Subshell capacities are s:2,p:6 and d:10 electrons. Fill 1s,2s,2p,3s,3p,4s,3d,4p,5s through the first 38 elements, allowing Cr=[Ar]3d54s1 and Cu=[Ar]3d104s1 exceptions. Remove 4s electrons before 3d in transition-metal cations: Fe2+ is [Ar]3d6.
Absorption promotes an electron to a higher allowed energy state when photon energy matches the gap. A downward transition emits a photon. Different elements have different energy gaps, giving characteristic wavelengths. Flame colours provide limited identification, while line spectra are more specific. Emission and absorption involve electron-state changes, not changes to the atomic nucleus.
Example 1
Write configurations for calcium and Ca2+.
Ca (20 electrons):1s22s22p63s23p64s2, or [Ar]4s2. Ca2+ loses its two 4s electrons, leaving [Ar].
Example 2
Explain why emission lines occur at selected wavelengths.
Only particular electron energy differences are allowed. Each downward transition emits a photon of that energy, giving discrete lines rather than every possible wavelength.
Watch out: Moving higher requires energy absorption; emission accompanies a transition to a lower allowed energy state.