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1. P R E S E N T E D B Y : A K S H A T S H R I V A S T V A
C L A S S : I X

2. • On over 500 BC, Maharishi Kanad, made
the postulate that if we go on dividing the
matter it would finally reach a stage where
it would be indivisible. At this stage he
termed it as parmanu.
• Democritus on the same era also discovered
the indivisibility of matter and termed the
smallest particle as matter.

3. • We know that, atoms are either positive or negative or
neutral.
• The electrons carries negative charges.
• Protons carries positive charges.
• And neutrons are neutral.
• If there are more electrons than the protons then the atom
is negatively charged.
• And if there are more protons then it is charged positively.

4. •A
• J.J. Thomson
• Ernest Rutherford
• Neils Bohr

5. • J.J. Thomson proposedthe model of atomto be similar as
plumpudding where electrons were like currants.
• He proposed that:
• An atom consists of a positively
charged sphere and the electrons are
embedded inside it.
• The negative and the positive charges
are equal in magnitude. So the atom
as a whole is electrically neutral.

6. • Although his model explained that the atom is
electrically neutral, but the results of experiments
carried out by other scientist could not be explained
by this model.

7. • He selected a gold foil because he wanted as thin as possible.
• α particles are doubly charged helium ions. Since, they have a mass of
40, the fast moving α particles have a considerable amount of energy.
• It was expected that a particles would be deflected by the sub-atomic
particles in gold atoms.
• Since the α particles were much heavier
than the protons he did not expect to see
larger deflections.

8. • Most of the fast moving alpha particles passed straight
through the gold foil.
• Some of the alpha particles were deflected by the foil by
small angles.
• Surprisingly one out of every 12000
particles appeared to rebound.

9. • The positive charge and most of the mass of the atom was densely
concentrated in a extremely small region. This very small portion of the
atom was called the nucleus by Rutherford. The size of the nucleus is
very small as compared to the size of the atom.
• The nucleus is surrounded by electrons that move around the nucleus
with a very high speed in circular paths called orbits. Rutherford’s
model of atom resembles solar system in which the sun was nucleus and
the electrons were planets.
• Electrons and the nucleus are held together by electrostatic force of
attraction.

10. • The orbital revolution of the electrons is not expected
to be stable. Any particle in a circular orbit would
undergo acceleration. During acceleration, charged
particles would radiate energy. Thus, the revolving
electron would loose energy and finally fall into the
nucleus. If this happens the atom would be highly
unstable. We know that, the atom is quite stable.

11. • In order to overcome the objections raised
against the model of Rutherford, Neils
Bohr put up the following postulates about
the model of atom:
• Only certain special orbits known as
discrete orbits are allowed inside the atom.
• While revolving around the nucleus the
electron should not radiate energy.

12. • Mainly for hydrogen.
• Could not explain the spectra of mutli-electron
atoms.
• In Bohr’s model, an electron is regarded as a charged
particle moving in a well defined circular orbit
around the nucleus.

13. • The following rules are followed for writing the number of
electrons in different energy levels or shells:
1. The maximum number of electrons present in a shell is given by the
formula 2n2, where ‘n’ is the orbit number or energy level index,
1,2,3,…. Hence the maximum number of electrons in different shells are
as follows: first orbit or K-shell will be = 2 × 12 = 2, second orbit or L-
shell will be = 2 × 22 = 8, third orbit or M-shell will be = 2 × 32 = 18,
fourth orbit or N-shell will be = 2 × 42= 32, and so on.
2. The maximum number of electrons that can be accommodated in the
outermost orbit is 8.
3. Electrons are not accommodated in a given shell, unless the inner
shells are filled. That is, the shells are filled in a step-wise manner.

14. • The combining capacity of the atoms of other
elements, that is, their tendency to react and form
molecules with atoms of the same or different
elements is known as valency.

16. • The atomic no. of a element is the no. of protons
present in the nucleus of the element. All the atoms
of an element have same atomic no.
• Eg: hydrogen->1 (1 proton)
helium->2 (2 protons)
lithium-> 3 (3 protons)

17. • The mass of an element is the sum of the no. of proton
and neutron present in the nucleus of an atom of the
element.
• The mass of an atom is mainly the sum of protons and
neutrons in the nucleus on the atom.
• Eg: Carbon: (6 protons +6 neutrons)=12u
Aluminium: (13 protons+14 neutron)=27u
Sulphur: (16protons +16 neutrons)=32u
• In the notation of an atom the
atomic no. and mass no. is written
as, 12
7 N
Mass no.
Atomic no.
Symbol
of
element

18. • Isotopes are the atoms which have same atomic no.s
but have different mass no.s.
• Eg: Hydrogen has three isotopes. They are protium (1 1
H), deuterium ( 2 1 H or D) and tritium ( 3 1H or T).
• carbon has two isotopes,12 6 C and 14 6 C,
• chlorine has two isotopes, 35 17 Cl and 37 17 Cl

19. • Isobars are atoms of two different elements having
different atomic no.s but same mass no.s.
• These pair of elements have the same number of nucleons.
• Eg: calcium, atomic number 20, and argon, atomic
number 18. The number of electrons in these atoms is
different, but the mass number of both these elements is
40.