STRUCTURE OF ATOM

                1. Introduction

The existence of atoms has been proposed since the time of early Indian and Greek philosophers (400 B.C.) who were of the view that atoms are the fundamental building 

blocks of matter. According to them, the continued subdivisions of matter would ultimately yield atoms which would not be further divisible. The word ‘atom’ has been 

derived from the Greek word ‘a-tom io’ which means ‘uncut-able’ or ‘non-divisible’. These earlier ideas were mere speculations and there was no way to test them experimentally. These ideas remained dormant for a very long time and were revived again by scientists in the 

nineteenth century. The atomic theory of matter was first proposed on a firm scientific basis by John Dalton, a British school 

teacher in 1808. His theory, called Dalton’s atomic theory, regarded the atom as the ultimate particle of matter. Dalton’s atomic theory was able to explain the law of conservation of mass, law of constant 

composition and law of multiple proportion very successfully. However, it failed to explain the results of many experiments, for example, it was known that substances like glass or ebonite when rubbed with silk or fur get electrically charged. 

In this unit we start with the experimental

observations made by scientists towards the end of nineteenth and beginning of twentieth century. These established that atoms are made of sub-atomic particles, i.e., electrons, protons and neutrons — a concept very different from that of Dalton.

     2.1  DISCOVERY OF SUB-ATOMIC

                        PARTICLES

An insight into the structure of atom was

obtained from the experiments on electrical

discharge through gases. Before we discuss

these results we need to keep in mind a basic

rule regarding the behaviour of charged

particles : “Like charges repel each other and

unlike charges attract each other”.

 » Discovery of Electron  :

In 1830, Michael Faraday showed that if

electricity is passed through a solution of an

electrolyte, chemical reactions occurred at

the electrodes, which resulted in the

liberation and deposition of matter at the

electrodes. He formulated certain laws which

you will study in class XII. These results

suggested the particulate nature of

electricity. In mid 1850s many scientists mainly  Faraday began to study electrical discharge in partially evacuated tubes, known as cathode ray discharge tubes. A cathode ray tube is made of glass containing two thin pieces of metal, called electrodes, sealed in it. The electrical discharge through the gases could be observed only at very low pressures and at very high voltages. The pressure of different gases could be adjusted by evacuation of the glass tubes. When sufficiently high voltage is applied across the electrodes, current starts flowing through a stream of particles moving in the tube from the negative electrode (cathode) to the positive electrode (anode). These were called cathode rays or cathode ray particles. The flow of current from 

cathode to anode was further checked by

making a hole in the anode and coating the

tube behind anode with phosphorescent

material zinc sulphide. When these rays, after

passing through anode, strike the zinc

sulphide coating, a bright spot is developed

on the coating. 

The results of these experiments are

summarised below:

(i) The cathode rays start from cathode and

move towards the anode.

(i i) These rays themselves are not visible but

their behaviour can be observed with the

help of certain kind of materials

(fluorescent or phosphorescent) which

glow when hit by them. Television picture

tubes are cathode ray tubes and television pictures result due to fluorescence on the television screen coated with certain fluorescent or phosphorescent materials. 

(i i i) In the absence of electrical or magnetic

field, these rays travel in straight lines .

(i v) In the presence of electrical or magnetic

field, the behaviour of cathode rays are

similar to that expected from negatively

charged particles, suggesting that the cathode rays consist of negatively charged particles, called electrons. 

(v) The characteristics of cathode rays

(electrons) do not depend upon the material of electrodes and the nature of the gas present in the cathode ray tube. Thus, we can conclude that electrons are basic constituent of all the atoms.

» Charge to Mass Ratio of Electron

In 1897, British physicist J.J. Thomson

measured the ratio of electrical charge (e) to

the mass of electron (me ) by using cathode ray

tube and applying electrical and magnetic field

perpendicular to each other as well as to the

path of electrons. When only electric field is applied, the electrons deviate from their 

path and hit the cathode ray tube at point A .

Similarly when only magnetic field 

is applied, electron strikes the cathode ray tube

at point C. By carefully balancing the electrical

and magnetic field strength, it is possible to

bring back the electron to the path which is

followed in the absence of electric or magnetic

field and they hit the screen at point B.

Thomson argued that the amount of deviation

of the particles from their path in the presence

of electrical or magnetic field depends upon:

(i) the magnitude of the negative charge on

the particle, greater the magnitude of the

charge on the particle, greater is the interaction with the electric or magnetic 

field and thus greater is the deflection.

(i i) the mass of the particle — lighter the

particle, greater the deflection.

(i i i) the strength of the electrical or magnetic

field — the deflection of electrons from its

original path increases with the increase

in the voltage across the electrodes, or the

strength of the magnetic field.

By carrying out accurate measurements on

the amount of deflections observed by the

electrons on the electric field strength or

magnetic field strength, Thomson was able to

determine the value of e/me

 as: e /me = 1.758820 × 1011 C kg–1

Where me is the mass of the electron in kg and

e is the magnitude of the charge on the electron in coulomb (C). Since electrons are negatively charged, the charge on electron is –e.

2.2 Discovery of Protons and Neutrons

Electrical discharge carried out in the modified

cathode ray tube led to the discovery of canal

rays carrying positively charged particles. The

characteristics of these positively charged

particles are listed below:

(i) Unlike cathode rays, mass of positively

charged particles depends upon the nature of gas present in the cathode ray tube. These are simply the positively charged gaseous ions. 

(ii) The charge to mass ratio of the particles

depends on the gas from which these

originate.

(iii) Some of the positively charged particles

carry a multiple of the fundamental unit

of electrical charge.

(iv) The behaviour of these particles in the

magnetic or electrical field is opposite to

that observed for electron or cathode

rays.

The smallest and lightest positive ion was

obtained from hydrogen and was called

proton. This positively charged particle was

characterised in 1919. Later, a need was felt

for the presence of electrically neutral particle

as one of the constituent of atom. These

particles were discovered by Chadwick (1932)

by bombarding a thin sheet of beryllium by

α-particles. When electrically neutral particles

having a mass slightly greater than that of

protons were emitted. He named these

particles as neutrons. The important

properties of all these fundamental particles. 

            2.2 ATOMIC MODELS

Observations obtained from the experiments

mentioned in the previous sections have

suggested that Dalton’s indivisible atom is

composed of sub-atomic particles carrying

positive and negative charges. The major

problems before the scientists after the

discovery of sub-atomic particles were:

• to account for the stability of atom,

• to compare the behaviour of elements in

terms of both physical and chemical

properties,

• to explain the formation of different kinds

of molecules by the combination of different

atoms and,

• to understand the origin and nature of the

characteristics of electromagnetic radiation

absorbed or emitted by atoms.

Different atomic models were proposed to

explain the distributions of these charged

particles in an atom. Although some of these

models were not able to explain the stability of

atoms, two of these models, one proposed by

J.J. Thomson and the other proposed by

Ernest Rutherford.