An element of nuclear number A and proton number Z emits an alpha particle from its nucleus. The nucleon…

Analyze the question: An alpha particle is emitted from a nucleus. We need the new nucleon number (mass number) and proton number (atomic number). The question says "nuclear number A", which here means nucleon (mass) number.

Concept: An alpha particle is a helium nucleus, ⁴₂He. It contains 2 protons and 2 neutrons, so it has a mass number of 4 and a proton number of 2.

When a nucleus emits an alpha particle, it loses both of these:

ᴬZ X → ᴬ⁻⁴(Z−2) Y + ⁴₂He

Step by step:

• Nucleon number: A − 4 (the alpha particle carries away 4 nucleons)
• Proton number: Z − 2 (the alpha particle carries away 2 protons)

Example: Uranium-238 (Z = 92) emits an alpha particle and becomes Thorium-234 (Z = 90). Here 238 − 4 = 234 and 92 − 2 = 90.

Answer: The new element has nucleon number (A − 4) and proton number (Z − 2), which is option E.

Tip: Remember that alpha decay reduces the mass number by 4 and the atomic number by 2. Beta decay is different. It leaves the mass number unchanged and increases the atomic number by 1. Options with "+" are never correct for alpha emission, because the nucleus only loses particles.