
Atoms are made up of three types of
smaller particles, namely protons,
neutrons and electrons. These particles
are known as subatomic particles.
SUBATOMIC
PARTICLE
SYMBOL
RELATIVE MASS
RELATIVE
ELECTRIC
CHARGE
PROTON
P
1
+1
NEUTRON
n
1
0
ELECTRON
e
0 (almost)
-1
Protons and neutrons are found in the nucleus of
an atom while electrons surround the nucleus.

Each element is given a name and a symbol.
Some examples of elements and their symbols
are shown below.
ELEMENT
SYMBOL
ELEMENT
SYMBOL
Hydrogen
H
Sodium
Na
Helium
He
Magnesium
Mg
Lithium
Li
Aluminium
Al
Beryllium
Be
Silicon
Si
Boron
B
Phosphorus
P
Carbon
C
Sulphur
S
Nitrogen
N
Chlorine
Cl
Nucleon number
Proton
number
Proton number = An elements is the
number of protons in its atom.
 Since atoms are neutral, the proton
number is also the number of electrons in
the atom.

Proton number = electron number

Each elements has its own proton
number.
The nucleon number is also known as the
mass number.
 The relative mass of an atom is almost
the same as its nucleon number.
 Protons and neutrons are collectively
called nucleons because protons and
neutrons occupy the nucleus.


Nucleon number: total number of protons
and total number of neutrons in its atom.

From definition,
Nucleon number = number of proton + number of neutrons
or
Number of neutrons = nucleon number – number of proton

Isotopes are atoms of the same element with
the same number of protons but different
numbers of neutrons.

Isotopes of an element have the same:
(a) number of protons in an atom.
(b) number of electrons in an atom.
(c) electron arrangement.
(d) chemical properties because they have
the same electron arrangement.

Isotopes of an element have different:
(a) nucleon number.
(b) number of neutrons in an atom.
(c) physical properties (such as density,
melting point and boiling point), though
these different are very small.

Eg: 3He, 4He, 12C, 14C, 235U, and 239U,
respectively)
Atomic Structure Worksheet
Fill in the blanks for the elements in this chart. For the purposes of this chart,
round all atomic masses to the nearest whole number.
Element
lithium
carbon
chlorine
silver
lead
calcium
Number of Number of Number of
Protons
Neutrons Electrons
Atomic
Mass
Atomic
Number
ANSWER
Element
Number of
Protons
Number of
Neutrons
Number of
Electrons
Atomic
Mass
Atomic
Number
lithium
3
4
3
7
3
carbon
6
6
6
12
6
chlorine
17
18
17
35
17
silver
47
61
47
108
47
lead
82
125
82
207
82
calcium
20
20
20
40
20

The electron configuration (electron arrangement)
of an atom is the arrangement of electrons in shells
(orbits) around the atom's nucleus.

Example
SODIUM
Proton number = 11
Electron number = 11
Electron configuration = 2.8.1

The electrons in the last shell of an atom are
known as valence electrons. They
determine the chemical properties of the
element.
Electron configuration = 2.8.4
Valence electron = 4
Electron configuration = 2.7
Valence electron = 7
Electron configuration = 2.4
Valence electron = 4

Helium has a complete shell of 2 valence electrons
or duplet condition.

Similarly, neon and argon also have 8 valence
electrons or the octet condition.

These elements are monoatomic and are very
stable gases.
Helium
2
Neon
2.8
Argon
2.8.8



Number of shells = Period in Periodic Table
Valence electron (1 and 2) = Group in Periodic Table
Valence electron (3 – 8) = Valence electron + 10
= Group in P.Table
eg: Valence electron =7
So, Group 17
1
8
2
SODIUM
Proton number = 11
Electron number = 11
Electron configuration = 2.8.1
Position in Periodic Table
Period: 3
Group : 1
Element
Proton Number
number of
electrons
Number of elements in shell Electron
Number of
arrangement valance
1
2
3
4
electrons
Oxygen
8
8
2
6
2.6
6
Fluorine
9
9
2
7
2.7
7
Neon
10
10
2
8
2.8
8
Sodium
11
11
2
8
1
2.8.1
1
Magnesium
12
12
2
8
2
2.8.2
2
Aluminium
13
13
2
8
3
2.8.3
3
Silicon
14
14
2
8
4
2.8.4
4
1.
Draw electron arrangement for element
Na, Ar, Mg, Al, Cl, O, C, N.
2.
State the valence electron for each
elements
3.
State the position in Periodic Table