SOME BASIC CONCEPTS OF CHEMISTRY
Matter and Elements
The term matter refers to anything that occupies space and has mass—in
other words, the “stuff” that the universe is made of. All matter is made up of
substances called elements, which have specific chemical and physical
properties and cannot be broken down into other substances through ordinary
chemical reactions. Gold, for instance, is an element, and so is carbon. There are
118 elements, but only 92 occur naturally. The remaining elements have only been
made in laboratories and are unstable.
Each element is designated by its chemical symbol, which is a single capital
letter or, when the first letter is already “taken” by another element, a
combination of two letters. Some elements follow the English term for the element,
such as C for carbon and Ca for Calcium. Other elements’ chemical symbols come from
their Latin names; for example, the symbol for sodium is Na, which is a short form of
Natrium, the Latin word for sodium.
The four elements common to all living organisms are oxygen (O), carbon (C),
hydrogen (H), and nitrogen (N), which together make up about 96% of the human
body. In the non-living world, elements are found in different proportions, and some
elements common to living organisms are relatively rare on the earth as a whole.
THE STRUCTURE OF THE ATOM
An atom is the smallest unit of matter that retains all of the chemical
properties of an element. An atom consists of two regions.
1. The tiny atomic nucleus, which is in the centre of the atom and contains
positively charged particles called protons and uncharged particles called
neutrons.
2. Electrons, negatively charged particles that orbit around the nucleus.
The attraction between the positively charged protons and negatively charged
electrons holds the atom together.
Electron Shells
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Most atoms contain all three of these types of subatomic particles—
protons, electrons and neutrons.
Hydrogen (H) is an exception because it typically has
one proton and one electron, but no neutrons.
Symbol
Nature
Mass
Actual mass
Charge
Electron
eNegatively charged
negligible
9.1×10-31kg
-1
Proton
p+
Positively charged
1
1.67×10-27kg
+1
Neutron
n
Neural
1
1.67×10-27kg
No charge
Atom as a whole is neutral. In an atom, the number of
electrons orbiting the nucleus is equal to the number of protons
inside the nucleus. The positive and negative charges cancel
out, leading to an atom becoming electrically neutral.
Protons, neutrons, and electrons are very small, and most of the volume of an
atom—greater than 99 percent—is actually empty space. With all this empty
space, you might ask why so-called solid objects don’t just pass through one
another. The answer is that the negatively charged electron clouds of the atoms
will repel each other if they get too close together, resulting in our perception
of solidity.
Atomic Number and Mass Number
Atomic number is the fundamental property of an atom. Every atom is
identified by its unique atomic number. Atomic number is denoted by ‘
Z’.
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Atomic number is defined as the number of protons present in the
nucleus of an atom. Since an atom is electrically neutral, the number of
protons and electrons are equal in number.
Atomic Number = Number of protons = Number of electrons
Example:
Atomic number of carbon is 6. Thus, it has 6 protons and 6 electrons.
Mass number(A) of an atom is defined as the sum of the number of
protons and number of neutrons.
Mass number = Number of protons + Number of neutrons
Example:
Lithium has 3 protons and 4 neutrons. Thus its mass number is 7
THE PERODIC TABLE
In our daily life, we like materials to be arranged in a systematic pattern.
For example, arrangements of books in a library, arrangement of clothes in a
cupboard in a definite pattern, attendance register etc. All these are arranged
in some logical order or groups. In attendance register, the names are
arranged in alphabetical order. It is a human instinct to classify i.e. to put
things of one kind in one group and of another kind in another group.
Towards the end of the 18th and the beginning of the 19th century more and
more elements began to be discovered and their individual properties were
studied. There was a need to classify these elements so as to make their
comparative study easier. To make the study of elements easier, scientists
started arranging elements in a tabular form known as the periodic table.
Periodic means an occurrence of an event at a particular interval of time.
Table means a set of systematic arrangement of substances in columns and
rows. So, the periodic table of elements is an arrangement of elements in a
repetitive manner based on certain properties.
Modern Periodic Table
A tabular arrangement of elements in groups and periods which
highlights the regular trends in properties of elements is defined as the
periodic table.
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It is a table in which the elements are arranged in order of increasing
atomic number. Elements with similar properties are arranged in the same
column and elements with the same number of shell are arranged in the
same row.
There are eighteen vertical columns known as groups in the modern
periodic table which are arranged from left to right and seven horizontal
rows which are known as periods.
