# What is an atom ?

What is an atom ? or Define atom

The smallest particle of any substance is known as atom.

An atom consists of electron, proton and neutron.

Electron:

Electron was discovered by  J.J. Thomson in cathode rays experiment in 1897.

The mass of an electron is

$9.11&space;\times&space;10^{-31}$ kg (or) 0.005486 a.m.u.

The charge of an electron is $1.602$ $\times&space;10^{^{-19}}$ coulombs (or) -4.8 $\times&space;10^{-10}$ e.s.u.

The charge of an electron was discovered by Millikan in oil drop experiment.

The charge of one mole of electrons is 96500 coulombs or 1 Faraday.

Proton:

Proton was discovered in 1911 by anode rays experiment.

Anode rays also called as positive rays were discovered by E. Goldstein.

The mass of a proton is $1.672$ $\times&space;10^{-27}$ kg (or) 1.00728 a.m.u.

The charge of a proton is $+&space;1.602$ $\times&space;10^{-19}$ coulombs (or) 4.8 $\times&space;10^{-10}$ e.s.u

The specific charge of a proton is $9.58$ $\times&space;10^{7}$ coulombs/kg.

The charge of one mole of the proton is 96500 coulombs.

Neutron :

Neutron was discovered by James Chadwick in 1932 in the bombardment of Beryllium with $\alpha$– particles.

The mass of a neutron is $1.675$  $\times&space;10^{-27}$ kg (or) 1.008665 a.m.u.

The specific charge of a neutron is zero.

Neutron is the heaviest, unstable particle and disintegrates into electron, proton and neutrino.

What is Atomic Number ?

The number of protons present in the nucleus of an atom is known as its Atomic number.

It is generally denoted by the letter Z.

This was proposed by Moseley on X-ray spectral studies $\sqrt{\nu&space;}&space;=&space;a&space;(Z&space;-&space;b)$

Where

V = frequency of X-ray of element

Z = atomic numbers of the element

a and b are constants

What is Mass Number ?

The sum of the number of protons and neutrons in nucleus of an atom is known as its mass number.

It is generally denoted by the letter A.

Mass number (A) = number of protons + number of neutrons

A = Z + number of neutrons

(or)

Number of neutrons = A – Z