During gamma decay of an excited atomic nucleus, how do its mass number and atomic number change as a result of emitting a gamma photon?

Introduction / Context:
Radioactive decay processes are commonly classified as alpha, beta and gamma decays. Each type of decay changes the nucleus in a different way. Alpha decay emits a helium nucleus and changes both mass number and atomic number. Beta decay changes a neutron into a proton or vice versa and alters the atomic number while keeping the mass number the same. Gamma decay, however, involves emission of high energy photons from an excited nucleus. This question asks you specifically how the mass number and atomic number of a nucleus are affected during gamma decay, which is important for interpreting nuclear reaction equations and decay schemes.

Given Data / Assumptions:
• The nucleus undergoes gamma decay, emitting a gamma photon. • Gamma radiation consists of high energy electromagnetic photons. • No nucleons (protons or neutrons) are emitted in pure gamma decay. • We assume the nucleus transitions from an excited state to a lower energy state of the same nuclide.

Concept / Approach:
Gamma decay occurs when a nucleus in an excited energy state releases excess energy by emitting a gamma photon. The internal arrangement of nucleons can change from a higher energy configuration to a lower one, but the numbers of protons and neutrons do not change. Since mass number A is the total number of protons plus neutrons, and atomic number Z is the number of protons, both A and Z remain unchanged in pure gamma emission. The process is analogous to an atom emitting a photon when an electron falls from an excited energy level to a lower one, which does not change the number of electrons or the nuclear charge. Therefore, gamma decay is an isomeric transition that alters only the energy of the nucleus, not its mass number or atomic number.

Step-by-Step Solution:
Step 1: Recall that alpha decay emits a helium nucleus containing two protons and two neutrons, reducing mass number by four and atomic number by two. Step 2: Recall that beta decay converts a neutron to a proton or a proton to a neutron, changing the atomic number by one while keeping mass number constant. Step 3: Recognise that gamma decay instead emits a photon, which is a quantum of electromagnetic energy with no rest mass and no charge. Step 4: Understand that during this emission, the nucleus transitions between energy levels but does not lose or gain any nucleons. Step 5: Conclude that the number of protons remains the same, so the atomic number Z is unchanged. Step 6: Since neither protons nor neutrons are added or removed, the mass number A also remains unchanged.

Verification / Alternative check:
Nuclear decay schemes often show an excited nucleus, indicated by an asterisk, transforming into the same nuclide without the asterisk plus a gamma photon. For example, cobalt 60 in an excited state decays to cobalt 60 in a lower energy state by emitting gamma rays, but the nuclide remains cobalt 60 with the same mass number and atomic number. This is distinct from alpha and beta changes that create different elements or isotopes. Nuclear reaction equations for gamma decay show identical nuclides on both sides, confirming that A and Z are unchanged. This observational evidence backs up the theoretical expectation based on the nature of photons.

Why Other Options Are Wrong:
Option A suggests that mass number changes while atomic number does not, which resembles alpha emission but not gamma decay. Option B suggests only atomic number changes, similar to beta decay processes where a neutron converts to a proton or vice versa. Option C suggests that both mass number and atomic number change, which may occur in combined processes but not in pure gamma emission. None of these patterns matches the specific role of gamma radiation, which carries away only energy, not nucleons or charge.

Common Pitfalls:
Students sometimes assume that all forms of radioactive decay must change the element or isotope, leading them to overlook the special role of gamma decay as an energy adjustment. Another pitfall is confusing the high energy of gamma photons with mass, even though photons have no rest mass and cannot carry away nucleon count. Remember to distinguish between mass number and energy; gamma decay reduces the energy content of the nucleus but not its nucleon count or charge. Keeping these distinctions clear helps in balancing nuclear equations correctly in exams.

Final Answer:
The correct choice is Neither the mass number nor the atomic number of the nucleus changes, because gamma decay involves emission of a photon that carries away only energy, leaving the number of protons and neutrons in the nucleus unchanged.