This document discusses the structure of the atom. It describes the discovery of subatomic particles like electrons, protons, and neutrons through experiments. This led to the failure of Dalton's atomic theory which stated that atoms were indivisible. It discusses J.J. Thomson's "plum pudding" model and Rutherford's gold foil experiment, which resulted in Rutherford's nuclear model of the atom with electrons orbiting a small, dense nucleus. Later, Bohr improved on this model by proposing that electrons can only orbit in fixed, quantized energy levels. The document also discusses concepts like atomic number, mass number, isotopes, and isobars.

1. STRUCTURE OF ATOM
Subatomic Particles of Atom, Discovery of Electron, Protons and
Neutrons, J.J. Thomson Model of Atom, Rutherford Model of Atom,
Bohr’s Atomic Model, Valency and Valence Electrons, Atomic
Number and Mass Number, Isotopes and Isobars.
Topics to be covered

2. SUBATOMIC PARTICLES OF ATOM
Dalton’s atomic theory in suggested that an atom was indivisible. However, the
discovery of three fundamental particles named as electrons, protons and neutrons,
inside the atom, led to the failure of Dalton’s atomic theory.
ELECTRONS PROTONS NEUTRONS

3. DISCOVERY OF ELECTRON - DISCHARGE TUBE EXPERIMENT
In 1879, William Crooks studied the conduction of electricity through gases at low pressure.
He performed the experiment in a discharge tube which is a cylindrical hard glass tube
about 60 cm in length. It is sealed at both the ends and fitted with two metal electrodes.

4. DISCOVERY OF CATHODE RAYS {ELECTRONS}
● At very low pressure and at very high
voltage, electrical discharge is
observed.
● The pressure of different gases is
regulated by vacuum pump attached to
the discharge tube.
● When a high voltage is applied across
the electrodes, current starts flowing
from the negative electrode (cathode)
to the positive electrode (anode). These
were called cathode rays giving green
inflorescence.

5. PROPERTIES OF CATHODE RAYS { ELECTRONS }
● Cathode rays travel in straight line.
● They start from cathode and move towards the anode.
● These rays are not visible but due to phosphor and zinc
sulfide in the discharge tube which glow when hit by
them.
● Cathode rays are negatively charged particles. When
electric field is applied on the cathode rays with the help
of a pair of metal plates, these are found to be deflected
towards the positive plate indicating the presence of
negative charge.

6. DISCHARGE TUBE EXPERIMENT

7. e/m RATIO FOR ELECTRONS
● R.A. Millikan devised a method known as oil drop experiment to determine the charge
on the electrons.
● J. J. Thomson experimentally determined charge/mass ratio called e/m ratio for the
electrons.
● For this, he subjected the beam of electrons released in the discharge tube as
cathode rays to influence the electric and magnetic fields. These were acting
perpendicular to one another as well as to the path followed by electrons.

8. DISCOVERY OF PROTONS { CANAL OR ANODE RAYS }
● In 1886, Goldstein modified the
discharge tube by using a perforated
cathode.
● On reducing the pressure, he observed
a new type of luminous rays passing
through the holes or perforations of the
cathode and moving in a direction
opposite to the cathode rays.
● These rays were named as positive rays
or anode rays or canal rays. Anode rays
are not emitted from the anode but
from a space between anode and
cathode.

9. PROPERTIES OF CANAL RAYS { PROTONS }
● The rays are streams of positive ions of
the gas.
● They are deflected by electric and
magnetic fields.
● They travel in straight lines.
● The velocity of canal rays is much
smaller than the velocity of cathode
rays.

10. JJ THOMSON MODEL OF ATOM- PUDDING MODEL
● An atom consists of a uniform sphere of positive
electricity in which the electrons are distributed more or
less uniformly.
● The negative and the positive charge are equal in
magnitude. Thus, the atom as a whole is electrically
neutral.
DRAWBACKS
● Thomson's atomic model failed to explain how the positive charge holds on the
electrons inside the atom.
● It failed to explain an atom's stability.
● It could not explain the scattering experiment by Rutherford.

11. RUTHERFORD SCATTERING EXPERIMENT
Apparatus Used-
● He used 𝛂 particles which are positively charged
Helium atoms {He2+} carrying +2 charge and 4 unit of
mass.
● A 1000 atom thin foil of gold and zinc sulfide screen at
its back.
Observations -
● Most of the 𝛂-particles (nearly 99%) passed through
the gold foil undeflected.
● Some of the 𝛂-particles (about one in every 20,000)
were deflected by small angles.
● A few 𝛂 particles (1 in about 106) were either deflected
by very large angles or were actually reflected back
along their path.

