1. ATOMIC STRUCTURE

2. DEVELOPMENT OF ATOMIC MODEL
Scientist Atomic Atomic Model
Structure
1808- suggested that
matter is made of tiny
particles which are
indivisible and
indestructible
John Dalton An atom is like a miniature
(1766-1844) billiard ball
1897- put forward the
“plum pudding” model
of the atom. The
atom is like a sphere
with small electrons Plum Pudding Model
embedded in a
J.J Thompson positively charged -discovery of electron
(1856-1940) mass

3. Scientist Atomic Atomic Model
Structure
1911- shows that:
 atom is spherical &
-
has a centre or -
nucleus with a dense
-
positive charge
Ernest Protons are the -
Rutherford -
positively charged
(1871-1937) particles in the
nucleus
A system of
electrons rotates Rutherford’s Solar
around the nucleus System Model
much like the planets
orbiting around the
sun

4. Scientist Atomic Atomic Model
Structure
1913- showed that:
Electrons orbit in
specific energy levels
called shells
Each shell is capable
Niels Bohr Bohr’s electrons orbiting in
of holding a certain
(1885-1962) number of electrons. shells
1932- discovered that:
A particle in the
nucleus which has no
electric charge and
called it neutron
James All nuclei (except
Chadwick
hydrogen) contain
(1891-1974) neutrons as well as
protons

5. SUBATOMIC PARTICLES
electron
{
proton
nucleon + shell
neutron
+
Atomic structure of an atom
 Atoms are made of three different types of subatomic particles called
protons, neutrons and electrons
 Protons and neutrons are found in the nucleus and they are known as
nucleons
 Nucleus is positively charged because the protons are positively charged
and the neutrons are electrically neutral
 Electrons rotate around the nucleus in a certain orbit called shells, just
like planets rotate around the sun

6. THE PROPERTIES OF SUBATOMIC PARTICLES
Subatomic Proton Neutron Electron
particles
Symbol p n e¯
Relative +1 0 -1
electric
charge
Relative 1 1 _1__
mass 1840
 An atom is neutral when it has the same number
of protons and electrons

7. PROTON NUMBER
 The number of protons in an atom of an element
Proton number = number of protons

8. NUCLEON NUMBER
 The sum of protons and neutrons in an atom
element
Nucleon number = number of protons + number of neutrons

9. NUMBER OF NEUTRONS
 the difference between the nucleon number and the
proton number
Number of neutrons = nucleon number – proton number
 A neutral atom has the same number of protons and the
number of electrons
 Different elements have different numbers of protons

10.  The number of protons, electrons and neutrons in a
particular atom are given in the Periodic Table by the
nucleon number and proton number
A
X
Z
A : proton number
Z :nucleon number
X: symbol of an element
 The number of protons, electrons and neutrons can be
determined from the nucleon number and proton number

11. Example: (a) Sulphur
32
S
16
 the symbol of the element above shows that it is sulphur
 Nucleon number = 32
 Proton number = 16
 Number of neutron = 32-16 = 16
 So, sulphur atom has 16 protons, 16 electrons and 16
neutrons

12. Example: (b) Aluminium
27
Al
13
 The symbol of the element above shows that it is
aluminium
 Nucleon number = 27
 Proton number = 13
 Number of neutron = 27 – 13= 14
 Aluminium atom has 13 protons, 13 electrons and 14
neutrons

13. Example: (c) Sodium
23
Na
11
Number of protons = 11
Number of neutrons
= 23 - 11
= 12
So, number of electrons = 11

14. Exercises
1. What is matter?
2. What are atoms, molecules and ions?
3. How does the state of matter change with the change
in temperature?
4. How does the kinetic theory of matter explain the inter-
conversion of the state of matter?
5. How was the atomic model developed?
6. What are the main subatomic particles of an atom?
7. What is proton number?
8. What is nucleon number?