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Structure of the Atom : Chapter Notes


Matter is made up of tiny particles called atoms. Atoms are further made of three fundamental particles or sub – atomic particles called electron, proton and neutron.

Earlier Dalton postulated that atom is indivisible i.e. cannot be further divided which proved to be wrong by discovery of sub – atomic particles.

Types of Sub - atomic Particles

Note -

  • Protons and neutrons are almost of same mass.
  • The mass of proton is approximately 2000 times as that of the electron.

Discovery of Sub- atomic Particles

(a) Electron (e-):

These negatively charged particles were discovered by J. J. Thomson in 1897. In the experiment, a gas at low pressure was taken in a discharge tube made up of glass. At the ends of the discharge tube two electrodes were placed, connected to a battery for high voltage supply. The electrode connected to the negative end was known as cathode and that to the positive as anode. He then observed a stream of negatively charged particles coming out of cathode towards the anode. These particles were called electrons.

The collection of negatively charged particles emitted from cathode of discharge tube is called cathode rays.

(b) Proton (p+):

In 1886, Goldstein observed in the same experiment, with different situations that the anode emitted positive particles which were called

Canal rays: The positively charged radiations produced in the discharge tube from anode are called Canal rays.

(c) Neutron (n):

These neutrally charged particles were discovered by James Chadwick in 1932. Neutrons are present in atoms of all elements except Hydrogen.

Activities related to charged particles

Activity 1 - Comb rubbed with dry hair attracts paper pieces.


  • Rub a comb on your dry hair.
  • On rubbing some electrons will move from hair to comb.
  • Thus the comb has negative charge.
  • Now bring the comb close to some paper pieces.
  • The negatively charged Comb will attract the paper pieces.

Activity 2 - Glass rod rubbed with a silk cloth attracts an inflated balloon.


  • Rub a silk cloth on a glass rod.
  • On rubbing some electrons will move from glass rod to the silk cloth.
  • Thus the glass rod becomes positively charged.
  • Now bring the glass rod close to an inflated balloon.
  • This charged glass rod now attracts the inflated balloon.

The Structure of an Atom

Following are the various models explaining structure of an atom:

1. Thomson’s Model of an atom

It is also known as Thomson’s plum pudding model. In 1903 J. J. Thomson proposed the structure of an atom similar to that of a Christmas pudding.

He proposed:

  • An atom is a positively charged sphere and the electrons are embedded in it.
  • The Magnitude of positive and negative charge is same inside atom, so net charge inside an atom is zero.

2. Rutherford’s “α- particle scattering experiment”

Rutherford designed an experiment, He used:

  • α - Particles are doubly charged helium ion. Since they have a mass of 4 u, the fast-moving  α - particle have a considerable amount of energy.
  • A thin gold foil with thickness of about 1000 atoms.


A lot of fast moving  α - particles were bombarded on thin gold foil. After passing the foil the α – particles, hit the screen.

Following are the observations and inferences made:

Rutherford on the basis of this experiment concluded The Nuclear model of an atom. According to Nuclear model of an atom:

  1. Most of the space inside the atom is empty.
  2. At the Centre of an atom, a small, heavy, positively charged nucleus is present.
  3. Electrons revolve around the nucleus.
  4. Total positive charge in nucleus is same as total negative charge on all electrons of atom as atom has net zero charge.


According to Rutherford model, electrons revolve around positive charged nucleus. According to the, Theory of Maxwell, if any charged particle does accelerated motion it must radiate energy. Therefore, if a charged body (e-) rotates around another charged body (nucleus), it will radiate energy. Due to energy loss through the radiations, speed of electrons will decrease and eventually it will fall into the nucleus. But such collapse does not occur and atoms were found to be quite stable.

Note -

  • Rutherford after this experiment gets the credit of discovery of nucleus.
  • The protons and neutron, collectively known as ‘Nucleons’ were present in the nucleus.
  • It was found that almost all mass of an atom resides in nucleus.

3. Bohr’s Model of Atom

In 1913, Neil Bohr overcame the limitations of Rutherford model and proposed a model of atomic structure.

