MES INDIAN SCHOOL, DOHA-QATAR
NOTES-1- 2025-26
Section : Boys/Girls Date:27.04.2025
Class & Div.:XI (All divisions) Subject: Chemistry
Lesson / Topic: Chapter 2 – STRUCTURE OF ATOM
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Atom :
It is the smallest particle of matter that takes part in a chemical reaction.
Most of the elements are monatomic, oxygen, hydrogen, nitrogen and halogens are diatomic.
Phosphorous is tetra atomic and Sulphur octa atomic.
Sub atomic particles : Electron , Proton and Neutron.
An electron : Discovered by J.J Thomson electron name given by Stony.
Electron is the fundamental particle which carries 1 unit negative charge (1.6 x 10 -19C) and has a mass
nearly equal to 1/1837 th of that hydrogen atom(9.11x10 -31 kg).
Proton : Discovered by Goldstein
Proton is the fundamental particle which carries 1 unit Positive charge (1.6 x 10 -19 C) and has a mass
nearly equal to of that hydrogen atom(1.672× 10–27kg).
Neutron : Discovered by James Chadwick
Neutron is the fundamental particle which carries neutral charge and has a mass nearly equal to that
of hydrogen atom(1.672× 10–27kg)
Representation of atom : ZXA
Where : A is Mass number, Z is Atomic number, X Symbol of atom.
Atomic Number : It is represented by Z. The number of protons present in the nucleus of an atom is called
atomic number of an element. It is also known as nuclear charge.
For neutral atom : Number of proton = Number of electron
For charged atom : Number of electron = Z + (charge on atom}
Mass Number : It is represented by capital A. The sum of number of neutrons and protons is called
the mass number of the element. It is also known as number of nucleons because neutron & proton
are present in nucleus.
A = number of protons + number of neutrons
Isotopes : Given by Soddy
They are the atoms of a same element which have the same atomic number (Z) but different mass
number(A).
They have same Nuclear charge (Z) but different number of Neutrons (A–Z).
Isotopes have same chemical property but different physical property.
F 061, Rev 01, dtd 10th March 2020 1
C- 12 C-13 C-14
e=6 e=6 e=6
p=6 p=6 p=6
n=6 n=7 n=8
Isobars : Given by Aston
They are the atoms of different elements which have the same mass number (A) but different
atomic number (Z).
They have different number of Electron, Protons & Neutrons , but the sum of number of neutrons &
Protons (number of nucleons) remains same.
For Eg. 19 K40 20 Ca40
THOMSON'S MODEL OF ATOM [1904] :- 'Plum-Pudding model'.
Thomson proposed that an atom consists of a uniform sphere of positive charge in which the electrons are
present at some places.
RUTHERFORD's MODEL OF ATOM
The positively charged heavy mass, which occupies only a small volume in an atom, is called nucleus. It
is supposed to be present at the centre of the atom.
Electrons revolve round the nucleus in closed orbits with high speeds. This model was similar to the
solar system, the nucleus representing the sun and revolving electrons as planets.
Drawbacks of Rutherford model :
This theory could not explain the stability of an atom. According to Maxwell electron loses its energy
continuously in the form of electromagnetic radiations. As a result of this, the electron should lose energy
at every turn and move closer and closer to the nucleus following a spiral path. The ultimate result will be that
it will fall into the nucleus, thereby making the atom unstable.
If the electrons loss energy continuously, the observed spectrum should be continuous but the actual observed
spectrum consists of well-defined lines of definite frequencies (discontinuous). Hence, the loss of energy by
electron is not continuous in an atom.
Electromagnetic waves (EM waves) or Radiant Energy/Electromagnetic radiation
It is the energy transmitted from one body to another in the form of waves and these waves travel in the space
with the same speed as light ( 3 × 108 m/s) and these waves are known as Electromagnetic waves or radiant
energy.
Electromagnetic spectrum or EM spectrum :
The arrangement of arranging various types of EM waves in order of their increasing frequency or
decreasing wave length is called as EM SPECTRUM.
F 061, Rev 01, dtd 10th March 2020
