CDS Chemistry · Teaching notes

Atomic Structure and Periodic Classification — CDS Chemistry

Atomic Structure and Periodic Classification has 26 past-year questions in CDS General Knowledge, from 2018 (I) to 2026 (I), and none of them is HARD. The older papers ask single facts: who found the nucleus, which isotope is radioactive. The newer ones put four statements about the periodic table in one item, so the trends in size and metallic character pay off as rules. The isotope and isobar definitions are the one pair to learn exactly, since CDS swaps them in the options.

Every subtopic, worked example, formula and trap in one printable document — answers shown, ready to share.

All 26 past questions CDS home

Subtopic notes

Formula & revision sheet

2 formulas · 7 reference tables · 12 gotchas across all subtopics — the exam-eve cheat-sheet

Atomic Models: Dalton to Bohr

Reference tables (2)

Dalton's atomic theory and its limits4 rows

Statement	Explained by Dalton's theory?
Law of conservation of mass	Yes
Law of constant composition	Yes
Law of multiple proportions	Yes
Atoms contain electrons, protons and neutrons	No — the atom is indivisible in this theoryQ CDS 2025 (I): the presence of sub-atomic particles is what Dalton's theory does not explain.

Rutherford's gold-foil experiment and its limit4 rows

Scientist	Key result
J. J. Thomson	Discovered the electron; plum-pudding model
Ernest Rutherford	Gold-foil experiment: discovered the atomic nucleusQ Asked three times: CDS 2019 (II), 2020 (I) and 2025 (I). The answer is always the nucleus, never the proton or neutron.
Niels Bohr	Fixed orbits; explains why the atom is stableQ CDS 2025 (I): Rutherford's model could not explain the stability of the atom.
James Chadwick	Discovered the neutron

Watch out for (3)

Dalton's symbols versus today's symbols
→ Dalton's atomic theory and its limits
Gold foil found the nucleus, not the proton
→ Rutherford's gold-foil experiment and its limit
Rutherford's weakness is stability
→ Rutherford's gold-foil experiment and its limit

Atomic Number, Isotopes and Isobars

Formulas (1)

Counting protons, neutrons and electrons · Mass number and electron count $A = Z + N \qquad e^{-} = Z - q$

Reference tables (2)

Isotopes and isobars3 rows

Term	Same	Different	Example
Isotopes	Atomic number Z	Mass number A	³⁵Cl and ³⁷Cl (both Z = 17)Q
Isobars	Mass number A	Atomic number Z	⁴⁰Ar (Z = 18) and ⁴⁰Ca (Z = 20)Q CDS 2019 (II): 'isobars have the same atomic number' was the incorrect statement.
Hydrogen isotopes	Z = 1	A = 1, 2, 3	Protium, deuterium, tritium; tritium is radioactiveQ

Isoelectronic species2 rows

Electrons	Species
10	N³⁻, O²⁻, F⁻, Ne, Na⁺, Mg²⁺, Al³⁺
18	P³⁻, S²⁻, Cl⁻, Ar, K⁺, Ca²⁺Q CDS 2022 (II): Cl⁻ is not isoelectronic with Mg²⁺, which has only 10 electrons.

Watch out for (4)

A positive ion has FEWER electrons
→ Counting protons, neutrons and electrons
Isobars share A, not Z
→ Isotopes and isobars
Hydronium is not an isotope
→ Isotopes and isobars
Count the ion's electrons, not the atom's
→ Isoelectronic species

Electron Configuration and the Periodic Table

Formulas (1)

Electron configuration and valency · Valency from valence electrons $\text{valency} = \begin{cases} v & v \le 4 \\ 8 - v & v \ge 4 \end{cases}$

Reference tables (3)

How the modern periodic table is built4 rows

Fact	Answer
Who showed atomic number is more fundamental than atomic mass	Henry MoseleyQ
Sm, Cs, Os, Np	Lanthanoid, alkali metal, transition metal, actinoidQ CDS 2026 (I) match-the-list.
Naturally occurring elements	94Q
Symbols of gold, tin, lead	Au, Sn, PbQ

Trends in atomic size and metallic character3 rows

Property	Down a group	Across a period
Atomic radius	Increases	DecreasesQ CDS 2024 (I): Na > Li > Be > O.
Metallic character	Increases	DecreasesQ CDS 2026 (I): Be < Ca < Sr < Ba is the right order of metallic character.
Ionisation energy	Decreases	Increases

Noble gases, alkali metals and other element facts5 rows

Element	Fact
Helium	Monatomic gas (full shell)Q CDS 2019 (II) asked which is monatomic; CDS 2026 (I) had 'helium : diatomic gas' as the wrong pair.
Caesium	Timekeeper in atomic clocksQ
Nitrogen	Three unpaired electrons (2p³)
Carbon	Tetravalent
Phosphorus, sulphur	Polyatomic: P₄, S₈

Watch out for (5)

Chemistry comes from electrons, not the nucleus
→ Electron configuration and valency
Sn is tin, Sb is antimony
→ How the modern periodic table is built
Lanthanoids fill 4f, actinoids 5f
→ How the modern periodic table is built
Radius falls across a period
→ Trends in atomic size and metallic character
Noble gases are single atoms
→ Noble gases, alkali metals and other element facts

PYQ weightage by concept

9 concepts · 26 PYQs — where the marks actually sit, so you know what to drill first

Atomic Models: Dalton to Bohr6 PYQs · 23%

Concept	PYQs	Share
Rutherford's gold-foil experiment and its limit	4	15%
Dalton's atomic theory and its limits	2	8%

Atomic Number, Isotopes and Isobars8 PYQs · 31%

Concept	PYQs	Share
Isotopes and isobars	5	19%
Counting protons, neutrons and electrons	2	8%
Isoelectronic species	1	4%

Electron Configuration and the Periodic Table12 PYQs · 46%

Concept	PYQs	Share
How the modern periodic table is built	4	15%
Electron configuration and valency	3	12%
Noble gases, alkali metals and other element facts	3	12%
Trends in atomic size and metallic character	2	8%

Test yourself on Atomic Structure and Periodic Classification

10 past CDS questions from this chapter, timed at 10 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

Take the Atomic Structure and Periodic Classification chapter test

Send them this chapter's questions. It's free for them too.