So alpha decay causes both the mass and charge of the nucleus to decrease.

The idea

An alpha particle is a helium nucleus: two protons and two neutrons, so it has mass number 4 and charge 2⁺. When an unstable nucleus emits an alpha particle, it loses those two protons and two neutrons, so its mass number falls by 4 and its proton number falls by 2. For example, uranium-238 decays to thorium-234: ²³⁸₉₂U → ⁴₂He + ²³⁴₉₀Th. The mass number decreases because nucleons leave the nucleus, and the charge decreases because two positive protons leave. The nucleus becomes a different element because its proton number changes. Alpha emission is common in very heavy nuclei and is stopped by a few centimetres of air or a sheet of paper.

What earns the marks

• An alpha particle consists of 2 protons and 2 neutrons, so it has mass number 4 and charge 2⁺.
• Emitting an alpha particle removes 2 protons and 2 neutrons from the nucleus.
• The mass number of the nucleus decreases by 4 because 4 nucleons are lost.
• The proton number (atomic number) decreases by 2, so the nuclear charge decreases by 2 units.
• The nucleus changes into a different element because its proton number changes.
• In a nuclear equation, the total mass number and total proton number must balance on both sides.

Common mistakes and how to put them right

• Thinking the mass number decreases by 2 rather than 4.

  Put it right by counting both protons and neutrons in the alpha particle.

• Believing the charge stays the same because neutrons are also lost.

  Put it right by noting only protons carry nuclear charge, and 2 protons are lost.

• Writing the alpha particle as ⁴₂He with the wrong charge or mass.

  Put it right by remembering it is a helium nucleus, mass number 4 and charge 2⁺.

Exam tips

• Write the nuclear equation and check that mass numbers and proton numbers balance on both sides.
• State clearly that the proton number decreases by 2, so the charge decreases by 2 units.
• Use the term nucleon when explaining why mass number falls by 4.