THE STABILITY OF ISOTOPES
5 marks · HardA nucleus of potassium-40 () decays by emitting a beta minus particle to form a stable isotope.
Write a balanced nuclear equation for this decay, including the atomic and mass numbers of all particles.
Explain, using your answer, what has changed inside the nucleus.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) · must show Both atomic and mass numbers correct for all particles, Correct symbol for beta minus particle
(b) — · must show State a neutron changes into a proton and electron, Mass number stays the same, Atomic number increases by 1
Common mistakes: incorrect atomic or mass numbers in nuclear equation; not identifying neutron-to-proton transformation
Show mark scheme and worked solution
Marking points
A1 (a) · 2 marks · 1 mark for correct atomic and mass numbers on all particles; 1 mark for correct process/symbols for beta minus decay
B1 (b) · 1 mark · States a neutron changed into a proton and an electron
B2 (b) · 1 mark · Mentions atomic number increased by 1
B3 (b) · 1 mark · Mentions mass number stayed the same
Worked solution
(a) The nuclear equation is:
(b) Inside the nucleus, a neutron changes into a proton and an electron. The proton remains in the nucleus, raising the atomic number by 1, while the electron is emitted as a beta particle. The mass number stays the same because the total number of nucleons does not change.