Chapter 20 — Chapter 20 — Atomic and Nuclear Physics

Rutherford's α-scattering (gold foil): most α-particles passed straight (atom mostly empty space), a few deflected and very few bounced back → atom has a tiny, dense, positively charged nucleus containing nearly all mass; electrons revolve around it. Nucleus: protons and neutrons (nucleons). Atomic number Z = number of protons; mass number A = protons + neutrons; N = A − Z; notation ᴬ_Z X. Chemical properties depend on electrons (equal to Z in neutral atom), especially valence electrons. We don't pass through walls because electron clouds of atoms repel electrically (and Pauli exclusion), even though atoms are mostly empty.