Class 9 Science

# Structure of The Atom

Introduction

Matters are made of tiny particles called atom. Atom is made of three particles; electron, proton and neutron. These particles are called fundamental particles of an atom or sub atomic particles.

## Electron (e–)

Electron is denoted by ‘e’ and is a negatively charged particle. The absolute charge over an electron is equal to 1.6xx10^-19 of negative charge and is considered equal to – 1.

The relative mass of electron is 1/1836. Since the mass of an electron is very small, thus it is considered equal to 0. Electrons revolve round the nucleus of atoms.

## Proton (p+)

Proton is denoted by ‘p’ and is positively charged particle. The absolute charge over proton is 1.6xx10^-19 coulomb of positive charge and it is considered as unit positive charge. Thus absolute charge over a proton is equal to +1.

The absolute mass of a proton is equal to 1.6xx10^-24 g and considered equal to 1 as it is equal to the mass of 1 hydrogen atom. Proton is present in the nucleus of atom.

## Neutron (n)

Neutron is denoted by ‘n’ and is a neutral particle.

The absolute mass of neutron is 1.6 xx 10^ -24 g. The relative mass of neutron is equal to 1. Neutron is presents in the nucleus of atom.

Nucleus – The centre of atom is called nucleus. Nucleus comprises of neutron and proton. Nucleus of an atom contains the whole mass of an atom.

## Discovery of Electron

In 1897; J. J. Thomson, a British physicist, proposed that atom contains at least one negatively charged particle. Later this particle was named as electron. Thomson called those particles ‘corpuscles’.

## Discovery of Proton

Ernest Goldstein in 1886 discovered the presence of new radiation in gas discharge tube even before the identification of electron. He called these rays as Canal Rays. His experiment led to the discovery of proton.

## Discovery of Neutron

In 1932 J. Chadwick discovered another subatomic particle called neutron. Neutron is present in the nucleus of all atoms.

## Thomson’s Model of Atom

J. J. Thomson proposed the model of atom similar to a Christmas Pudding or similar to a water melon. His model of atom is generally called plum and pudding model of atom.

He proposed that electrons are embedded the way black seeds of water melon are embedded; in the sphere of positive charge. According to Thomson

(a) An atom consists of positively charged sphere in which electrons are embedded.

(b) The quanta of negative and positive charges are equal. The equal number of negative charge and positive charge makes an atom electrically neutral.

## Rutherford’s Model of Atom

Ernest Rutherford in 1909 with his team bombarded very thin gold foil with α – particles. He found that

• Most of the α – particles passed without any hindrance.
• Some of the α – particles deflected from their original path at noticeable angle.
• Very few of the α – particles bounced back at their original path.

On the basis of his observation, he proposed the model of atom. The Rutherford's Model of Atom is as follows:

• 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 round the nucleus.

The Rutherford’s Experiment is also known as Geiger-Marsden Experiment.

### Drawbacks of Rutherford Model

• According to Rutherford’s Model, electron revolves round the positively charged nucleus which is not expected to be stable. But a charged particle in an accelerated motion along a circular path would lose energy because of radiation and finally would fall into nucleus. This makes an atom unstable while atoms are quite stable.
• If atoms were not stable no matter would exist in nature.
• Rutherford model could not solve the problem of atomic mass of atom as it proposed only the existence of protons in the nucleus.

However, the problem of atomic mass could be solved after the discovery of neutron.