electron configuration

If electrons all crowded as close to the nucleus as possible, every element would behave the same. They do not. Electrons stack into a set of allowed energy levels called shells and subshells, a bit like guests filling a theatre from the front rows outward: the cheap front seats fill first, and only when a level is full do electrons move to the next. The electron configuration is simply the seating chart — which levels are filled and how full each one is.

The shells are numbered 1, 2, 3, ... outward, and each contains subshells labelled s, p, d, f that hold at most 2, 6, 10, and 14 electrons respectively. Electrons fill from lowest energy upward. Sodium, with 11 electrons, is written 1s2 2s2 2p6 3s1: two in the first shell, eight in the second, and a lone electron starting the third. That lonely outer electron is exactly what makes sodium reactive and metallic.

The configuration is the bridge from a bare proton count to real chemistry. Elements with similar outer configurations behave alike, which is the whole reason the periodic table has columns. A filled outer shell (as in neon, 1s2 2s2 2p6) is unusually stable and unreactive — the atom has no incentive to bond, which is why the noble gases are chemically aloof.