How are neutrons found?

Subtract the atomic number from the mass number, so 23 minus 11 gives twelve for sodium.

How many electrons does a neutral atom have?

The same as its protons, that is the atomic number, so oxygen with eight protons has eight electrons.

S4 Chemistry: Atomic Structure (Paper 1)

2026-10-31

Ten original questions start with the three numbers: sodium has atomic number 11 and mass number 23, so twelve neutrons; magnesium with twelve electrons arranges as 2,8,2; chlorine as 2,8,7. The middle items check that a neutral atom has as many electrons as protons, then form ions: sodium loses one electron to give Na+, potassium fills four shells and oxygen gains two electrons to give O2-. The challenge item works back from mass number 23 and twelve neutrons to atomic number 11, and the paper ends with the common error of treating the mass number as the electron count.

Direct answer　Write the atomic number as the proton count, subtract it from the mass number for neutrons, fill shells from two upwards and change only the outermost shell for ions.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 5 · Intermediate 3 · Challenge 1 · 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)

FoundationA sodium atom has atomic number 11. How many protons does it contain?
FoundationThe mass number of sodium is 23. How many neutrons does it contain?
FoundationWrite the electron arrangement of magnesium, atomic number 12.
FoundationHow many electrons are in the outermost shell of chlorine, atomic number 17?
FoundationA neutral oxygen atom has eight protons. How many electrons does it have?
IntermediateA sodium atom loses one outermost electron. Give the ion symbol and the electron arrangement.
IntermediatePotassium has atomic number 19. How many electron shells does it have?
IntermediateAn oxygen atom, atomic number 8, gains two electrons. Give the ion symbol.
ChallengeAn atom has mass number 23 and 12 neutrons. What is its atomic number?
Error redoError repair: a student uses the mass number 23 as the electron count of sodium. What is correct?

03　Answers and one-line notes

#	Answer	One-line note
1	11 protons	The atomic number is defined as the number of protons, so sodium has eleven.
2	12 neutrons	Neutrons equal mass number minus atomic number, so 23 minus 11 is 12.
3	2,8,2	Twelve electrons fill two in the first shell and eight in the second, leaving two outermost.
4	7 electrons	The arrangement is 2,8,7, so the outermost shell holds seven electrons.
5	8 electrons	A neutral atom has as many electrons as protons, so eight electrons.
6	Na+, arrangement 2,8	Losing one electron gives a single positive charge, so the symbol is Na+ and the arrangement 2,8.
7	4 shells	The arrangement is 2,8,8,1, four shells, so potassium sits in period four.
8	O2-	Starting from 2,6 it gains two electrons to reach eight and carries two negative charges, written O2-.
9	11	Protons equal mass number minus neutrons, so 23 minus 12 gives eleven.
10	11 electrons	Neutral atoms have electrons equal to protons, that is eleven; the mass number includes neutrons and cannot be used.

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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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.