Read the Half-Filled and Fully-Filled Exceptions Before Ranking IE
First ionisation energy rises across a period and falls down a group — but the single most-tested exception is that half-filled and fully-filled subshells are extra stable, breaking the smooth increase at specific points (N > O, Be > B, Mg > Al, P > S, Zn > Ga).
First Ionisation Energy Trends Across Periods and Groups is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
The half-filled/fully-filled exception, applied consistently
N (half-filled 2p3) has higher IE than O (2p4, where the 4th electron enters an already-occupied orbital, increasing repulsion); Be (filled 2s2) has higher IE than B (2p1, easier to remove since it's alone in a higher-energy orbital). The same logic gives Mg > Al and P > S — always check whether either element being compared sits at a half-filled or fully-filled configuration before applying the plain 'IE rises across a period' rule.
Down a group, IE falls — with d-block and coinage-metal wrinkles
IE falls down a main group as the outermost electron gets farther from the nucleus and more shielded — but coinage metals (Cu, Ag, Au) don't follow a simple monotonic trend: Au's IE exceeds Ag's due to poor shielding by the filled 4f subshell (a relativistic/lanthanide-contraction-adjacent effect), giving Cu > Ag < Au rather than a smooth decrease.
Comparing IE across elements with very different configurations
When elements from different blocks or groups are compared together (like Ca, Ba, S, Se, Ar in one ranking), work out each one's actual position (period, group, block) and apply BOTH the period trend and group trend together, rather than trying to memorise the combined ranking as a single fact — noble gases like Ar sit at the high end of their period, discontinuously higher than neighbouring groups.
OlympIQ exam lens
- ✓IE rises across a period, falls down a group — but check for a half-filled or fully-filled exception before applying this directly.
- ✓Half-filled (p3, d5) and fully-filled (p6, d10, s2) configurations are extra stable, giving that element higher IE than its immediate neighbour.
- ✓Coinage metals (Cu/Ag/Au) don't follow a smooth group trend — Au's IE exceeds Ag's due to poor 4f shielding.
- ✓When comparing elements across different groups/periods/blocks at once, apply period and group trends together rather than memorising the combined order.
Don't apply 'IE always rises across a period' blindly — check first whether either element sits at a half-filled (N, P) or fully-filled (Be, Mg, Zn) configuration, since these break the smooth trend at exactly the point most commonly tested.
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