Radioactivity
5 marks · NormalA radioactive isotope of nitrogen, , decays by beta minus emission. A scientist is studying nuclear stability and writes an equation for this decay.
Write the balanced nuclear equation for the beta minus decay of .
State, with reasons, how the numbers of protons and neutrons change in the nucleus as a result of this decay.
Explain why some isotopes with the same number of protons as are stable, while others are unstable.
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)
(b) During beta minus decay, a neutron in the nucleus changes into a proton and emits an electron (beta particle). Therefore, the number of protons increases by 1 and the number of neutrons decreases by 1.
(c) Stability depends on the number of neutrons: isotopes with the right neutron-to-proton ratio are stable; too many or too few neutrons make a nucleus unstable.
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Marking points
A1 (a) · 1 mark · Writes correct product isotope symbol in the balanced nuclear equation ().
A2 (a) · 1 mark · Writes correct beta particle symbol (), balancing nucleon and charge numbers.
A3 (b) · 1 mark · Explains a neutron becomes a proton during beta minus decay.
A4 (b) · 1 mark · States/proves number of protons increases by 1 and neutrons decreases by 1, with reasons related to the nuclear change.
A5 (c) · 1 mark · Relates nuclear stability to neutron-to-proton ratio and identifies this as the reason for differences in stability among isotopes.
Worked solution
a) The nuclear equation is:
b) In beta minus decay, a neutron is converted into a proton. Thus, the proton number (atomic number) increases by 1 (7 8), while the neutron number decreases by 1. The mass number stays the same.
c) Stability in isotopes depends on the balance of neutrons to protons. If the balance is not right for a nucleus, it becomes unstable and may decay. Nitrogen-14 is stable, but nitrogen-16 has too many neutrons, making it unstable.