Radioactivity
4 marks · NormalPotassium-40 is an unstable isotope present in the human body. Its nucleus can decay by emitting a beta minus particle, turning into calcium-40.
Write a balanced nuclear equation for this beta minus decay. Supply all atomic numbers and mass numbers.
Describe the changes in the atomic number and mass number that occur in the nucleus during this decay.
Explain why potassium-40 is unstable in terms of neutrons and nuclear stability.
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.
Show answer
(a) · must show Correct atomic numbers and mass numbers for both isotopes and beta particle
(b) Atomic number increases by 1, mass number remains the same. · must show state effect on A and Z
(c) An imbalance between neutrons and protons makes the nucleus unstable; potassium-40 has more neutrons than stable potassium isotopes, causing decay. · must show reference to imbalance of neutrons and protons causing instability
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Correctly writes Ca as product with correct numbers
A1 (a) · 1 mark · Complete equation with all symbols, including beta and energy
B1 (b) · 1 mark · States atomic number up by 1, mass unchanged (or equivalent)
B2 (c) · 1 mark · Refers to neutron-proton imbalance and instability
Worked solution
a) Nuclear equation:
b) Atomic number increases by 1 (from 19 to 20), mass number remains at 40.
c) Potassium-40 is unstable because it contains more neutrons than the most stable isotope, making the nucleus prone to beta decay to reach a more stable balance.