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Group
1
Group Name
Alkali metals
Property
They form strong alkalis with water
Alkaline earth
metals
Boron family
Carbon family
They also form alkalis but weaker than
group I elements
Boron is the first member of this family
Carbon is the first member of this family
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Nitrogen family
Nitrogen is the first member of this family
16
Oxygen family
Oxygen is the first member of this family
17
Halogen family
They are also known as Halogens
18
Zero group
They are called noble gases (or inert gases)
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Classifications of Elements in the periodic table
The elements of group 1, 2, 13, 14, 15, 16, and 17 are known as the Main
group elements or Normal elements. The elements of groups 3, 4, 5, 6, 7, 8, 9,
11 and 12 are known as the Transition elements. The group 18 is called
the Noble gases or Inert gases. The two rows of 14 elements at the bottom of
the periodic table are the Lanthanides and the Actinides.
Shell and shell configuration
An electron shell is the outside part of an atom around the atomic nucleus.
The maximum number of electrons in a shell = 2n2,
where n is the shell number ( K shell, n= 1; L shell, n= 2
M shell, n=3 etc.)
First shell (K) has maximum 2 electrons.
Second shell (L) has maximum 8 electrons.
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Valence shell, valence electrons and valency
The outermost orbital /shell of an atom is called its valence shell.
The electrons in the valence shell are called valence electrons. The number of
electrons lost or gain by an atom to get the stable octet is called valency.
(Octet means eight electron in the valence shell)
For Example,
1. SODIUM- Na
Atomic number- 11
Electronic configuration- K-2, L-8, M-1
Atom of every element wants to have eight electrons in its valence shell. As
you can see, Sodium has one electron in its valence shell. In order to get a
completely filled outermost shell it has to lose this one electron. So, Sodium
will have a valency of 1.
2. SULPHUR - S
Atomic number- 16
Electronic configuration- K-2, L-8, M-6
As Sulphur has six electrons in its valence shell. It has to gain 2 more
electrons to have a full valence shell . So, Sulphur will have a valency of 2.
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SYMBOLS OF ELEMENTS
Symbols are used for representing elements. The symbols are either the
first letter or first two letters of the name of the elements. First letter of
the symbol is always in capitals followed by the small letter. For example:
H for Hydrogen, Al not AL for Aluminum, Co for Cobalt, not CO.
Some symbols are derived from the first and third letter of anelement’s
name For example: Cl for Chlorine, Zn for zinc, Mn for Manganese, Mg for
Magnsium etc.
Some of the elements have their symbols derived from Latin or Greek. For
example: Na (Natrium) for sodium, Fe (Ferrom) for iron, K (kalium) for
potassium.
Element
Latin/ Greek
name
Symbols
Sodium
Natrium
Na
Potassium
Kalium
K
Iron
Ferrum
Fe
Copper
Cuprum
Cu
Argentum
Ag
•
Silver
Tin
Stannum
Sn
•
Antimony
Stibium
Sb
•
Tungsten
W
Gold
Wolframium
Aurum
Mercury
Hydrargyrum
Hg
Lead
Plumbum
Pb
Au
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The atomic number, symbols, shell configuration and valency of the first 20
elements in the periodic table are given below:
Atomic number
Element
Symbol
1
Hydrogen
H
2
Helium
He
3
Lithium
4
5
Shell
configuration
Valency
1
1
2
0
Li
2, 1
1
Beryllium
Be
2, 2
2
Boron
B
2, 3
3
6
Carbon
C
2, 4
4
7
Nitrogen
N
2, 5
3
8
Oxygen
O
2, 6
2
9
Flourine
F
2, 7
1
10
Neon
Ne
2, 8
0
11
Sodium
Na
2, 8, 1
1
12
Magnesium
Mg
2, 8, 2
2
13
Aluminium
Al
2, 8, 3
3
14
Silicon
Si
2, 8, 4
4
15
Phosphorus
P
2, 8, 5
3,5
16
Sulphur
S
2, 8, 6
2
17
Chlorine
Cl
2, 8, 7
1
18
Argon
Ar
2, 8, 8
0
19
Potassium
K
2,8,8,1
1
20
Calcium
Ca
2, 8, 8, 2
2
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Mnemonics for first 20 elements
Happy Henry Lives Beside Boron Cottage
Near Our Friend Nelly
Nancy Mg Allen Silly Patrick Stays Close
to Arthur and Kind to Carrie.
Symbols of other elements:
No
Elements
1.
Zinc
2.
Iodine
3.
Bromine
4.
5.
Symbols
No
Elements
Symbols
Zn
6. Radium
Ra
I
7. Platinum
Pt
Br
8. Chromium
Cr
Nickel
Ni
9. Titanium
Ti
Cobalt
Co
10. Manganese
Mn
IONS
Metals and non-metals can form charged species called ions.
