Elastic scattering changes velocity direction Steps:

• Alpha particle approaches gold nucleus and experiences Coulomb repulsion.
• For deflection >90°, impact parameter is small, resulting in significant trajectory bend.
• Interaction is elastic, conserving kinetic energy and internal structure.
• Velocity vector changes direction, but magnitude remains constant.

Why B is correct:

• Velocity changes as a vector quantity due to the deflection altering the direction of motion, per the laws of classical mechanics in Rutherford scattering.

Why the others are wrong:

• A: Charge is fixed at +2e for alpha particle; electrostatic repulsion doesn't alter it.
• C: Nucleon number (mass number 4) unchanged, as no nuclear reaction occurs.
• D: Proton number (atomic number 2) unchanged, preserving helium nucleus identity.

Final answer: B