Following are the postulates:

  1. Electrons revolve around a centrally located heavy small and positively charged nucleus in certain discrete orbits.
  2. While revolving in discrete orbits the electrons do not radiate energy.
  3. These discrete orbits or shells are called energy levels. These orbitals or shells are represented by the letters K, L, M, N… or the numbers, n = 1, 2, 3, 4…

Atomic Number

The number of protons present in the nucleus of an atom is called atomic number. It is denoted by Z.

Atomic number of some common elements

For atoms:

Atomic number = number of proton = number of electron

For ions:

Atomic number = number of proton ≠ number of electron

For example, in aluminium atom number of electrons is equal to atomic number but in aluminium ion it is not so.

Mass number

Mass number is equal to the number of nucleons present inside the nucleus of an atom. It means it is the sum of number of protons and neutrons present in the nucleus of an atom. It is denoted by letter A.

Mass number of element = Atomic mass of element = number of protons + number of neutrons

For example -

Carbon   : Mass number  =  12  (no. of p= 6, no. of  n = 6 )
Nitrogen : Mass number  =  14  (no. of p+ = 7, no. of  n  = 7 )
Fluorine  : Mass number  =  19  (no. of p+ = 9, no. of  n = 10 )

The symbol of the element with atomic number and mass number -

For example -
Nitrogen is written as _7^{14}N, where 14 is the mass number and 7 is the atomic number.


Isotopes are atoms of same element having same number of protons but different number of neutrons. Isotopes have similar chemical properties but different physical properties.

For example -

1. Hydrogen - It exists in three different isotopic forms:

2. Carbon - It exists in three different isotopic forms:

Uses of Isotopes:

1. An Isotope of uranium \left( {_{92}^{235}Ur} \right) is used in nuclear power plants to generate electricity.

2. Used for medical purposes:

  • An Isotope of cobalt is used in the treatment of cancer.
  • An isotope of iodine is used in the treatment of goiter.

Fractional Mass number of elements: If an element is a mixture of isotopes, it may have fractional mass number.

For example, chlorine exists in two different isotopic forms i.e.  _{17}^{35}Cl and _{17}^{37}Cl in the ratio of 3 : 1 i.e. 75 % and 25 %. The varying mass numbers are 35 and 37 respectively. For calculating Mass number of chlorine, percentage of the isotopic form is used to calculate the average mass.


Isobars are atoms having different number of protons but same number of nucleons (number of protons + number of electrons). For example, Argon and Calcium have different atomic number but same mass number.

The Electron distribution in orbitals

The distribution of electrons into different orbits of an atom was suggested by Bohr and Bury and is known as electronic configuration.

Following are the three major rules for electron distribution :

1. An orbit can have a maximum of  electrons , where ‘n’ is the orbit number therefore,  maximum number of electrons in different shells are

2. Orbit are filled from inside to outside. First, n = 1 shell is filled, then n = 2, and so on.

3. The outermost occupied shell of an atom can have a maximum of 8 electrons even if it can accommodate more electrons.

For example -

1. Sulphur - It has 16 electrons. Therefore the electronic configuration is 2, 8, 6.

n = 1   or   K shell    :  2   electrons
n = 2    or  L shell   :  8   electrons
n = 3    or  M shell   :  6   electrons

2. Sodium - It has 11 electrons. Therefore the electronic configuration is 2, 8, 1.

n = 1    or    K shell    :    2 electrons
n = 2    or    L shell    :    8 electrons
n = 3    or    M shell   :    1 electron


Valency of an element is the number of electrons that its atoms should give away or take to attain stable electronic configuration i.e. the atom should accommodate 8 electrons in the outermost shells or valence shells except the K shell which can accommodate 2 electrons to the maximum.

The electrons present in the outermost orbit of an atom are known as valence electrons.

Three ways to obtain stable electronic configuration:
(a) By losing electrons
(b) By Gaining electrons
(c) By sharing electrons

For example -

Hydrogen molecule: Hydrogen has only one electron in its outermost orbit thus requires one more electron to complete its outermost orbit (K shell). For this, hydrogen atom shares one electron with another Hydrogen atom and forms H2 (Hydrogen molecule).

Valency of atoms of first ten elements with Electronic configuration

Note - Atoms do chemical reactions to attain stable configuration. But noble gases are inert as they already have stable electronic configuration.


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