 # Structure of Atom

## 10 Neutrons

#### THE NEUTRAL SUB-ATOMIC PARTICLE-NEUTRON

• The discovery of neutron was done by James Chadwick in 1932.
• The neutron is a neutral particle found in the nucleus of an atom.
• It is represented by symbol ‘n’.
• Protons and neutrons are collectively known as nucleons.
• Neutrons carry no charge.
• Atoms of all the elements contain neutrons except ordinary hydrogen atom which does not contain ant neutron.

#### DISCOVERY OF NEUTRON

• Rutherford discovered that except the hydrogen atom, the atomic mass of other elements could not be explained on the basis of only electrons and protons.
• For instance, the atomic mass of a helium atom should be double that of a hydrogen atom. When the e/m ratio (charge to mass ratio) was calculated, it was discovered that the mass of helium ion was four times the mass of hydrogen atom.
• So, Rutherford predicted the presence of a particle other than electrons and protons in an atom to explain this anomaly.
• According to him such a particle should be electrically neutral and have the mass equal to that of a proton.
• James Chadwick later confirmed the presence of neutrons from his experiment based on the interaction of alpha particles with beryllium.

#### CHARACTERISTICS OF NEUTRONS

• Mass of neutron – The mass of a neutron is equal to that of a proton. Electrons are considered to have negligible mass, the mass of an electron is 1/1840 times that of the proton.
• Thus, Mass of neutron = 1.676 x 10-27 kg = 1.676 x 10-24 g
• The charge of a neutron – Neutrons do not carry any charge. Hence they are electrically neutral in nature.

#### DIFFERENCE BETWEEN VALENCY, A AND Z:

• To find out the valency, electrons only in the outermost shell of the atom are considered.
• But to determine the atomic number or the mass number, the total number of protons and neutrons are considered.

#### NOTATION OF ATOM:

• An atom is represented in the following way
• So, the notation of Carbon is:

#### CALCULATING NUMBER OF NEUTRONS:

• The number of neutrons can be calculated by subtracting its atomic number from its mass number.