2.2.3 · D5Periodic Trends
Question bank — Ionic radius — cation - parent atom, anion - parent atom; isoelectronic series
Before the questions, we build the three levers visually and quantitatively so the answers below rest on something concrete, not slogans.
First: what the words and symbols mean on this page
True or false — justify
Every answer must name which lever moved: protons (), electron count, shielding (), or number of shells.
Removing an electron always makes a species smaller than its parent atom.
True in direction — with fewer electrons, shielding drops and the surviving electrons feel a higher , so they pull in; if a whole outer shell vanishes (e.g. Na → Na⁺) the shrink is dramatic.
An anion is larger than its parent because the added electron carries negative charge that repels the nucleus.
False — the nucleus is attracted to any electron, not repelled. The anion swells because one more electron shares the same (lower each) and adds electron–electron repulsion, not because "negative repels."
In an isoelectronic series, the species with the most electrons is the largest.
False — by definition every member has the same electron count, so you can't compare on electrons; you compare protons, and fewer protons means larger.
Na⁺ and Ne have identical radii because both are with 10 electrons.
False — same 10 electrons, but Na⁺ has 11 protons vs Ne's 10, so Na⁺ pulls the cloud tighter and is smaller ( pm vs pm).
Cl⁻ is bigger than K⁺ only because chlorine is further left in the period.
Misleading — both are 18-electron (Ar-like); the real reason is proton count: Cl has 17 protons, K has 19, so K⁺'s stronger pull makes it smaller.
Adding two electrons (O → O²⁻) increases the number of repelling electron pairs far faster than adding one.
True and countable — repulsion scales with the number of pairs : going electrons adds 8 new pairs, but adds new pairs. So the second electron piles on more new repulsion than the first, and per electron keeps dropping — O²⁻ ( pm) ends up much larger than O.
For a cation, higher charge (Mg²⁺ vs Na⁺) always means smaller size even across different elements.
Not automatically — it's true here because Mg²⁺ and Na⁺ are isoelectronic (10 e⁻) so Mg's 12 protons beat Na's 11. Compare non-isoelectronic ions and you'd have to weigh shells and separately.
The isoelectronic rule "" also predicts trends when electron counts differ.
False — the rule assumes and electron count are fixed so that . Change the electron count and changes too, breaking the clean proportionality.
Spot the error
Each line states a flawed piece of reasoning. Reveal the fix.
"F and F⁻ are the same element, so they're about the same size."
Wrong — F⁻ ( pm) is nearly double F ( pm). One extra electron on a small, high- shell drops the per-electron pull sharply and adds repulsion, so the swell is large in proportion.
"S²⁻ has 18 electrons and Ca²⁺ has 18 electrons, so their sizes are basically equal."
Wrong — equal electrons is exactly when protons decide: S has 16 protons, Ca has 20, so Ca²⁺ pulls far harder and is much smaller ( pm vs S²⁻ pm).
"Al³⁺ is the smallest of N³⁻…Al³⁺ because it has the biggest positive charge."
The charge is a symptom, not the cause. Al³⁺ is smallest because it has the most protons (13) squeezing the shared 10-electron cloud; the +3 label just records that it lost 3 electrons.
"To rank Na⁺, Mg²⁺, and Al³⁺, look at their charges: +1, +2, +3, so radius decreases."
Right answer, incomplete reason — it works only because they're isoelectronic (all 10 e⁻), which makes charge track proton count. Always check isoelectronicity first, then it's really protons doing the work.
"Removing an electron lowers , so the cation feels less pull and should grow."
Wrong — removing an electron does not change (proton count). stays fixed; it's that drops, so rises and the ion shrinks.
"The cation shrinks because losing negative charge makes the atom less crowded."
Loose but fixable — the useful statement is: fewer electrons → less shielding and repulsion → higher per remaining electron → tighter, smaller. "Less crowded" alone doesn't explain the tighter pull.
Why questions
Why does removing the single electron from Na cause such a huge drop (186 → 102 pm)?
Because that electron was the only occupant of the shell; losing it removes an entire outer shell, leaving the compact core — a change in shell count, not just a small squeeze.
