According to Dalton’s atomic theory, the atom is considered to be the smallest indivisible constituent of all matter. This theory could explain the law of conservation of mass, law of constant composition and law of multiple proportions.

Sir J.J.Thomson discovered that when very high voltage was passed across the electrodes in the cathode ray tube, the cathode produced rays that travel from cathode to anode and were called cathode rays.

Eugen Goldstein discovered anode rays by using a perforated cathode (a cathode having holes in it) in the discharge tube filled with air at a very low pressure. The discovery of anode rays established the presence of positively charged proton in the atom.

According to Thomson’s plum-pudding model, atoms can be considered as a large sphere of uniform positive charge with a number of small negatively charged electrons scattered throughout it.

The α-ray scattering experiment performed by Geiger and Marsden led to the failure of Thomson’s model of atom. The results of α-ray scattering experiment were explained in terms of Rutherford’s model. According to which the atom contains a dense and positively charged region called nucleus at its centre and the negatively charged electrons move around it. All the positive charge and most of the mass of atom is contained in the nucleus.

In 1932, James Chadwick discovered an electrically neutral particle in atom and named it as neutron. 

The number of protons in an atom is called the atomic number and is denoted as Z. On the other hand the number of nucleons (protons plus neutrons) in the nucleus of an atom is called its mass number and is denoted as A.

The electrons are distributed in different shells in the order of increasing energy. The distribution is called electronic configuration. The maximum number of electrons present in a shell is given by the formula 2n2, where n is the number of the orbit or the shell.

The valence is the number of chemical bonds that an atom can form with univalent atoms. If the number of valence electrons is four or less, then the valency is equal to the number of the valence electrons. On the other hand, if the number of valence electrons is more than four, then generally the valency is equal to 8 minus the number of valence electrons.