up of both positive and negative ions).
• They do not conduct electricity in the solid state but conduct in the molten or aqueous (dissolved in
water. Latin : aqua = water) state.
• During conduction, they decompose (break down; de- = opposite, Latin : com- = together, p¢nere =
put.).
What is electrolysis ?
• Electrolysis is the decomposition of an electrolyte by the passage of electric current through the
electrolyte in the molten or aqueous state (lysis :Greek , lusis = to loosen)
Examples of conductors, insulators and electrolytes :
Conductors (elements)
Insulators
(elements or compounds)
Electrolytes (compounds)
mercury
iron
copper
lead
graphite (non-metal)
sugar
distilled water
plastics
wax
sulphur (an element)
sodium chloride
lead(II) bromide
copper(II) sulphate
copper(II) chloride
potassium iodide
F3 Chemistry Notes – Page 20
How do electrolytes conduct electricity ?
•
•
•
•
•
Electrolytes are made up of charged particles called ions. Thus they are ionic compounds.
Ions are electrically charged particles made from atoms or groups of atoms.
Positively charged ions are also called cations.
Negatively charged ions are also called anions.
On passing electricity through molten or aqueous electrolytes, the (+ve) cations move to the negative
electrode (cathode) and gain electrons from the electrode. The (-ve) anions move to the positive
electrode (anode) and lose electrons to the electrode.
• Electrons do not flow directly through the electrolyte, but electric current is carried through the
electrolyte by the movement of ions.
• Movement of ions in the liquid state or the molten state is also known as the migration of ions.
Why electrolytes cannot conduct electricity in the solid state ?
• In the solid state, the cations (+ve) and anions (−ve) attract each other strongly. The ions are held
together firmly in fixed positions. The ions are therefore not free to move and the electrolyte cannot
conduct electricity.
Why can electrolytes conduct electricity in the molten or aqueous state ?
• In molten or in aqueous state, the strong attractive forces between ions are weakened. Thus, the ions
become free to move (i.e., become mobile) and conduct electricity.
An example of the conduction of electricity through an electrolyte  molten lead(II) bromide
• The positive lead ions gain electrons from the cathode and form neutral lead atoms.
is seen around the cathode.
positive lead ions + electrons
→
(electrons from cathode)
Metallic lead
neutral lead atoms
• The negative bromide ions lose electrons to the anode and form neutral bromine atoms, which then
further reacted to become bromine molecules. Red bromine vapour is seen around the anode.
negative bromide ions
→
neutral bromine atoms + electrons
(electrons carried away by the anode)
F3 Chemistry Notes – Page 21
9. Chemical Bonding (Ionic Compounds and Covalent Substances)
Why do elements combine to form compound ?
Example : Two elements, sodium and chlorine, combine to form sodium chloride.
• When sodium is ignited and lowered into a jar of chlorine, it continues to burn and heat is given out.
A white solid is left behind which is sodium chloride.
• Sodium and chlorine have higher potential energy compared with sodium chloride. When ignited,
the elements combine and release energy. A chemical bond which links sodium and chlorine atom
together is formed. A compound, sodium chloride (i.e. the white salt), is also formed which has
lower potential energy and is more stable than the two elements.
sodium + chlorine
→
(elements : higher potential energy)
sodium chloride
(compound : lower potential energy)
IONIC BONDING
What are ionic compounds ?
Compounds made up of both positive and negative ions are called ionic compounds.
Are there any evidence for the existence of ions ?
Evidence of the existence of ions
 migration of ions
F3 Chemistry Notes – Page 22
• In the above experiment, the U-tube is filled with a solution of copper(II) sulphate and potassium
dichromate in gel. Water containing some dilute sulphuric acid for better conduction of electricity is
added carefully to each limb of the U-tube, so as to avoid mixing of the layers.
• The circuit is closed. After some time, a blue colour appears around the cathode region and an orange
colour appears around the anode region.
• The reason for the above observation: cations (+ve), Cu2+ ions, move towards the cathode (-ve) and the
anions (-ve), Cr2O72- ions, move towards the anode (+ve).
How and why ions are formed ?
