Standard Grade Chemistry
Summary Notes
Topic 3 : Atoms And The Periodic Table
Learning Outcomes
General
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Credit
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Classify elements as natural / man-made; solid/liquid/gas; metal/ non-metal
Elements are arranged in the Periodic table because chemists have classified
them
A group is a column of elements in the Periodic table.
Elements in the same group have similar chemical properties.
Identify the following chemical families
 halogens
 alkali metals
 noble gases
 transition metals
Noble gases are unreactive metals
Every element is made up of small particles called atoms
Atoms have a small positively charged nucleus with negatively charged
electrons moving around outside the nucleus
An atom is neutral because the positive charge of the nucleus is equal to the
sum of the negative charges of the electrons
State that atoms of different elements are different and have a different
number on the Periodic table called the atomic number.
Give the electron arrangements of the first twenty elements
Explain the structure of the Periodic table in terms of the atomic number
and chemical properties of the elements
Elements with the same number of outer electrons have similar chemical
properties
Atoms of different elements vary in size and mass
Describe the location and charge of the proton, neutron and electron in the
atom
State the masses of the proton, neutron and electron
Atoms are neutral because they have the same number of protons and
electrons
Atoms of different elements have different numbers of protons and this is
called the atomic number
Electrons are arranged in energy fields
An atom has a mass number which equals the number of protons plus
neutrons
Calculate the number of n, p and e from the mass number and atomic number
and vice versa
Calculate the number of n, p and e in atoms and ions
Most elements exist as a mixture of isotopes
Explain that isotopes are atoms with the same atomic number but different
mass numbers
Explain what is meant by relative atomic mass
Explain why the average atomic mass of an element is rarely a whole number
Elements
Everything in the world is made up of about 100 simple substances called elements.
Elements cannot be made into anything simpler.
In music – eight notes can be made into many different tunes.
The alphabet – 26 letters can be made into millions of words.
In Chemistry – about 100 elements make millions of different materials – in fact every
material in the world!!
Different Types Of Elements
1.
Metals and Non-metals
Elements can be classified as metal or non-metal.
Metals
usually shiny
good conductors of
electricity and heat
Non-metals
usually dull
NOT good conductors of
electricity and heat
On the Periodic Table there is a black zig zag line on the chart. This line
separates metal elements from non-metal elements. The metals are on the left
of the line, the non-metals are on the right.
2.
Solids, Liquids & Gases
Elements can be classified as solids, liquids and gases.
3.
Natural and Man-made Elements
Most elements occur naturally in the world. A few do not – they did not exist
until they were made by scientists. These elements are man-made or
synthetic. All the elements with Atomic Number greater than 92 (Uranium)
are man-made.
Periodic Table
Elements are found listed in the Periodic Table. They are listed in a special way and this
allows us to find out about different elements.
Each horizontal row of elements makes up a period.
Each vertical column of elements makes up a group.
Elements in the same group of the Periodic Table have similar properties, that is, their
chemical behaviour is similar.
More About Atoms
All elements have just one kind of atom in them.
Atoms are built of three kinds of even smaller particles – protons, electrons and
neutrons.
Name
proton
electron
neutron
Symbol
p
e
n
Charge
+1
-1
0
Mass (amu)
1
1/2000
1
The Structure Of The Atom
At first it was thought that atoms were a ‘plum-pudding’ mixture of protons, neutrons
and electrons.
In 1911, it was found that when alpha particles were fired at a thin gold foil most of the
particles passed straight through, although a few were severely deflected.
Ernest Rutherford explained these observations by suggesting that most of the mass of
the atom is concentrated at the centre in the positively charged nucleus. The lighter,
negatively charged electrons surround the nucleus at some distance.
More About Protons
Protons are positively charged and are found in the nucleus. It is the number of protons
that make an element what it is.
The atomic number of an element is the number of protons it has in its nucleus.
Each element has a different atomic number, therefore each element has a different
number of protons in its nucleus.
atomic number = number of protons = number of positive charges in an atom
More About Electrons
The atomic number of an atom tells us how many protons are in its nucleus. Atoms have
the same number of protons and electrons, so the atomic number also tells us how many
electrons an atom has.
atomic number = number of protons = number of electrons
Electrons are negative and found outside the nucleus.
Electron Arrangement
We know that electrons are found outside the nucleus.
