Nature of α-particles : 

(1) An alpha particle is a helium nucleus, i.e., a doubly ionized helium atom. It consists of two protons and two neutrons. 

(2) Mass of the α-particle ≅ 4u Charge on the α-particle = 2 × charge on the proton

Properties of α-particles :

(1) Of the three types of radioactive radiations, α- particles have the maximum ionizing power. It is about 100 times that of β-particles and 104 times that of γ-rays.

(2) They have the least penetrating power, about 100 times less than that of β-particles and 10⁴ times less than that of γ-rays. They can pass through very thin sheets of paper but are scattered by metal foils and mica. Since αparticles produce intense ionization in a medium, they lose their kinetic energy quickly. As a result, they do not penetrate more than a few centimetres (about 2.7 cm to 8.6 cm) in air under normal conditions.

(3) They are deflected by electric and magnetic fields since they are charged particles. Their deflection is less than that of β-particles in the same field.

(4) They affect photographic plates. 

(5) They cause fluorescence in fluorescent materials such as zinc sulphide.

(6) They emerge from the nuclei with tremendous speeds in the range of 1/100th to 1/10th of the speed of light in free space.

(7) They destroy living cells.

[Note : α-rays and β-rays were discovered by Henri BecquereL]