Nuclear facts: pick the wrong statement about radioactivity and nuclear particles.

Introduction / Context:
This item mixes fundamental statements from nuclear physics and chemistry. The task is to identify the one incorrect statement among generally correct facts about decay, particle properties, and neutron absorption.

Given Data / Assumptions:

• Thermal effects on decay rates are negligible (radioactive decay is a nuclear, not chemical, process).
• Elementary particle properties are at textbook values.
• Typical neutron absorbers (like cadmium, boron) are well known.

Concept / Approach:
Deuterium (hydrogen-2) has one proton and one neutron in the nucleus. Therefore, any statement asserting ”one proton and two neutrons” describes tritium (hydrogen-3), not deuterium. Heating has no practical effect on decay constant; electrons carry −1 elementary charge and very small rest mass; cadmium is a strong thermal-neutron absorber used in control rods and shielding.

Step-by-Step Solution:

Verification / Alternative check:
Standard nuclear data tables confirm deuterium's composition as 1 proton + 1 neutron.

Why Other Options Are Wrong:

• (a) True: decay is governed by nuclear forces; macroscopic heating doesn't alter λ.
• (b) True: electron charge is −1e; mass ≈ 9.11×10^−31 kg.
• (d) True: cadmium is a classic neutron absorber (e.g., Cd control blades).

Common Pitfalls:
Confusing deuterium with tritium; overinterpreting minor environmental effects on decay rates.

Final Answer:
A deuterium atom has one proton and two neutrons in its nucleus.