• Except helium, all atoms of noble gases have 8 electron in the outermost shell, i.e. an octet structure
of electrons in the outermost shell. It is believed that noble gases are unreactive because their
electronic structures are very stable.
• Atoms of elements other than noble gases do not have such stable electronic arrangements. They can
become stable by gaining or losing electron(s) to attain the nearest noble gases structure.
• A simple ion is formed when an atom loses or gains electron(s).
• Ions are electrically charged particles.
Positive ions (Cations)
• Metal atoms lose their outermost shell electron(s) to form positive ion.
• The positive metal ion has the stable electronic structure of the nearest noble gas.
• e.g., each sodium atom loses one electron and becomes a sodium ion.
...
..
11p
12n
..
... .
sodium atom, Na
(2, 8, 1)
loses
1 electron
...
..
11p
12n
..
+
...
+ 1e-
sodium ion, Na+
(2, 8)
11 protons = 11+
10 electrons = 10__________________
overall charge = +1
Negative ions (Anions)
• Non-metal atoms gain electron(s) until there are eight electrons in the outermost shell to form negative
ions (except hydrogen).
The negative non-metal ion has the stable electronic structure of the nearest noble gas.
F3 Chemistry Notes – Page 23
• e.g., each chlorine atom gains one electron to become a chloride ion.
x x
x x
x x
x x
x
17p
18n
-
x x
x x
x
x x
x
+ 1e-
gains
1 electron
x x
x
x x
x x
x x
17p
18n
.
x x
x
x
x x
x x
chloride ion, Cl(2, 8, 8)
chlorine atom, Cl
(2, 8, 7)
17 protons = 17+
18 electrons = 18__________________
overall charge = -1
What is an ionic bond ?
• An ionic bond is the strong electrostatic attractive force between cations (+ve) and anions (-ve).
• A complete transfer of electrons from a metal atom to a non-metal atom can form an ionic bond.
• Example: sodium chloride
1. The sodium ions Na+ and chloride ions Cl- in sodium chloride are formed by the transfer of electrons
from sodium atoms to chlorine atoms.
structures.
Both the ions formed have stable noble gas electronic
2. The positive Na+ ions and the negative Cl- ions are then attracted and held together by strong
electrostatic forces.
3. Electronic diagram for sodium chloride :
...
..
Na
..
... .
sodium atom
Na
.
(2, 8, 1)
x x
x x
x xx
x
x
Cl
xx
xx
...
..
Na
...
x x
x x
..
chlorine atom
sodium ion
xx
x
Cl
xx
x
x
(2, 8, 7)
-
x x
x x
+
.
x xx
x
x
Cl
xx
xx
x x
x x
chloride ion
.
xx
[Na]+
x
[ x Cl x ]xx
(2, 8)
(2, 8, 8)
Notice that in the second representation above, only the outermost shells are shown.
F3 Chemistry Notes – Page 24
•Example: Magnesium fluoride
x x
x
...
..
Mg
..
x
x
F
x
x x
x x
.....
...
x x
x
x
F
x
x
..
Mg
..
.
x
2+
x
...
x
F
x
x x
x x
.
x
x x
x
x
F
x
x x
Magnesium atom
.
Mg
(2, 8, 2)
fluorine atoms
xx
2
x
F
xx
(2, 7)
x
x
fluoride ions
magnesium ion
[Mg]2+
2
(2, 8)
.
xx
[x F
xx
x
x
]-
(2, 8)
In the space below, sketch electronic diagram to show how the compound Al2O3 could be formed from
its elements Al and O.
Electronic diagram of Al2O3
What is a simple ion ?
• A simple ion is a monatomic ion formed from a single atom, e.g., Cl-, Na+.
What is a polyatomic ion ?
• A polyatomic ions is an ion made from two or more atoms, e.g., OH-, SO42-.
