2.2 · Atomic StructurePractice · 原子结构

The nucleus is made of protons and neutrons: a proton carries one unit of positive charge and a neutron has none. Electrons are negative and move outside the nucleus. The nucleus as a whole is positive because of its protons.

Most alpha particles weren’t deflected at all, so they passed through almost nothing: an atom is mostly empty space. The few that bounced back sharply showed that the positive charge and nearly all the mass are packed into a tiny nucleus.

E = hν = hc/λ, so the shorter the wavelength, the more energetic the photon. Violet has the shortest wavelength of visible light (about 410 nm) and red the longest (about 650 nm), so violet photons carry the most energy.

c = λν, so ν = c/λ. Convert the wavelength to meters first: 600 nm = $6.00 \times 10^{-7}$ m. ν = $3.00 \times 10^{8}$ ÷ $6.00 \times 10^{-7}$ = $5.00 \times 10^{14}$ Hz.

Higher levels have higher energy. Dropping from n = 3 to n = 1 releases the difference as a photon whose energy equals the gap between the levels. Moving up takes energy instead.

A d sublevel has 5 orbitals, each holding at most 2 electrons of opposite spin, so 5 × 2 = 10. The s, p, and f sublevels hold 2, 6, and 14.

Fill 11 electrons from lowest energy up: 2 in 1s, 2 in 2s, 6 in 2p, and the last one in 3s: 1s²2s²2p⁶3s¹. A p sublevel holds at most 6 and an s sublevel at most 2, which rules out the others.

P is 1s²2s²2p⁶3s²3p³. The outermost level (n = 3) holds 3s² and 3p³, so 5 valence electrons. For main-group elements this matches the group (group 15).

The number of dots equals the number of valence electrons: Mg 2, O 6, Al 3, Ne 8. Only Mg (3s²) has 2.

The filling order is 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p … 3d is slightly higher in energy than 4s but lower than 4p, so it fills right after 4s. That is why the transition metals begin at Sc in period 4.