Cation is formed
Anion is formed
These charged species can be either positively charged called Cation or
negatively charged called Anion. Example: In sodium chloride (NaCl), Na+ is
cation and Cl- is anion.
As a mnemonic device, you can imagine
that the "t" is a "+"for cations, (ca+ion )
and "I" is a "-"for anions, (an-I-on)
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They can be further classified into following two types:
Ions possessing only one atom are termed as monoatomic ions. For
example, Na+, K+ etc. Ions possessing more than one atom are termed as
polyatomic ions. For example, CO32-, NO3-etc.
Names and symbols of some ions:
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Writing chemical formula- Criss- Cross Method
RULE I: Cross multiply the valencies of the elements to form the formula of the
respective compound.
RULE II: If a compound consists of both metallic as well non-metallic
elements then name or symbol of the metal is considered first.
where Mg (magnesium) is a
metal and Cl (chlorine) is a
non-metal.
RULE III: In case compounds are formed from polyatomic ions then the ion is
enclosed in a bracket before writing the number to indicate the ratio. But in
case the number of polyatomic ion is one then there is no need for bracket.
Since the number of polyatomic ion is one
therefore no brackets are required.
The formula for calcium hydroxide is Ca(OH)2.
The brackets around OH with subscript 2 imply
the presence of two hydroxyl group joined to one
calcium atom.
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Write the chemical formula for the following compound
S.No. Name of compound
Chemical
formula
Oxides
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
Sodium oxide
Calcium oxide
Potassium oxide
Magnesium oxide
Carbon dioxide
Sulphur dioxide
Nitrogen dioxide
Halides
Sodium chloride
Calcium chloride
Potassium iodide
Magnesium chloride
Hydrogen chloride
Hydroxides
Sodium hydroxide
Aluminium hydroxide
Potassium hydroxide
Magnesium hydroxide
Calcium hydroxide
Sulphates
Copper sulphate
Iron(II) sulphate
Hydrogen sulphate
Sodium sulphate
Magnesium sulphate
Carbonates
Sodium carbonate
Calcium carbonate
Ammonium carbonate
Sodium bicarbonate
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CHEMICAL EQUATIONS
A chemical equation is an expression for a given chemical change in terms of
formulae of the reactants and products. The substances that undergo chemical
change in the reaction are called reactants, while new substances formed
during the reaction, are called products.
As you know, when magnesium ribbon is burnt in oxygen, it gets converted
into magnesium oxide. It can be expressed in the form of word equation as
follows:
Magnesium + Oxygen → Magnesium oxide
A word equation shows change of reactants to products through an arrow
placed between them. The reactants are written on the left hand side with
a plus sign (+) between them. Similarly, products are written on the right
hand side with a plus sign (+) between them. The arrow head points
towards the products, and shows the direction of the reaction. In the above
mentioned word equation, Magnesium and oxygen are reactants, while
magnesium oxide is the product.
Writing chemical equation
A chemical equation is a shorter way of representing a chemical
reaction. It can be made more concise and useful if we use chemical formulae
instead of words. If you recall formulae of magnesium, oxygen and magnesium
oxide, the above word equation can be written as –
Mg + O₂
MgO
This is a skeletal chemical equation for the burning of magnesium in
air since the equation is unbalanced. Here, the number of atoms of each
element is not the same on both sides. Such a chemical equation is called
skeletal chemical equation for a reaction.
A chemical equation must always be balanced as shown below.
2Mg + O₂ → 2MgO (Balanced Chemical Equation)
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Balancing chemical equation
1. ____H2 +____ N2 → NH3
2. H2 + Cl2 → HCl
3. H2 + O2 → H2O
4. CaO + H2O → Ca(OH)2
5. CaCO3 → CaO + CO2
6. Zn + HCl → ZnCl2 + H2
7. Mg + O2 → MgO
8. Fe + HCl → FeCl2 + H2
9. Al+ O2 → Al2O3
10. Na + O2 → Na2O
11. K + O2 → K2O
12. Fe + CuSO4 → FeSO4 +Cu
Exercise Questions
1. What do you understand by reactants and products in a chemical
equation?
2.If the atomic number of an element is 11, write the electronic
configuration.
3. Write the symbol of the following elements.
a) Sodium
b) Chlorine
c) Pottasium
4. Write the balanced equation for the following chemical reactions.
a) Nitrogen + Hydrogen
Ammonia
b) Aluminium + Oxygen
Aluminium oxide
c) Hydrogen + Oxygen
Water
d) Sodium + Oxygen
Sodium oxide
5. Name the subatomic particles of an atom.
6. Draw the atomic structure of Magnesium.
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