12. RUTHERFORD SCATTERING EXPERIMENT

13. RUTHERFORD SCATTERING EXPERIMENT
Conclusions -
● Most of the space inside the atom is empty because
most of the alpha-particles passed through the gold foil
without getting deflected.
● Very few particles were deflected from their path,
indicating that the positive charge of the atom
occupies very little space
● A very small fraction of a-particles was deflected by
180°, indicating that all the positive charge and mass of
the gold atom were, concentrated in a very small
volume within the atom.

14. RUTHERFORD MODEL OF ATOM
● Most of the part in an atom is empty.
● There is a positively charged center in atom,
which contains nearly the whole mass of atom.
The centre is called nucleus.
● The size of nucleus is very small compared to an
atom.
● Electrons revolve around the nucleus.

15. DRAWBACK OF RUTHERFORD MODEL OF ATOM
● Rutherford’s model could not explain the stability of the atom. Whenever a charged
particle like an electron revolves around a central force like that of a nucleus, it loses
energy continuously in the form of radiations. Thus, the orbit of the revolving electron
will keep on becoming smaller and smaller and ultimately the electron should fall into
the nucleus.
● Rutherford model could not solve the problem of atomic mass of atom as it proposed only
the existence of protons in the nucleus.

16. DISCOVERY OF NEUTRONS
● By the bombardment of thin
sheets of beryllium with fast
moving alpha-particles he
observed that highly
penetrating rays consist of
neutral particles which were
named neutrons.
● It is discovered by
Chadwick.

17. BOHR’S MODEL OF ATOM
● Electron revolves around nucleus only in
certain selected circular orbits with
definite energies and are called energy
shells or energy levels.
● While revolving around the nucleus in an
orbit, an electron does not lose energy nor
does it gain energy.
● Different shells or orbits are numbered as
1, 2, 3, 4 or designated as K, L, M, N
● Every orbit is associated with a fixed
amount of energy.

18. DISTRIBUTION OF ELECTRONS IN DIFFERENT SHELLS OR ORBITS
The distribution of the electrons in the shells is known as electronic configuration. It is based
on certain guidelines or rules given by Bohr and Bury. This is known as Bohr-Bury scheme.
According to this scheme,
1. The maximum number of electrons present in a shell is given by the formula 2n2, where ‘n’ is
the orbit number of energy level index, 1, 2. 3,etc:
● First orbit or K-shell will be = 2 x 12 = 2,
● Second orbit or L-shell will be = 2 x 22 =8 ,
● Third orbit or M-shell will be = 2 x 32 = 18,
● Fourth orbit or N-shell will be = 2 x 42 = 32 and so on.
3. The maximum number of electrons that can be accommodated in the outermost orbit is 8.
4. 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.

19. VALENCE ELECTRONS AND VALENCY
● Valency: The combining capacity of an atom is called its valency,
● Valence Electrons: The electrons present in the outermost shell of the atom.
These electrons determine the valency of an atom.
● For the atoms having valence electrons less than or equal to 4, valency is same
as that of the number of valence electrons in that atom.
● For example, valency of Magnesium (2, 8 , 2) = 2
● If number of valence electrons exceed 4, then valency = 8 – Number of valence
electrons.
● For example, valency of Oxygen (2, 6) = 8 – 6 = 2.
● Valency of atoms with 8 valence electrons is zero as they have fully filled
valence shell and cannot gain or lose electrons to form, molecules or
compounds.

20. ATOMIC NUMBER AND MASS NUMBER
● The number of protons present in the nucleus of an atom.
● Since an atom is electrically neutral, thus number of protons and number of electrons are equal,
therefore:
● Atomic number (Z) = number of protons = number of electrons.
● The total number of the protons and neutrons present in the nucleus of an atom is called mass
number. It is denoted by A.
● The protons and neutrons together are called nucleon.
● The number of neutrons present in the nucleus of an atom is represented by N.
ATOMIC NUMBER
MASS NUMBER

21. DISTRIBUTION OF ELECTRONS IN DIFFERENT SHELLS OR ORBITS

22. ISOTOPES AND ISOBARS
● Atoms of the same element having same atomic number but different mass numbers are known as
Isotopes.
● Applications of Isotopes
a. Isotope of uranium is used as a fuel in nuclear reactor
b. Isotope of cobalt is used in treatment of cancer
c. Isotope of iodine is used in treatment of goitre.
● Atoms of different elements having same mass numbers are known as Isobars,
● For example:
Potassium, K and Argon. Ar both have the mass numbers equal to 40.
ISOTOPES
ISOBARS