1) An atom is made up of protons, neutrons, and electrons. Protons and neutrons are located in the nucleus, while electrons orbit the nucleus. 2) Rutherford's gold foil experiment showed that the mass and positive charge of an atom are concentrated in a small nucleus. 3) Bohr's model improved upon Rutherford's by proposing that electrons can only orbit in discrete, fixed energy levels rather than any path, resolving the issue of electrons radiating energy in orbits.

1. STRUCTURE OF THE ATOM
CLASS – 9

2. WHAT IS AN ATOM ?
Atoms are defined as “Basic Building Blocks of Matter”. It is the Smallest Constituent
Unit of Matter that posses the properties of the chemical element.
Atoms are very small. Atomic Radius is measured in Nanometers(1 m =109 nm).
Atom is made up of three Sub – Atomic Particles :-
 Protons – It is found in the Nucleus with a Positive Charge and mass of 1 A.M.U.
 Neutrons – It is found in the Nucleus with a Neutral Charge and mass of 1 A.M.U.
 Electrons – It is a Sub – Atomic Particle with Electrically Negative Charge and of
Negligible Mass (
1
2000
Mass of the Proton).

3. The Sub – Atomic Particles

4. PROTONS
In 1886 E. Goldstein discovered evidence
for the existence of this Positively Charged
Particle (Protons) . He used a Cathode Ray
Tube with Holes. Then he noticed that these
rays were travelling in the Opposite
Direction from the Cathode Rays. He called
them as the Canal Rays and showed that
they were composed of Positively Charged
Particles. The mass of the Proton is
approximately 2000 times as that of the
Electron. In general, Proton is represented
as ‘p+’. Its mass is taken as 1 Atomic Mass
Unit and the Charge of +1.

5. NEUTRONS
In 1932, J. Chadwick discovered the Sub –
Atomic Particle which had Neutral Charge
and a Mass of 1 A.M.U. which is equal to the
mass of the Protons. It was named as
Neutron. Neutrons are present in the nucleus
of all atoms, except Hydrogen. In general, a
Neutron is represented as ‘n’.
(Atomic Mass = No. of Protons + No. of
Neutrons)

6. ELECTRONS
In 1897, J.J. Thompson discovered
Electrons which had a Negative Charge and
mass which is
1
2000
than that of the Protons.
In general, Electron is represented as ‘e-’.
(Atomic Number = No. of Protons = No. of
Electrons)

7. DALTON’S ATOMIC THEORY
The Atomic Theory of John Dalton suggested that the
atom was Indivisible and Indestructible. But the
discovery of Sub – Atomic Particles led to the failure of
Dalton’s aspect. Then, it was considered necessary to
know how electrons and protons are arranged within an
atom. For explaining this, many scientists proposed
various atomic models. J.J. Thomson was the first one
to propose a model for the structure of an atom. Then,
Ernest Rutherford proposed a model for the structure of
an atom by doing Certain Experiments. After that, Neils
Bohr postulated a model of an atom which overcome
the Objections raised by Rutherford.

8. J.J. THOMSON’S MODEL
J.J. Thomson proposed the model of an atom to be
similar to that of a Christmas pudding. The
Electrons, in a sphere of Positive Charge, were like
Currants (dry fruits) in a Spherical Christmas
pudding. It was just like a Watermelon where the
Red Part was the Positive Sphere and the Seeds
are the Electrons which are embedded in it.
Thomson proposed that :-
 An atom consists of a positively charged sphere
and the electrons are embedded in it.
 The Negative and Positive Charges are equal in
Magnitude. So, the atom as a whole is Electrically
Neutral.

9. RUTHERFORD’S MODEL
Ernest Rutherford was interested in knowing how the electrons are arranged within
an atom. So, he carried out an experiment in which fast moving Alpha (α)-Particles
were made to fall on a thin Gold Foil.
 He selected a Gold Foil because he wanted as thin a layer as possible. This gold
foil was about 1000 Atoms Thick.
 α-Particles are Doubly-Charged Helium Ions. As, they have a mass of 4 u, the
fast-moving α-particles have a considerable amount of energy.
 It was expected that α-particles would be deflected by the sub-atomic particles in
the gold atoms. Since the α-particles were much heavier than the protons, he did
not expect to see large deflections.

