Comparing masses of fundamental particles

Steps:

• Recall that an alpha particle is a helium nucleus (2 protons + 2 neutrons).
• Note electron is a fundamental lepton with negligible mass compared to nucleons.
• Identify proton and neutron as baryons with similar masses around 1 atomic mass unit (u).
• Compare: electron mass ≈ 0.00055 u; proton/neutron ≈ 1 u; alpha ≈ 4 u.

Why B is correct:

• The electron's rest mass is 9.11 × 10⁻³¹ kg, defined as the lightest charged particle in standard atomic structure.

Why the others are wrong:

• A: Alpha particle mass ≈ 6.64 × 10⁻²⁷ kg (four times a proton's mass).
• C: Neutron mass ≈ 1.675 × 10⁻²⁷ kg (nearly identical to proton).
• D: Proton mass ≈ 1.673 × 10⁻²⁷ kg (basic unit for nuclear masses).

Final answer: B