CDS Chemistry · Atomic Structure and Periodic Classification

Electron Configuration and the Periodic Table

How electrons fill shells and set valency, how the modern periodic table is built on atomic number, how size and metallic character change across it, and the element facts CDS repeats.

All Atomic Structure and Periodic Classification notes

Why this matters

Twelve CDS questions, the largest page in the chapter, and five of them from 2025 and 2026. The newer questions mix four statements about the table in one item, so the trends are worth knowing as rules rather than as single facts.

Concept 1 of 4: Electron configuration and valency

Electrons fill shells from the inside out: 2 in the first, 8 in the second, 8 in the third for the first twenty elements. The outermost (valence) electrons are the ones an atom gains, loses or shares, so they decide its chemistry and its valency.

Definition

The rules for the first twenty elements:

Shells fill in the order K = 2, L = 8, M = 8, then the fourth shell.
The valence electrons (outermost shell) decide chemical properties. The nucleus decides mass and identity.
Valency = number of valence electrons if it is 1–4; = 8 − valence electrons if it is 4–8.
Transition (d-block) elements fill the d subshell of the shell below the outermost one: general configuration (n−1)d¹⁻¹⁰ ns⁰⁻².

Valency from valence electrons

\text{valency} = \begin{cases} v & v \le 4 \\ 8 - v & v \ge 4 \end{cases}

$v$ number of electrons in the outermost shell

Worked example

An element has atomic number 16. Write its electron configuration, name it and give its valency.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

CDS · 2025 · CDS (I) 2025 — General Knowledge · Q53Easy

Example 1 · Atomic Structure and Periodic Classification · Electron Configuration and the Periodic Table

If atomic number of an element is 11, the correct name, symbol and valency are :

Chemistry comes from electrons, not the nucleus

Protons fix which element it is and neutrons add mass, but chemical properties come from the valence electrons.

Drill 2 more on electron configuration and valency

Concept 2 of 4: How the modern periodic table is built

Mendeleev arranged elements by atomic mass. Moseley showed that atomic number is the more fundamental property, and the modern table is arranged by it. Every column is a group of elements with the same outer configuration.

Definition

The structure:

Moseley (X-ray spectra) showed that atomic number is more fundamental than atomic mass. The modern periodic law is based on atomic number.
18 groups, 7 periods; blocks s, p, d, f by the subshell being filled.
Alkali metals: group 1 (Li, Na, K, Rb, Cs). Transition metals: d-block (Fe, Cu, Os, Pt).
Lanthanoids (4f): Ce to Lu, including Sm. Actinoids (5f): Th to Lr, including U, Np, Pu.
94 elements occur naturally (some, such as neptunium and plutonium, only in traces); the rest are made artificially.
Several symbols come from Latin names: Au (aurum, gold), Ag (argentum, silver), Sn (stannum, tin), Pb (plumbum, lead), Na (natrium), K (kalium), Fe (ferrum), Cu (cuprum), Hg (hydrargyrum).

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

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

CDS · 2026 · CDS (I) 2026 — General Knowledge · Q21Moderate

Example 2 · Atomic Structure and Periodic Classification · Electron Configuration and the Periodic Table

Match List-I with List-II and select the answer using the codes given below the Lists :

List-I (Symbol of Element)	List-II (Group of Element)
A. Sm	1. Transition Elements
B. Cs	2. Alkali Metals
C. Os	3. Actinoid Elements
D. Np	4. Lanthanoid Elements

Codes :

Sn is tin, Sb is antimony

Tin is Sn (stannum). Sb is antimony, and Ga is gallium, not gold. Gold is Au.

Lanthanoids fill 4f, actinoids 5f

Samarium (Sm) is a lanthanoid; neptunium (Np) is an actinoid. Osmium (Os) is a d-block transition metal and caesium (Cs) an alkali metal. Match-the-list options swap the two f-series.

Drill 3 more on how the modern periodic table is built

Concept 3 of 4: Trends in atomic size and metallic character

Going down a group adds a new shell, so atoms get bigger and lose electrons more easily: more metallic. Going across a period adds protons to the same shell, which pulls the electrons in: atoms get smaller and less metallic.

Definition

The two directions:

Down a group: atomic radius increases; metallic character increases (Be < Mg < Ca < Sr < Ba).
Across a period (left to right): atomic radius decreases; metallic character decreases.
So in period 2, Li > Be > … > O in size, and Na (period 3) is bigger than Li.
Isotopes of an element have the same atomic number, so they sit in the same place in the table.
Germanium and tellurium are metalloids in the p-block, not transition metals.

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

Practice this conceptself-check · 3 quick reps

The same idea in a real exam question:

CDS · 2024 · CDS (I) 2024 — General Knowledge · Q77Moderate

Example 3 · Atomic Structure and Periodic Classification · Electron Configuration and the Periodic Table

The correct order of atomic radius of Li, Na, Be and O is

Radius falls across a period

'Atomic radius increases from left to right' is false. More protons in the same shell pull the electrons closer, so size decreases across a period.

Drill 1 more on trends in atomic size and metallic character

Concept 4 of 4: Noble gases, alkali metals and other element facts

A few element facts come back in CDS with a twist. Noble gases already have full shells, so they exist as single atoms. Caesium keeps time in atomic clocks. Phosphorus and sulphur form big molecules, not pairs.

Definition

The facts:

Noble gases (He, Ne, Ar …) are monatomic: single atoms, because their shells are full. Helium is not diatomic.
Hydrogen, nitrogen, oxygen and chlorine are diatomic (H₂, N₂, O₂, Cl₂). Phosphorus is P₄ and sulphur S₈.
Caesium-133 is the timekeeper in atomic clocks; the SI second is defined by it.
Nitrogen (2p³) has three unpaired electrons; carbon is tetravalent; chlorine has two stable isotopes.

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

Practice this concept4 quick reps

The same idea in a real exam question:

CDS · 2026 · CDS (I) 2026 — General Knowledge · Q24Moderate

Example 4 · Atomic Structure and Periodic Classification · Electron Configuration and the Periodic Table

Which one among the following pairs is not correctly matched ?

Noble gases are single atoms

He, Ne and Ar exist as single atoms. A pair that says 'helium : diatomic gas' is the wrongly matched one.

Drill 2 more on noble gases, alkali metals and other element facts

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

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

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 Drill all 12 questions