Atoms, Elements & the Periodic Table

The number of protons, which defines the element — 6 protons is always carbon.

Atoms of the same element with different neutron counts (e.g. carbon-12 vs radioactive carbon-14). They behave identically chemically.

An atom that has gained or lost electrons, giving it a charge — Na⁺ lost one electron, Cl⁻ gained one.

s = 2, p = 6, d = 10, f = 14 — matching the widths of the table's blocks.

Periods are electron shells; groups are elements with the same number of valence electrons, hence similar chemical behaviour.

The lone outer electron sits further from the nucleus and is screened by more inner shells, so it's easier to lose — caesium is explosive.

Each is one electron short of a full p subshell, so they grab electrons greedily; fluorine is the most aggressive oxidiser.

Their outer shells are already full — nothing to gain or give — so they're used in lightbulbs and welding shields.

Not because atoms are packed full of matter — atoms are mostly empty space — but because electron clouds repel each other electrically.

1s² 2s² 2p⁶ 3s¹ — one loose outer electron, so sodium readily gives it away (e.g. to chlorine, forming NaCl).