Atomic structure & periodicity — questions and answers
4 high-yield, syllabus-aligned questions on Atomic structure & periodicity, covering 12 individually creditable mark points. Each one shows the answer that scores full marks under our guide, which phrase earns which mark, and a common incomplete answer — so you can see the difference rather than guess at it.
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Every question and solution is reviewed by examiners versed in WAEC and JAMB, each with more than thirty years of experience.
Mark allocations are TopMarks’ own, written against the published syllabus and Chief Examiners’ reports. They are a guide to how these answers are usually credited, not an official marking scheme.
Explain3 marks
Define isotopy and explain why the isotopes of an element have identical chemical properties.
TopMarks model answer
Isotopy is the existence of two or more atoms of the same element that have the same atomic number but different mass numbers. The difference in mass number arises because the atoms contain different numbers of neutrons. Isotopes have identical chemical properties because chemical properties depend on the number and arrangement of the electrons, and the isotopes of an element have the same number of electrons in the same configuration.
Suggested marking guide
1Same element and same atomic number, different mass numbers
2The difference is in the number of neutrons
3Chemical properties depend on the electrons
Why it scores
The third mark is the one the question is really testing. Chemistry happens in the outermost electrons; the nucleus is a spectator. Isotopes differ only in the nucleus, so their chemistry cannot differ. State that reasoning explicitly — “because they have the same electronic configuration” — rather than merely asserting that the properties are the same.
A common incomplete answer
“Isotopes are atoms of the same element with different mass numbers.”
What it costs: Two marks of three. The definition is credited, but the neutrons are not mentioned and the question’s actual demand — WHY the chemistry is identical — is never addressed.
Key ideas to include: atomic number, mass number, neutron, electronic configuration.
Deduce4 marks
An element has atomic number 17. Write its electronic configuration and use it to deduce its period, its group and its valency.
TopMarks model answer
The electronic configuration is 2, 8, 7. The atom has three occupied shells, so the element is in Period 3. It has seven electrons in its outermost shell, so it is in Group VII. To attain a stable octet it gains one electron, so its valency is 1 and it forms a singly charged negative ion.
Suggested marking guide
1Correct configuration 2, 8, 7
2Period 3 deduced from the number of shells
3Group VII deduced from the outer electrons
4Valency 1
Why it scores
Each deduction must be tied to the evidence: the PERIOD comes from the number of shells, the GROUP from the number of outer electrons, the VALENCY from how many electrons are needed to reach eight. Writing the three answers without their reasons risks all three, because the command word is “deduce”.
A common incomplete answer
“The configuration is 2, 8, 7 so the element is in Group 7.”
What it costs: Two marks of four. Configuration and group are correct, but the period is never deduced and the valency is never given.
State three properties that change gradually across a period of the Periodic Table from left to right.
TopMarks model answer
Atomic radius decreases across a period, because the increasing nuclear charge pulls the electrons in the same shell closer to the nucleus. Ionisation energy increases across a period, and electronegativity also increases, because the atoms hold their outermost electrons more tightly. The character of the elements changes from metallic on the left to non-metallic on the right.
Suggested marking guide
1Atomic radius decreases
2Ionisation energy or electronegativity increases
3Metallic character changes to non-metallic
Why it scores
Each mark needs the property AND the direction. “Atomic radius” on its own is not a trend; “atomic radius decreases” is. Whenever a question says “changes gradually”, the examiner is waiting for the word increases or decreases in every line.
A common incomplete answer
“The atomic radius decreases across a period.”
What it costs: Two marks of three. Correct and properly directed, but only one of the three trends asked for.
Explain why the atomic radius of sodium is larger than that of chlorine, although both elements are in Period 3.
TopMarks model answer
Sodium and chlorine both have their outermost electrons in the third shell, so the shielding effect of the inner electrons is the same in both. Chlorine has the greater nuclear charge, seventeen protons against sodium’s eleven, so it attracts its outer electrons more strongly and pulls them closer to the nucleus, giving the smaller atomic radius.
Suggested marking guide
1Same shell, so the same shielding
2Greater nuclear charge attracts the outer electrons more strongly
Why it scores
A comparison across a period is always an argument about two competing factors — shielding and nuclear charge — and the mark is for showing that one is CONSTANT while the other CHANGES. An answer that mentions only the nuclear charge has not explained why the shells do not compensate for it.
A common incomplete answer
“Because chlorine is a smaller atom than sodium.”
What it costs: Both marks. This restates the question as though it were the answer. Neither shielding nor nuclear charge is mentioned, so there is no explanation at all.
Practise Atomic structure & periodicity on real questions
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