F3 Chemistry Notes – Page 25
NAMES AND FORMULAE OF COMMON IONS
Cations
Anions
Formula
Name
Colour
Formula
Name
Colour
Na+
sodium ion
colourless
Cl-
chloride ion
colourless
K+
potassium ion
colourless
Br-
bromide ion
colourless
colourless
-
iodide ion
colourless
hydroxide ion
colourless
nitrate ion
colourless
permanganate ion
purple
nitrite ion
colourless
hydrogencarbonate ion
colourless
hydrogensulphate ion
colourless
hypochlorite ion
colourless
hydride ion
colourless
oxide ion
colourless
carbonate ion
colourless
sulphate ion
colourless
dichromate ion
orange
chromate ion
yellow
sulphide ion
colourless
sulphite ion
colourless
+
Ag
+
silver ion
I
-
H
hydrogen ion
colourless
OH
NH4+
ammonium ion
colourless
NO3-
+
Cu
+
Hg
copper(I) ion
mercury(I) ion
red
colourless
MnO4
NO2
-
-
HCO3HSO4
OCl
-
-
HMg2+
2+
Ca
2+
Ba
magnesium ion
calcium ion
barium ion
colourless
colourless
colourless
O2CO3
SO4
2-
2-
2+
lead(II) ion
colourless
Cr2O7
Fe2+
iron(II) ion
pale green
CrO42-
Pb
2+
Co
Ni
2+
cobalt(II) ion
pink
S
2-
22-
nickel(II) ion
blue / green
SO3
Mn2+
manganese(II) ion
colourless
(Mn2+ ions are actually very pale pink in colour)
Cu2+
copper(II) ion
blue
Zn
zinc ion
colourless
Hg2+
mercury(II) ion
colourless
Al3+
aluminium ion
colourless
2+
3+
iron(III) ion
brown
3+
chromium(III) ion
green
Fe
Cr
PO43-
phosphate ion
3+
(Dilute Fe
colourless
ion solutions are yellow in colour)
Formulae and naming of ionic compounds
How can the formulae of an ionic compound be accurately predicted ?
• The formula of an ionic compounds shows the ratio of different ions in the compound.
• The symbol of the cation (+ve) is always written first, followed by the symbol of the anion (-ve).
• All ionic compounds are overall electrically neutral, thus in an ionic compound the total positive
charge must be equal to the negative charge.
F3 Chemistry Notes – Page 26
Examples:
(a) sodium sulphate is made up of Na+ and SO42- ions.
One negative SO42- ion needs two positive
Na+ ions to make the compound overall electrically neutral.
Ions
Na+
SO42Ratio
Formula
2
:
1
Na2SO4
Note :
the number “2” in the above formula is called the subscript. The subscript
tells how many times the ion occurs in each formula.
(b) magnesium hydroxide contains Mg2+ and OH- ions. One positive Mg2+ ion needs two negative
hydroxide ions to make the overall charges become zero. As the OH- ion is a polyatomic ion,
the symbol of the OH- ion is placed in brackets and the subscript is written outside the
bracket.
Ions
Mg2+
OHRatio
Formula
1
:
2
Mg(OH)2
Note : the bracket is needed when both of the following conditions are satisfied -1. the ion in the bracket is a polyatomic ion, and
2. the ion occurs more than once in each formula of the coumpound.
Do not use a bracket if any one of the above conditions are not satisfied.
(c) Some more examples (Can you explain why these compounds should have such formulae ?) :
K2Cr2O7
(NH4)2SO4
Al2O3
Pb(NO3)2
How do we give names to ionic compounds ?
• The cation (+ve) is named first, followed by the name of the anion (-ve).
• The name of a simple non-metal ion always ends in the letters -ide. For example, ZnO is called zinc
oxide, FeS is called iron sulphide, NaCl is called sodium chloride.
• The name of polyatomic ions containing oxygen always end in -ate or ite. For example, MgSO4 is
called magnesium sulphate, KMnO4 is called potassium permanganate, (NH4)2Cr2O7 is called
ammonium dichromate.
• Many metals, especially transition metals, can form ions with different ionic charges. The charge is
then indicated by Roman numerals written in brackets after the name of the metal. For example,
Fe(OH)2 is iron(II) hydroxide and Fe(OH)3 is iron(III) hydroxide.
F3 Chemistry Notes – Page 27
COVALENT BONDING
How and why are covalent bonds (covalent molecules) formed ?
• Instead of transferring electrons to become ions, non-metal elements can also get the stable electronic
structure of noble gas by sharing electrons.
• Bonds formed by sharing electrons are called covalent bonds.
• Covalent bonds are found in non-metallic elements and compounds formed between non-metallic
elements.
