Read Successive IP Jumps as a Signal of Valence Electron Count
A big jump between successive ionisation energies (IE1→IE2→IE3...) marks exactly where the next electron has to come from a completed inner shell — reading these jumps tells you the element's valence electron count directly, without needing to identify the element by name.
Successive and Exceptional Ionisation Energies is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Locating the valence electron count from a jump
For IE values 100, 150, 1500 eV, the huge jump between IE2 and IE3 signals that the element has exactly 2 valence electrons (easy to remove) before hitting the tightly-bound core — this identifies the element as being in group 2 (like Be), without needing any other information.
Second ionisation energy has its own period trend, distinct from IE1
Second IP order across Na, Mg, Al, Si doesn't simply mirror the IE1 trend — removing a SECOND electron from Na (already Na+, with a stable noble-gas-like configuration) costs far more than removing a second electron from Mg (Mg+ still has one more valence electron to lose easily) — always re-derive the specific ion's own configuration rather than assuming IE2 tracks IE1's ranking.
Comparing an isoelectronic series' second ionisation energies
For C, N, O, F's second ionisation energies (removing an electron from C+, N+, O+, F+ respectively), the same half-filled-stability logic applies one level up — N+'s configuration situation differs from O+'s in exactly the way N and O themselves differ at the neutral-atom level, so work out each cation's own electron configuration before ranking.
He's second IE and other cross-atom comparisons
He's second ionisation energy (removing the last electron from He+, a hydrogen-like species with Z=2) is exactly 4× hydrogen's first IE, since IE for a one-electron species scales with Z² — this Z² scaling is worth recognising directly whenever a question compares a hydrogen-like ion to hydrogen itself.
OlympIQ exam lens
- ✓A large jump between successive IEs marks exactly where the next electron must come from a filled inner shell — locate the jump to find valence electron count.
- ✓IE2 has its own trend, not simply a shifted copy of IE1 — recompute from each specific ion's own configuration.
- ✓Half-filled/fully-filled stability effects apply to cations too (N+ vs O+), not just neutral atoms.
- ✓For hydrogen-like one-electron species, ionisation energy scales with Z² — He+'s IE is 4× H's IE.
Don't assume IE2's ranking across a row of elements mirrors IE1's ranking — always work out the specific singly-ionised species' own electron configuration (including whether IT sits at a half-filled/fully-filled point) rather than reusing the neutral-atom trend.
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