Question

1. What do you understand by the term 𝛼-particles as applied to radioactivity?
2. Why are these particles called helium nuclei?

Answer 1

α-particles are positively charged, high-energy particles emitted spontaneously from the nucleus of a heavy radioactive element. Each α-particle is a helium nucleus containing 2 protons and 2 neutrons, so it carries a charge of +3.2 × 10⁻¹⁹ C and has a rest mass of 6.64 × 10⁻²⁷ Kg. In alpha decay, the parent nucleus loses an α-particle, so its mass number A decreases by 4 and its atomic number Z decreases by 2:

$${}_Z^A \mathrm{X} \rightarrow {} _{Z-2}^{A-4} \mathrm{Y} + {}_2^4 \mathrm{He}$$

These particles are called helium nuclei because an α-particle is identical to the nucleus of a helium atom, $${}_2^4 \mathrm{He}$$. When an α-particle gains two electrons, it becomes a neutral helium atom. Thus, α-particles are simply helium nuclei ejected by radioactive substances.