How do isotopes differ?

They share the proton count and so are the same element, but different neutrons give different mass numbers, as chlorine-35 and chlorine-37 have eighteen and twenty.

Do protons change when an ion forms?

No. Only electrons are gained or lost, so a magnesium atom and Mg2+ both keep twelve protons.

S4 Chemistry: Atomic Structure (Paper 4)

2026-10-31

Tutor Prep Guide · Secondary Chemistry

Ten original questions close the topic with particle comparisons and isotopes: fluorine at atomic number 9 and mass number 19 gives ten neutrons and the arrangement 2,7 in period two group seven, potassium loses one electron for K+ at 2,8,8, and calcium arranges 2,8,8,2 in period four group two. The middle items recover atomic number 19 from mass number 39 with twenty neutrons, compare Na+ with Mg2+ at the same 2,8 but eleven and twelve protons, and contrast a magnesium atom with its ion that keeps twelve protons but loses two electrons. The challenge items compare chlorine-35 and chlorine-37 at eighteen and twenty neutrons, and turn mass number 24 with 2,8,2 into twelve neutrons. The paper ends with the error of quoting seventeen electrons for a chloride ion.

Direct answer　Split every particle into three columns: protons fix the element, neutrons come from mass number minus protons, and only electrons change with the charge.

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)

FoundationFluorine has atomic number 9 and mass number 19. How many neutrons does it contain?
FoundationWrite the electron arrangement of a fluorine atom with nine electrons and state its period and group.
FoundationPotassium, atomic number 19, loses one outermost electron. Give the ion symbol and arrangement.
FoundationHow many electron shells does calcium, atomic number 20, have, and where does it sit in the periodic table?
IntermediateAn atom has mass number 39 and twenty neutrons. Give its atomic number and arrangement.
IntermediateNa+ and Mg2+ both show the arrangement 2,8. Give their proton counts and say whether they are the same element.
IntermediateA magnesium atom has twelve electrons and the Mg2+ ion has ten. Do they share the same proton count, and how are the electrons arranged?
ChallengeChlorine-35 and chlorine-37 are atoms of the same element. Give their proton and neutron counts and explain why their chemistry is identical.
ChallengeAn atom has mass number 24 and the arrangement 2,8,2. Give its proton count, neutron count, period and group.
Error redoError repair: a student says a chloride ion has seventeen electrons because chlorine is atomic number 17. Give the correct count and arrangement.

03　Answers and one-line notes

#	Answer	One-line note
1	10 neutrons	Neutrons equal mass number minus atomic number, so 19 minus 9 gives ten.
2	2,7; period 2, group 7	Two shells give period two and seven outer electrons give group seven.
3	K+, arrangement 2,8,8	Losing one electron leaves eighteen electrons and a single positive charge, so the arrangement is 2,8,8.
4	4 shells; period 4, group 2	Twenty electrons arrange as 2,8,8,2, so four shells mean period four and two outer electrons mean group two.
5	atomic number 19, arrangement 2,8,8,1	Thirty-nine minus twenty gives nineteen protons, so nineteen electrons arrange as 2,8,8,1.
6	11 protons and 12 protons, so they are different elements	The same arrangement only means the same electron count; eleven protons against twelve make two different elements.
7	both have 12 protons; atom 2,8,2, ion 2,8	Forming an ion changes electrons but not protons, so the atom keeps 2,8,2 and the ion holds 2,8.
8	17 protons each; 18 and 20 neutrons; both have 7 outer electrons	Thirty-five minus seventeen gives eighteen neutrons and thirty-seven gives twenty; only neutrons differ, so the chemistry matches.
9	12 protons, 12 neutrons; period 3, group 2	The arrangement totals twelve electrons, so twenty-four minus twelve gives twelve neutrons and three shells with two outer electrons mean period three group two.
10	18 electrons, arrangement 2,8,8	Seventeen is the atom count; the negative charge adds one electron for eighteen, filling 2,8,8 while protons stay at seventeen.

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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