How do I recover the atom from an ion?

Add back the electrons a cation lost or remove those an anion gained, and the total is the atomic number.

What are the shell limits?

Two electrons for the first shell and at most eight after that, so 2,9 is impossible.

S4 Chemistry: Atomic Structure (Paper 3)

2026-10-31

Ten original challenge questions work from ions and arrangements back to the atom: an ion with three positive charges and ten electrons gives aluminium at atomic number thirteen, 2,8,7 gives chlorine in period three group seven, mass number 56 with thirty neutrons gives twenty-six protons, and a sulfide ion with eighteen electrons gives sulfur at sixteen. The middle items compare neon, Na+ and F- at ten electrons each, add two electrons back to Mg2+ for 2,8,2, and explain why 2,9 cannot exist. The final items turn period three group two into 2,8,2 with twelve protons, split one ion into atom and ion arrangements, and repair the error of using mass number 35 as the chlorine electron count.

Direct answer　Reverse the charge to recover the atom electrons, read the period from shells and the group from outer electrons, and respect the 2, 8, 8 limits.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S4 Chemistry: Atomic Structure
Level	Four basic, three advanced, two challenge and one error-repair item

02　Questions (10 items)

FoundationAn ion Y3+ has ten electrons. Give the atomic number, the element and the ion arrangement.
FoundationAn atom has the arrangement 2,8,7. Give its electron count, element, period and group.
FoundationAn atom has mass number 56 and thirty neutrons. Give its protons and electrons.
FoundationA sulfide ion has eighteen electrons. Give the atomic number of sulfur and the ion arrangement.
IntermediateNeon, the sodium ion and the fluoride ion all show 2,8. How many electrons does each hold?
IntermediateThe magnesium ion Mg2+ shows 2,8. Give the atom arrangement and proton count.
IntermediateA student writes the arrangement 2,9. Why is this impossible and what should it be?
ChallengeAn element sits in period 3 group 2. Give its arrangement, electron count and proton count.
ChallengeAn ion X- has eighteen electrons. Give the arrangement of the atom and of the ion.
Error redoError repair: a student uses the mass number 35 as the electron count of chlorine. What is correct?

03　Answers and one-line notes

#	Answer	One-line note
1	atomic number 13, aluminium, arrangement 2,8	The ion has three electrons fewer than the atom, so the atom had thirteen, which is aluminium with arrangement 2,8 as an ion.
2	17 electrons, chlorine, period 3 group 7	The electrons total seventeen, which is chlorine, and three shells with seven outer electrons mean period three group seven.
3	26 protons and 26 electrons	Fifty-six minus thirty gives twenty-six protons, and a neutral atom has the same number of electrons.
4	atomic number 16, arrangement 2,8,8	Two extra electrons mean the atom had sixteen, so the atomic number is sixteen and the ion fills 2,8,8.
5	ten electrons each	Neon has ten; sodium loses one of eleven and fluorine gains one on nine, so all three hold ten electrons.
6	arrangement 2,8,2 and 12 protons	The ion forms by losing two outer electrons, so adding them back gives 2,8,2, twelve electrons and atomic number twelve.
7	the second shell holds at most 8, so it should be 2,8,1	Each shell has a limit of two then eight, so 2,9 must be rewritten as 2,8,1.
8	2,8,2 with 12 electrons and 12 protons	Period three means three shells and group two means two outer electrons, giving 2,8,2 and twelve protons.
9	atom 2,8,7 and ion 2,8,8	One extra electron means the atom had seventeen, arranged 2,8,7, and gaining one completes 2,8,8.
10	17 electrons in the arrangement 2,8,7	Electrons equal protons at atomic number seventeen with arrangement 2,8,7; the mass number 35 includes eighteen neutrons.

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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