Discovery of neutron

In 1932, Chadwick discovered another sub-atomic particle which had no charge and a mass equal to that of a proton. It was eventually named as neutron. Neutrons are present inside the nucleus of atom. The mass of the atom is , therefore, given by the sum of the masses of protons and neutrons present in the nucleus.

Modern views about structure of atom

• An atom consists of a central part called nucleus and a number of electrons revolving around it in its orbit.
• Total charge on the nucleus is neutralized by the charge on the electrons in the orbits. (The proton carries a unit positive charge and the electron carries a unit negative charge. The number of protons in the nucleus is equal to the number of electrons in the orbits hence net charge on the atom is zero.)
• The mass of atom is almost entirely packed in the nucleus.
• Mass of a proton is nearly equal to the mass of a hydrogen atom.
• Mass of an atom is nearly equal to the mass of its protons and neutrons.
• An atom is the smallest particle of an element which retains its chemical identity throughout the physical and chemical changes.