Net changes in proton and neutron numbers from particle emissions

Steps:

• First α-emission: proton number decreases by 2, mass number by 4.
• β-emission (electron): proton number increases by 1, mass number unchanged.
• Second α-emission: proton number decreases by 2, mass number by 4.
• Net: proton number -3 (different element), mass number -8; neutron number = mass - protons, so original N decreases by 5.

Why B is correct:

• Proton number changes by -3, defining a new element; neutron number drops by 5 per N = A - Z formula.

Why the others are wrong:

• A: Proton number decreases, not increases.
• C: Same element requires identical proton number; change means different element.
• D: Isotopes share proton number but differ in neutrons/mass; here proton number changes.

Final answer: B