IGCSE Physics 0625: Why Radioactive Decay Is Random and Spontaneous

A group of students is investigating a small sealed sample of a radioactive isotope in a school laboratory. One student, Amir, suggests that if the sample were heated in an oven, or dissolved in acid to form a different chemical compound, its rate of radioactive decay would change.

(a) State what is meant by describing radioactive decay as random. [1]

(b) State what is meant by describing radioactive decay as spontaneous. [1]

(d) The sample decays by emitting alpha particles. State, in general terms, what happens to the nucleus of an atom of the original element when it emits an alpha particle. [1]

Part (a): Meaning of “random”

Random means that it is impossible to predict which particular nucleus in the sample will be the next one to decay, or exactly when any one nucleus will decay. Every nucleus of the isotope has the same chance of decaying in a given time interval, regardless of its “age” or history.

Part (b): Meaning of “spontaneous”

Spontaneous means the decay is not caused or triggered by anything external. It is not affected by physical conditions such as temperature or pressure, nor by chemical conditions such as which compound the atom is bonded into.

Part (c): Why Amir is incorrect

Amir’s suggestion assumes radioactive decay behaves like a chemical reaction, whose rate can be changed by heating or by a chemical environment such as an acid.

However, radioactive decay is a nuclear process happening deep inside the nucleus, not a chemical process involving the outer electrons:

Because it is spontaneous, the decay rate is not affected by external conditions such as temperature or the chemical compound the atom is part of. Heating the sample or dissolving it in acid changes only the electron arrangement/chemical bonding, not the nucleus.
Because it is random, there is no mechanism (such as added heat energy) that could make individual nuclei decay sooner or in a coordinated way.

So neither heating the sample nor changing its chemical form would change its rate of radioactive decay, and Amir is incorrect.

Part (d): Effect on the nucleus

When a nucleus emits an alpha particle, it loses two protons and two neutrons from the nucleus. This changes the number of protons in the nucleus, so:

$\text{the nucleus becomes the nucleus of a different element.}$

Final answers

(a) Random $=$ cannot predict which nucleus decays next / when
(b) Spontaneous $=$ not triggered or affected by external conditions
(c) Decay is unaffected by heat or chemical form, so Amir’s suggestion is incorrect
(d) The nucleus becomes the nucleus of a different element