What type of nuclear decay decreases the atomic number Z of a parent nucleus by 2 and decreases the mass number A by 4?

Answer and explanation

Correct answer: C. Alpha decay

  1. Step 1: An alpha particle is a helium-4 nucleus composed of 2 protons and 2 neutrons (^4_2 He).
  2. Step 2: Emission of an alpha particle reduces mass number A by 4 and atomic number Z by 2.
  3. Step 3: Beta-minus decay increases Z by 1 without changing A; gamma decay emits high-energy photons with no change in Z or A.
Answer
Alpha decay.
Why not the others: 'Beta-minus decay' increases atomic number Z by 1 with constant A; 'Beta-plus decay' decreases atomic number Z by 1 with constant A; 'Gamma decay' leaves both atomic and mass numbers unchanged.

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