Read Electron Configuration Before Naming Block, Group or Period
Block, group and period all follow mechanically from the electron configuration's last-filled subshell — write out the configuration first, then read off block/group/period, rather than trying to recognise the element by name.
Electronic Configuration and Block/Group/Period Identification is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Reading block from the last-filled subshell
The last electron entering an s or p subshell means s-block or p-block (representative elements); entering a d subshell means d-block (transition elements, general pattern (n-1)d₁-10 ns₁-2); entering an f subshell means f-block (inner transition, general pattern (n-2)f₁-14 (n-1)d₀-1 ns₂). A configuration like [Kr]4d₁₀ 5s₀ is d-block despite the empty s subshell shown — the LAST electron added still went into 4d.
Extending the periodic table past current elements
reasoning about hypothetical elements just means continuing the same n+l aufbau filling order past the known elements, not inventing new rules.
Group number needs care for d-block and f-block elements
For representative (s/p-block) elements, group number reads directly off the number of valence electrons — but d-block and f-block elements use different group-numbering conventions (old-style IIIB/IVB/etc. or IUPAC 1-18), and questions mixing block, group AND period together need each read off separately and carefully, not derived from one shortcut rule.
Doberiner's triads: the historical precursor to periodicity
Dobereiner's triads (three elements where the middle one's atomic mass/properties sit roughly midway between the other two) include classic examples like Ca-Sr-Ba, Cl-Br-I, and Li-Na-K, but P-As-Sb and S-Se-Te ALSO qualify — recognising which sets satisfy the 'middle element averages the outer two' criterion is a direct historical-context recall question.
OlympIQ exam lens
- ✓Block is determined by which subshell the LAST electron entered — s, p, d, or f — read directly from the configuration.
- ✓Continuing the periodic table past known elements just means continuing the same aufbau (n+l) filling order, nothing new.
- ✓Group numbering conventions differ for representative vs transition vs inner-transition elements — don't apply one universal rule.
- ✓Dobereiner's triads: the middle element's properties average the outer two — Ca-Sr-Ba, Cl-Br-I, P-As-Sb, S-Se-Te are classic examples.
Don't judge an element's block from which subshell appears LAST in the written configuration string — judge it from which subshell the LAST ELECTRON actually entered; [Kr]4d₁₀ 5s₀-style configurations can look s-block at a glance but are d-block.
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