Why can't you use "more electrons = bigger" as a universal size rule?
It only holds when the nucleus is fixed (comparing an atom to its own ion). Across different elements, more protons can overwhelm more electrons, and in an isoelectronic set the electron count is identical, so the rule gives no information.
Why is it only approximately true that stays constant across an isoelectronic series?
The gross shielding structure (same electron count, same configuration) is unchanged, so barely moves; but rising contracts the orbitals and slightly changes penetration, so drifts a little — enough that "" is a very good, not perfect, approximation.
Why is a cation's shrink often larger in magnitude than an anion's swell for the same element step?
Losing an electron can delete a whole shell (big geometric jump), whereas gaining one usually just loosens an existing shell; the shell-removal effect is more drastic than the loosening effect.
Why does higher imply smaller radius rather than just "more energy"?
In the hydrogen-like model , so a larger effective pull directly divides the most-probable radius down — the electrons sit closer on average, and radius measures that outer reach.
Why do we compare an ion to its neutral parent atom rather than to some fixed standard?
Because the only thing changing on ionization is the electron count; holding fixed and watching the size shift isolates the electron-count effect cleanly, which is exactly the concept being taught.
Edge cases
What is the radius trend if a species gains an electron into a new shell (unlike Cl⁻ which fills an existing one)?
The jump is even larger — starting to occupy a higher- shell adds a whole new, poorly-shielded outer layer, so the growth exceeds the modest swell seen when merely completing a subshell.
Is the isoelectronic rule meaningful for a set containing only one species?
No — "more protons → smaller" is a comparative rule; with a single species there's nothing to order, so the rule is vacuous until you have at least two members with equal electron counts.
Two species have equal protons but different electron counts (e.g. Fe²⁺ vs Fe³⁺). Which is smaller and why?
Fe³⁺ — same but fewer electrons means less shielding, higher per electron, tighter pull, so the more-positive ion is smaller (this is the "same nucleus, fewer electrons" case, the mirror of isoelectronic reasoning).
Where does the simple trend break down across the lanthanides?
In the lanthanide contraction: as you fill electrons, they shield poorly (diffuse, poorly-penetrating -orbitals), so creeps up faster than expected and radii shrink steadily across the series — leaving later elements (e.g. Hf) surprisingly small, almost matching the row above.
Why can transition-metal ionic radii defy a naive one-number estimate?
Because -electrons split into different orbital sets in a crystal field, and high-spin vs low-spin arrangements change how electrons occupy space — so the same ion (e.g. Fe²⁺) has different radii depending on spin state and coordination, something a single figure cannot capture.
Can two different elements ever have exactly equal ionic radii?
Yes, approximately — sizes are set by the balance of , electron count and shells, and different combinations can land near the same value; the trends are monotonic tendencies, not a law forbidding coincidental equality.
What happens to at the degenerate limit of removing all electrons?
With zero electrons there is no cloud to size and , so "radius" is undefined — the concept only applies while at least one electron remains to define an outer reach.
Is Ne (neutral, 10 e⁻) larger or smaller than F⁻ (10 e⁻), and why does a neutral atom sit inside an isoelectronic list?
Ne is smaller than F⁻ — Ne has 10 protons vs F's 9, so with equal electrons the more-proton Ne pulls tighter; neutral atoms belong in the list because "isoelectronic" cares only about electron count, not charge label.
Recall One-line diagnostic you can run on any ionic-radius question
Ask in order: (1) Same electron count? → compare protons, more protons = smaller. (2) Same element, different charge? → compare electron count/shielding, fewer electrons = smaller. (3) Different elements and different electron counts? → reason from shells first, then . Watch for the exceptions: lanthanide contraction and transition-metal spin states break the tidy single- picture.
Connections
- Parent topic (Hinglish) — the full derivation these traps test.
- Effective Nuclear Charge (Z_eff) — the lever behind every "why" here.
- Shielding and Penetration — where the Slater estimate comes from.
- Atomic Radius Trends — the neutral baseline the true/false items compare against.
- Ionization Energy — the energetics twin of the cation-shrink argument.
- Electron Affinity — the energetics twin of the anion-swell argument.