Trend Anomalies and Successive Ionization Energies

Beryllium has IE1 = 900 kJ/mol but boron only 801 kJ/mol, against the general increase. Beryllium removes an electron from a filled 2s subshell, while boron removes a 2p electron that is higher in energy and partly shielded by the 2s pair. The subshell change beats the extra proton.

Write the last occupied subshell before explaining a dip. Be is 2s² and B is 2s² 2p¹; the electron removed from B is a less penetrating, higher-energy 2p electron. Mg versus Al repeats the 3s²/3p¹ pattern. This mechanism is different from the N/O dip: there the electron is in the same p subshell but its pairing changes. Naming “extra repulsion” for every dip misses the distinction.

Consider X(g) + e⁻ → X⁻(g). When electron affinity is reported as the enthalpy change for this process, exothermic attachment has a negative value; more exothermic means more negative. Some tables instead report the positive energy released, so always read the convention before comparing numbers. A second electron added to an already negative ion is a different process and is generally endothermic for an isolated gaseous ion.

First electron-affinity trends are irregular because shell, subshell and electron-repulsion effects compete. Chlorine's electron attachment is more exothermic than fluorine's: the extra electron enters a roomier 3p cloud rather than compact 2p, reducing the repulsion penalty. Filled s subshells, half-filled p subshells and closed shells can make attachment less favorable than a simple across-period arrow suggests. These exceptions are reasons to inspect the electron configuration and supplied data, not to invent an exact affinity from position alone.

Each removal costs more than the one before, because the remaining electrons feel a higher charge-to-electron ratio. The informative feature is a sudden large jump: it appears when the valence shell is exhausted and the next electron must come from the core. Counting the removals before the jump gives the valence electron count, and therefore the group.

Successive removals involve different starting ions: IE₁ is X → X⁺, IE₂ is X⁺ → X²⁺, and IE₃ is X²⁺ → X³⁺, each in the gas phase with an electron produced. Later electrons are removed from increasingly positive species, so the values rise even within the same shell. A very large jump indicates a qualitative change to a tightly bound core electron. For a main-group atom with a jump between IE₂ and IE₃, two valence electrons were removed before the core was reached, so group 2 is the usual inference. This simple group assignment is not a general shortcut for every transition metal.