Trend Anomalies and Successive Ionization Energies

Dip one: group 2 to group 13

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.

Dip two: group 15 to group 16

Nitrogen has IE₁ = 1402 kJ/mol but oxygen only 1314 kJ/mol. Nitrogen is 2p³, with one electron in each p orbital. Oxygen is 2p⁴, so one orbital holds a pair, and the repulsion between those two same-orbital electrons makes one of them easier to remove.

Electron affinity is not a clean trend

Electron affinity becomes more exothermic across a period and less so down a group, but with exceptions. Chlorine, not fluorine, has the most exothermic first electron affinity, because fluorine is so small that adding an electron adds severe repulsion. Group 2 and group 18 elements have positive (endothermic) affinities, since the added electron must enter a new higher subshell or shell.

Successive ionization energies

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.

IE₁ < IE₂ < IE₃ < ...

Reading a data table

For a period 3 element with IE values 578, 1817, 2745 and then 11577 kJ/mol, three modest values are followed by a jump of about four times. Three valence electrons means group 13, and in period 3 that is aluminium.

Locate the jump before assigning a group

A large jump between IEₙ and IEₙ₊₁ means n electrons were removed before a core electron had to be taken. The jump position counts valence electrons, but it does not by itself name the element: a group 1 element in different periods can show the same pattern. To identify an element, combine the jump with another clue such as its period or the magnitude of the first IE. Compare ratios between consecutive values, not merely which IE is largest; later removals always cost more. Check that the proposed neutral configuration has n outer electrons.

Diagnose a dip before invoking the trend

A first-ionization-energy dip can come from two different orbital effects. Starting a higher-energy p subshell makes the first p electron easier to remove than an s electron in the preceding element. Pairing two p electrons adds repulsion, making one paired electron easier to remove than a single p electron in the preceding half-filled arrangement. State which orbital change occurs in the pair you compare.