An alpha particle (α) – this consists of two neutrons and two protons, it…

The idea

An alpha particle is a nuclear emission made of two protons and two neutrons, so it is identical to a helium-4 nucleus, ⁴₂He. It has a relative mass of 4 and a charge of 2⁺, because the two protons each carry 1⁺ and the neutrons are neutral. When a nucleus emits an alpha particle, its proton number decreases by 2 and its nucleon number decreases by 4, so the element changes. For example, radium-226 decays to radon-222: ²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂He. Alpha particles are highly ionising but have a short range in air, typically a few centimetres, and are stopped by paper or skin.

What earns the marks

• An alpha particle contains two protons and two neutrons, giving a nucleon number of 4 and a proton number of 2.
• Its charge is 2⁺ because the two protons each contribute 1⁺ and the neutrons contribute no charge.
• In alpha decay, the emitting nucleus loses 2 protons and 4 nucleons in total, so its proton number decreases by 2 and its nucleon number decreases by 4.
• A nuclear equation for alpha decay must balance nucleon numbers and proton numbers on both sides, for example ²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂He.
• Alpha particles are highly ionising and have a short range in air, being stopped by a few centimetres of air or a sheet of paper.

Common mistakes and how to put them right

• Thinking an alpha particle is a helium atom.

  Put it right: It is a helium nucleus, so it has no electrons and carries a 2⁺ charge.

• Writing the symbol as ⁴₂He²⁻ or ²₂He.

  Put it right: The mass number is 4, the atomic number is 2, and the charge is 2⁺, not 2⁻.

• Forgetting to change the element in alpha decay.

  Put it right: Losing two protons changes the proton number, so the daughter nucleus is a different element.

Exam tips

• When completing a nuclear equation, check that the total nucleon numbers and total proton numbers are equal on both sides before writing the final symbol.
• Use the terms proton number and nucleon number precisely; avoid saying mass number changes by 2 when it changes by 4.
• Link the short range of alpha radiation to its strong ionising ability, and use this to explain why alpha sources are handled with care but are stopped by paper.