They are arranged around the nucleus in different levels of energy.
Electrons in the same level have the same energy, but have different energies from
electrons in other levels.
The higher the level number, the greater the energy of the electrons.
Electrons always go to the lowest energy level where there is space.
The electron arrangement of any atom can be worked out if we know how many electrons
can fit into each level.
Level Number
Energy
1
2
3
lowest
higher
higher still
Maximum number of
electrons
2
8
8
An atom with atomic number 3 will have 3 protons and 3 electrons. 2 electrons will fit
into the lowest level – it can hold no more; 1 will be in the next level.
We write the electron arrangement : 2,1
2,1 means :
2 electrons are in the lowest level
1 electron is in the next level
If you know the electron arrangement of an atom, you can easily work out what atom it
is.
The electron arrangement of an atom is 2,7.
This means that there is a total of 9 electrons.
Therefore there are 9 protons.
Therefore the atomic number is 9.
The element is fluorine.
When the electron arrangements of all the elements are compared within the Periodic
Table it is seen that :
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levels correspond to the horizontal rows (periods) of the Periodic Table
1st row (period) – electrons occupy the first level
2nd row (period) – electrons occupy the second level
3rd row (period) – electrons occupy the third level
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Members of the same group (vertical column) have the same number of outer
electrons
Group I
1 outer electron
Group II 2 outer electrons
Group III 3 outer electrons
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The Noble gases have filled outer energy levels
They are also very unreactive.
This suggests that a filled outer energy level is the most stable
arrangement.
Why The Group Is Important
Elements in the same group have the same number of outer electrons. However, they
have more in common than the number of outer electrons.
Elements from the same Group all behave like each other. They behave like each other
because they have the same number of electrons in their outer energy levels. It is the
number of electrons in the outer shell that makes an element behave the way it does.
Atomic Number
The Atomic Number of an element tells us the number of protons in the nucleus of its
atoms.
Since an atom is neutral it also tells us the number of electrons.
Atomic number = number of protons = number of electrons
Mass Number
The mass number of an element is the number of protons PLUS neutrons in the nucleus
of an atom.
mass number = number of protons + number of neutrons
Notation
There is a special way of writing the symbol of an element to show the atomic number
and the mass number.
mass number
symbol
atomic number
This atom has
23
Na
11
11 protons
11 electrons
12 neutrons
(the same as the atomic number)
(the same as the atomic number)
(mass number-atomic number)
Ions
Atoms sometimes lose or gain electrons to try and have a full outer energy level of
electrons. When this happens, the number of electrons does not equal the number of
protons and the particle now has a positive or negative charge. These particles are called
ions.
Positive ions
Positive ions are formed when atoms have too many electrons in their outer energy level.
They lose electrons to have a full energy level. This means they now have more protons
than electrons and, therefore, have a positive charge.
mass number
symbol
atomic number
23
This ion has
11 protons
10 electrons
12 neutrons
Na+
positive charge
11
(the same as the atomic number)
(one electron has been lost)
(mass number –atomic number)
Negative Ions
Negative ions are formed when atoms need more electrons in their outer energy level to
make it full. They gain electrons. This means they now have more electrons than protons
and, therefore, have a negative charge.
mass number
symbol
atomic number
37
This ion has
17 protons
18 electrons
20 neutrons
Cl-
negative charge
17
(the same as the atomic number)
(one electron has been gained)
(mass number – atomic number)
Atoms And Isotopes
Each element has its own atomic number. It is the atomic number- the number of
protons that makes an element what it is.
All sodium atoms have atomic number 11 – they have 11 protons. If they do not have 11
protons, they are not sodium atoms.
Sometimes atoms of the same element may have different numbers of neutrons. Most
sodium atoms have 12 neutrons but some have 13. They still have 11 protons, so they are
still sodium atoms.
Atoms with the same number of protons but a different number of neutrons are called
isotopes.
Isotopes have the same atomic numbers but different mass numbers. Most elements
occur as a mixture of isotopes.
Relative Atomic Mass
If someone asks us what boron atoms weigh, we cannot say that they weigh 10 units of
mass because most of them weigh 11. Nor can we say that they weigh 11 because some of
them weigh 10. We need to work out an average which takes into account of how many
atoms there are and what they weigh.
This is called the relative atomic mass (RAM)
RAM is the average mass of all the isotopes of an element and it is rarely a whole
number.