How do I read period and group from an arrangement?

Atomic number 16; period 3, group VI

Why does radius shrink across a period?

It decreases: 186, then 160, 143 and 118 pm

What is the ionisation-energy order for potassium, sodium and lithium?

It falls down the group; more shells leave the outer electron farther out and better shielded

S4 Chemistry: Periodic Table and Trends Paper 4 (Error Redo)

2026-11-03

Tutor Prep Guide · S4 Chemistry

This paper reworks the misconceptions that cost marks in papers 1 to 3: reading the atomic number as the group, mixing the two trend directions, and ranking isoelectronic ions.

Quick answer Four foundation items on arrangement and trend direction, three intermediate items on ionisation-energy jumps and ion radii, two challenge items on strontium and neon, then one item correcting two errors at once.

01 Paper details (check answers afterwards)

Item	Detail
Questions	10 questions
Suggested time	20 minutes
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S4 Chemistry – Periodic Table and Trends
Level	Error redo

02 Questions (10 items)

FoundationElement A has the electron arrangement 2, 8, 6. Give its atomic number, period and group.
FoundationElement B has atomic number 20. Give its electron arrangement and say whether it is a metal.
FoundationThird-period radii in pm: Na 186, Mg 160, Al 143, Si 118. How does radius change left to right?
FoundationDown a group, how do atomic radius and metallic character change?
IntermediateAn element has first four ionisation energies of 738, 1451, 7733 and 10543 kJ/mol. Which group?
IntermediateFirst ionisation energies: potassium 419, sodium 494, lithium 520 kJ/mol. Explain the order.
IntermediateCa2+ 100 pm, K+ 138 pm and Cl- 181 pm all have 18 electrons. Order them and explain.
ChallengeElement C has the arrangement 2, 8, 18, 8, 2. Give its period and group, and predict whether it reacts with water more or less violently than calcium.
ChallengeNeon first ionisation energy is 2081 kJ/mol, above fluorine at 1681 in the same period. Why?
Error redoA student wrote that chlorine, atomic number 17, is in group 17 with 17 outer electrons, and that metallic character increases across a period. What is wrong twice?

03 Answers and one-line notes

#	Answer	One-line note
1	Atomic number 16; period 3, group VI	Total electrons give the atomic number; the shell count gives the period.
2	2, 8, 8, 2; a metal (calcium)	Only two outer electrons, easily lost, so it is a metal.
3	It decreases: 186, then 160, 143 and 118 pm	Nuclear charge rises while the shell count stays fixed, so electrons are pulled closer.
4	Radius increases; metallic character increases	An extra shell enlarges the atom and frees the outer electron more easily.
5	Group II; the jump comes at the third electron, so there are two valence electrons	The jump position minus one gives the valence count, the standard group test.
6	It falls down the group; more shells leave the outer electron farther out and better shielded	A lower ionisation energy means easier electron loss, so stronger metallic character.
7	Ca2+ then K+ then Cl-, smallest first; with equal electrons, more protons means a smaller radius	Proton counts are 20, 19 and 17, so the pull weakens in that order.
8	Period 5, group II (strontium); more violently than calcium	Metallic character rises down the group, so the reaction gets more violent.
9	Neon has a full outer shell and a smaller radius, so removing an electron costs more energy	A full shell is exceptionally stable, which is why group 0 is unreactive.
10	Chlorine is 2, 8, 7 with seven outer electrons in group VII; metallic character decreases across a period	The group comes from outer electrons, not the atomic number; stronger pull across a period makes electron loss harder.

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.

Find a tutor free Tutor Registration →

Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.