Structure Of Atom Class 9 Notes Science - Chapter 4

Chapter: 4

What Are Structure Of Atom ?

Charged Particle In Matter


Negatively charged sub-particles of atom is called electron.

  • An electron is represented as ‘e
  • The mass of an electron is considered to be negligible and its charge is minus one.


Positively charged sub-particles of atom is called proton.

  • A proton is represented as ‘p’.
  • The mass of a proton is taken as one unit and its charge is plus one.


Neutral sub-particles of atom is called electron.

  • A neutron is represented as ’n’.
  • A neutron has no charge and has mass nearly equal to that of a proton.

Structure Of Atom

Thomson’s Model of an Atom: This model is called Pudding model.

(i) An atom consists of a positively charged sphere and the electrons are embedded in it.

(ii) The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.

Rutherford’s Model: This experiment is α-particles scattering experiment. Important postulates of this model are:

(i) There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.

(ii) The electrons revolve around the nucleus in the well-defined orbits.

(iii) The size of the nucleus is very small as compared to the size of the atom.

Bohr’s Model: Neils Bohr put forward the following postulates about the model of an atom:

(i) Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.

(ii) While revolving in discrete orbits the electrons do not radiate energy. These orbits or shells are called energy levels.

(iii) These orbits or shells are represented by the letters K, L, M, N, ... or the numbers, n = 1, 2, 3, 4, ...

Distribution Of Electrons In Different Orbits

Bohr’s Bury Scheme

• The maximum number of electrons present in a shell is given by the formula 2n2, where ‘n’ is the orbit number.

• The maximum number of electrons that can be accommodated in the outermost orbit is 8.

• 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


  • 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.

• It is due to an attempt to attain a fully-filled outermost shell.

Atomic Mass

• It is the number of protons of an atom, which determines its atomic number.

• It is denoted by ‘Z’.


The atoms of the same element, having the same atomic number but different mass numbers

Example: There are three isotopes of hydrogen atom, namely protium, deuterium and tritium.

Mass Number

The mass number is defined as the sum of the total number of protons and neutrons present in the nucleus of an atom.


Atoms of different elements with different atomic numbers, which have the same mass number, are known as isobars.

Example: Calcium, atomic number 20, and argon, atomic number 18.

Notation For An Atom

$$\normalsiz\\ _{Mass Number}^{Atomic Number}\text{Symbol of element}=\space ^A_z\text{x}$$

$$\\ _{\footnotesize Mass Number}^{\footnotesize Atomic Number}\text{Symbol of element}=\space ^A_z\text{x}$$