10. RUTHERFORD’S OBSERVATIONS
From the 𝑎 – particles Experiment Rutherford concluded
that :-
 Most of the fast moving 𝑎 – particles passed straight
through the Gold Foil because most of the space
inside the atom is Empty.
 Some of the α-particles were deflected by the foil by
Small Angles, indicating that the Positive Charge of the
atom occupies Very Little Space.
 Surprisingly one out of every 12000 Particles
appeared to Rebound by 180º, indicating that all the
Positive Charge and Mass of the Gold Atom were
concentrated in a Very Small Volume within the atom.

11. DRAWBACKS OF RUTHERFORD’S MODEL
On the basis of his Experiment, Rutherford put forward the Nuclear Model of An
Atom, which had the following features:
 There is a Positively Charged Center in an atom called the Nucleus. Nearly all the
Mass of An Atom resides in the Nucleus.
 The Electrons revolve around the Nucleus in Circular Paths.
 The Size of the Nucleus is very small as compared to the Size of the Atom.
Drawbacks of Rutherford’s Model of the Atom
The Revolution of the Electron in a Circular Orbit 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 Loses Energy
and finally fall into the Nucleus. If this were so, the atom should be Highly Unstable.
But atoms are Quite Stable.

12. BOHR’S MODEL OF ATOM
In order to overcome the objections raised against
Rutherford’s Model of the Atom, Neils Bohr put
forward the following postulates about the model of
an atom:
 Only Certain Special Orbits known as Discrete
Orbits of Electrons, are allowed inside the atom.
 While revolving in Discrete Orbits the Electrons
Don’t Radiate Energy.
 These orbits or shells are also known as Energy
Levels.
 These orbits or shells are represented by the
Letters K,L,M,N or by the Numbers, n=1,2,3,4.

13. ELECTRONIC CONFIGURATION
The Electronic Configuration was suggested by Neils Bohr and Charles Rugeley
Bury.
The following rules are followed for writing the number of electrons in different
Discrete Orbits:
 The maximum number of electrons present in a shell is given by the formula 2𝑛2
,
where ‘n’ is the orbit number with 1, 2, 3, 4 increases according to the:
• 1st Orbit (K – Shell) : 2 * 1
2
= 2 Electrons
• 2nd Orbit (L – Shell) : 2 * 2
2
= 8 Electrons
• 3rd Orbit (M – Shell) : 2 * 32
= 18 Electrons
• 4th Orbit (N – Shell) : 2 * 4
2
= 32 Electrons
 The maximum number of electrons that can be accommodated in the Outermost

14. VALENCY
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 called
Valency.
Valency is determined by the outermost shell, which has to posses an Octet(8
Electrons). This is done by sharing, gaining or losing electrons. The number of
electrons gained, lost or shared so as to make the octet of electrons in the
outermost shell, gives us directly the Valency.
• If the element is a Non – Metal having 5,6,7- Electrons in the Outermost shell
then the valency is determined by subtracting No. of Electrons in Outermost Shell
from Octet.
• If the element is a Metal having 1,2,3 – Electrons in the Outermost shell then the
valency is said to be the No. of electrons in the Outermost Shell.

15. Composition of Atoms of the First Eighteen Elements
with Electron Distribution in Various Shells

16. ISOTOPES
Isotopes are atoms of the same element that have the same number of protons (i.e.,
atomic number, "Z") but a different number of neutrons, meaning that their mass
number, "A", varies. For example, take the case of hydrogen atom, it has three
atomic species, Protium (1
1 H), Deuterium (2
1 H), Tritium (3
1 H).
Applications
Since the chemical properties of all the isotopes of an element are the same,
normally we are not concerned about taking a mixture. But some isotopes have
special properties which find them useful in various fields. Some of them are :
(i) An isotope of uranium is used as a fuel in nuclear reactors.
(ii) An isotope of cobalt is used in the treatment of cancer.
(iii) An isotope of iodine is used in the treatment of goitre.

17. ISOBARS
Let us consider two elements — 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. That is, the total number of nucleons is the
same in the atoms of this pair of elements. Atoms of different elements with different
atomic numbers, which have the same mass number, are known as Isobars.