How do ionisation energies give the group number?

Variable oxidation states, common catalysts, and coloured compounds

X2+ has 18 electrons, what is Z?

More violently; rubidium sits below potassium, with a larger radius and lower ionisation energy

What features mark a transition metal?

Atomic number 20; period 4, group II (calcium)

S4 Chemistry: Periodic Table and Trends Paper 3 (Data Inference, Isoelectronic Series and Transition Metals)

2026-11-03

S · 4

This paper infers group number from ionisation-energy jumps, ranks a 10-electron isoelectronic series, and extends to transition-metal features and group-trend predictions.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10 questions
Suggested time	25 minutes
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S4 Chemistry – Periodic Table and Trends
Level	Challenge

02　Questions (10 items)

FoundationName three typical features that separate transition metals from main-group metals.
FoundationPredict whether rubidium reacts with water more or less violently than potassium, and explain.
FoundationThe ion X2+ has 18 electrons. What is the atomic number of X, and its period and group?
FoundationAn element has the arrangement 2, 8, 18, 8, 1. Give its period, group and type.
IntermediateNa+ 102 pm, Mg2+ 72 pm, Al3+ 54 pm, F- 133 pm. What do these ions share?
IntermediateWhy does metallic conductivity generally improve down a group?
IntermediateRadii: Na+ 102 pm, K+ 138 pm, Ca2+ 100 pm. Order them from smallest to largest with a reason.
ChallengeArgon first ionisation energy (1520 kJ/mol) far exceeds potassium (419) although potassium has the larger atomic number. Why?
ChallengeAn element has first four ionisation energies of 590, 1150, 4940 and 6480 kJ/mol. Which group?
Error redoA student wrote “radius shrinks down a group because nuclear charge increases”. Correct it.

03　Answers and one-line notes

#	Answer	One-line note
1	Variable oxidation states, common catalysts, and coloured compounds	Inner d electrons can take part, giving several oxidation states.
2	More violently; rubidium sits below potassium, with a larger radius and lower ionisation energy	Extrapolating a group trend is the standard prediction question.
3	Atomic number 20; period 4, group II (calcium)	Add the two lost electrons back to get 20, then write 2, 8, 8, 2.
4	Period 5, group I; a metal (rubidium)	Five shells means period 5; one outer electron means group I.
5	All are 10-electron isoelectronic species; the larger the nuclear charge, the smaller the radius	With equal electrons, proton count decides the pull.
6	Outer electrons are released more easily, giving more delocalised electrons and stronger metallic bonding	Metallic character and conductivity rise together from easier electron loss.
7	Ca2+ (100) < Na+ (102) < K+ (138); higher charge and fewer shells mean smaller	Compare both shell count and charge, not one alone.
8	Argon has a full outer shell and small radius; potassium has a single fourth-shell electron, far out and well shielded	Compare shell number and shielding, not atomic number alone.
9	Group II; the jump comes at the third electron, so there are two valence electrons	The jump index is the valence count plus one, the standard way to read the group.
10	Radius grows down a group; the extra shell outweighs the extra nuclear charge	Extra nuclear charge dominates across a period, but a new shell dominates down a group.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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