Relate the emission of alpha particles, beta particles, gamma radiation…

The idea

An unstable nucleus can shed energy and particles, and each emission changes the nucleus in a particular way. An alpha particle is a helium nucleus, 2 protons and 2 neutrons, so emitting one lowers the mass number by 4 and the proton (atomic) number by 2, reducing both mass and positive charge. A beta particle is a fast electron created when a neutron changes into a proton, so the mass number stays the same while the proton number rises by 1, increasing the positive charge. Gamma radiation is a high-energy electromagnetic wave with no mass and no charge, so it changes neither mass number nor proton number; it usually accompanies alpha or beta emission as the nucleus loses surplus energy. A neutron emission removes one neutron, lowering the mass number by 1 while the proton number is unchanged, so charge stays the same. Track mass number and proton number separately to predict the change.

What earns the marks

• States that alpha emission reduces the mass number by 4 and the proton number by 2, lowering both mass and positive charge.
• States that beta emission leaves the mass number unchanged and raises the proton number by 1, increasing positive charge.
• States that gamma emission changes neither the mass number nor the proton number, so mass and charge are unchanged.
• States that neutron emission lowers the mass number by 1 while the proton number and charge stay the same.
• Distinguishes the change in mass number from the change in proton number for each type of emission.

Common mistakes and how to put them right

• Claims that beta emission lowers the mass number.

  Put it right by noting that a neutron becomes a proton, so the total number of nucleons is unchanged.

• Treats gamma radiation as carrying away charge or mass.

  Put it right by stating that gamma is electromagnetic radiation with zero mass and zero charge.

• Assumes neutron emission changes the charge of the nucleus.

  Put it right by stating that only the neutron count, and so the mass number, falls.

• Confuses the proton-number change for alpha and beta emission.

  Put it right by linking alpha to a fall of 2 and beta to a rise of 1 in proton number.

Exam tips

• Write the mass number and proton number before and after each emission so the change is visible.
• Remember the alpha particle as a helium nucleus with 2 protons and 2 neutrons when working out the change.
• Check each option in a multiple-choice question against both mass number and proton number, not just one.
• Recall that gamma often accompanies alpha or beta decay but by itself alters neither number.