Chemistry IX | Structure of Atom | Part 1

Structure of Atom | Part 1


Atom is the smallest particle of an element, does not found in free state but takes part in chemical reaction.

Main Postulates of Dalton’s Atomic Theory

All matter is made up of very small particles known as atoms.

Atom can neither be created nor destroyed, it is the smallest particle that take part in a chemical reaction.

All atoms of an element are alike and of different element are different atoms.

Atoms combine together in the ratio of small whole numbers.

Drawbacks of Dalton Atomic Theory

It does not explain why atoms of different elements differs in their mass, valencies etc.

It does not explain the existence of isotopes and isobars.

It does not explain the reason for difference in atoms of different elements.

Discovery of electron, proton and neutron discarded the indivisible nature of atom proposed by Dalton.


Electron was discovered by J.J. Thomson in 1897.

Magnitude of charge on electron = 1.6 clip_image00310 – 19 coulomb

Mass of electron = 9.4 clip_image003[1] 10 – 28 gram.

When an electric discharge at high voltage is passed through a gas at low pressure, a stream of rays is emitted from the cathode surface. These are called as cathode rays which are made up of electrons.

Properties of Cathode Rays

Cathode rays always travel in straight line. Whenever an object is placed in the path of cathode rays, it forms a shadow on the walls opposite to the cathode.

Cathode rays consists of material particles and produce mechanical effects.

The beam of cathode rays are deflected towards the positive charged plate, hence the particles in the cathode rays carry negative charge.

Cathode rays are also deflected from their path in magnetic field. 

Cathode rays possess heating effect. When these strike a thin metal foil, it gets heated up.

It affects the photographic plates.

The cathode rays produce X-rays when they strike metals with high melting point like tungsten, etc.

The value of e/m was found to be 1.76 clip_image003[2] 108 coulomb per gram for electrons.

The charge of the electron was measured by R.A. Millikan.

Proton (1P1 or 1H1)

The existence of positively charged particles in an atom was discovered by E. Goldstein in 1886.

The magnitude of charge on proton is 1.6 clip_image003[3] 10 – 19 coulomb.

Mass of proton 1.6 clip_image003[4] 10 – 24 gram.

When a high voltage is supplied across the electrodes a new kind of rays emerges from the hole in the cathode opposite to the anode. These rays are anode rays.

Properties of Anode Rays

Anode rays travel in a straight line and cast shadow.

Anode rays are deflected by the magnetic and electric field and bent in the opposite direction of cathode rays.

The anode rays can also rotate the wheel placed in their path and also have heating effect.

The charge to mass ratio (e/m) for these rays is considerably smaller than electrons and is equal to 0.58clip_image003[5]104 coulomb / g and this ratio e/m for positive rays depends upon the nature of the gas taken in the tube.

Neutron (0n1)

The neutrons were discovered by James Chadwick.

These particles are neutral having charge 0.

Their mass is same as that of a hydrogen atom.

So, its mass = 1.66 clip_image003[6] 10 – 27 Kg.

Atomic Number

It was given by Moseley. The number of protons present in the nucleus or the number of electrons revolving around the nucleus is called Atomic number. Atomic number = number of protons in the nucleus = number of electrons.

Mass Number

The sum of number of protons and neutrons present in the nucleus is called mass number. It is a whole number.

Mass number (A)

= number of protons + number of neutrons

= number of electrons + number of neutrons

= Atomic number + number of neutrons.


Isotopes were discovered by F. Soddy.

Isotopes have the same atomic number but differ in their mass numbers.

Isotopes have identical chemical properties but differ in physical properties.

e.g., 1H1 (Protium), 1H2 (Deuterium) and 1H3 (Tritium), 8O16, 8O17 and 8O18


Isotones are the atoms of different elements possessing the same number of neutrons and have different mass numbers :

e.g., 1H3 and 2He4, 15P31 and 16S32, 19K39 and 20Ca40