NZ NCEA Level 3 Chemistry

## Energy levels, sublevels and orbitals Electrons in an atom occupy **energy levels** numbered $n = 1, 2, 3, \dots$, with the lowest number closest to the nucleus and lowest in energy. Each level is divided into **sublevels**, named s, p, d and f. An s sublevel holds one **orbital**, a p sublevel three, a d sublevel five, and each orbital holds at most two electrons, which must have opposite spins. So the sublevels hold 2 (s), 6 (p) and 10 (d) electrons. | Energy level | Sublevels present | Maximum electrons | |---|---|---| | 1 | 1s | 2 | | 2 | 2s, 2p | 8 | | 3 | 3s, 3p, 3d | 18 | | 4 | 4s, 4p, 4d, 4f | 32 | An electron configuration lists the occupied sublevels with the number of electrons as a superscript. In $3\mathrm{p}^4$ the 3 is the energy level, the p is the sublevel and the 4 is the number of electrons in it. ## The order of filling Electrons go into the lowest-energy sublevel available. Within the first 36 elements the order is $$1\mathrm{s} < 2\mathrm{s} < 2\mathrm{p} < 3\mathrm{s} < 3\mathrm{p} < 4\mathrm{s} < 3\mathrm{d} < 4\mathrm{p}$$ The 4s sublevel fills before the 3d because it is lower in energy in a neutral atom, so potassium and calcium add their outer electrons to 4s and the 3d sublevel starts filling at scandium. The 4p sublevel then fills from gallium to krypton. The periodic table follows the same order: groups 1 and 2 are the s block, groups 13 to 18 the p block, and the transition metals the d block. ## Writing configurations The number of…