• In the example below, each atomic nucleus (positively charged) has an attraction for the shared electron
pair (negatively charged). Therefore, the two atoms are held firmly together.
shared electron pair
: a covalent bond
H
.
+
x
H x
H
hydrogen atoms
or :
H
.
+
.
H
hydrogen molecule
x
.
H
H xH
or
H--H
What is a molecule ?
A molecule is formed when two or more atoms combine together by covalent bonds only.
What is the atomicity of a molecule ?
The number of atoms present in each molecule is called its atomicity.
• Molecules formed by combining two atoms only are called diatomic molecules.
e.g., O2, H2.
• Molecules formed by combining three atoms are called triatomic molecules.
• Molecules formed by combining many atoms together are called polyatomic molecules.
Examples for formation of covalent bond :
The chlorine molecule
• The electronic structure in notation of chlorine is 2, 8, 7. The outermost electronic shell of chlorine
needs one more electron in order to have a stable noble gas structure.
• Two chlorine atoms can each uses one electron for sharing. Each chlorine atom now has 8 electrons in
its outermost shell and it can then be considered to have a stable noble gas structure.
• The bond in the chlorine molecule is a single covalent bond (or single bond) since it is formed by one
shared electron pair (or two shared electrons).
..
or :
..
Cl
..
.......
Cl
x x
.
x
Cl
x x
chlorine atoms
+
x
x
..
..
x x
x
Cl
..
.......
.
Cl
x
x
x x
chlorine molecule
x
x x
Cl xx
x x
x
x x
x
Cl Clx x
x
or
......
x x
Cl--Cl xx
x x
F3 Chemistry Notes – Page 28
The oxygen molecule
• The electronic structure in notation of oxygen is 2, 6. The outermost shell of oxygen needs two more
electrons in order to have a stable noble gas structure.
• Two oxygen atoms can each use two electrons for sharing. Each oxygen atom now has 8 electrons in
its outermost shell and it can then attain the stable noble gas electronic structure.
• The bond in the oxygen molecule is a double bond since it is formed by two shared electron pairs (or
four shared electrons).
..
O
..
....
..
or :
..
x x
..
x
x
O
x
x
O
..
.. ..
..
x x
oxygen atoms
O
x x
..
O
x x
oxygen molecule
x
x
+
x x
O
O
x x
x
x
..
..
x x
O
x x
or
x
x
O = O
x
x
The nitrogen molecule
• The electronic structure in notation of nitrogen is 2,5. The outermost shell of nitrogen needs three
more electrons in order to have stable noble gas structure.
• Two nitrogen atoms can each use three electrons for sharing. Each nitrogen atom now has 8 electrons
in its outermost shell and it can attain the stable noble gas electronic structure.
• The bond in nitrogen molecule is a triple bond since it is formed by three shared electron pairs (or
six shared electrons).
..
N
..
.
x
x
x
x
N
nitrogen atoms
or :
.. ...
N
+
x
..
x
x
x
N
..
.
x
N
x
nitrogen molecule
x
x
x
N xx
.. ...
N
x
x
x
N
x
x
or
..
N
N
x
x
More examples :
The water molecules, H2O
• Hydrogen atom needs one more electron to become stable and the oxygen atom needs two more
electrons to become stable.
• A hydrogen atom and an oxygen atom each uses one electron for sharing. The hydrogen atom then
can be considered to have a stable structure as helium. The oxygen atom, however, needs one more
hydrogen atom for sharing electrons in order to have a stable noble gas structure. Thus, two single
bonds are formed .
• The electron pairs in the outermost shell not involved in the bonding are called lone pairs or
non-bonding electron pairs.
F3 Chemistry Notes – Page 29
H
.
bondpair
x
H
lone pair
xx
xx
x
O
.
H x
.
O
x x
.
x H
x x
The water molecule
has 2 bond pairs
and 2 lone pairs of
formation of water (H2O)
The CH4 and NH3 molecules
H
.
. .
.
x
H
x
C
x x
x H
H x
.
N
x
.
H
H
methane (CH4)
.
x H
x
ammonia (NH3)
End of F3 Chemistry Notes
F3 Chemistry Notes – Page 30