NDA Chemistry · Atomic Structure and Periodic Classification

Periodic Trends, Valency and Atomicity

Elements in the same group share a valency and react in patterned ways; the number of atoms in a free element's molecule is its atomicity, and properties like reactivity and oxidising power trend smoothly across the table.

All Atomic Structure and Periodic Classification notes NDA Chemistry strategy

Why this matters

The largest subtopic — 12 PYQs. It clusters into four ideas: valency (group → valency, ordering valencies), atomicity (mono/di/poly-atomic elements), periodic trends (halogen oxidising power, metal reactivity with water), and noble gases (inertness + uses). Each is recall or one-line reasoning; learn the four group facts and the answers fall out.

Concept 1 of 4: Valency, groups and the most fundamental property

Valency is the combining capacity of an element — the number of electrons in its outermost shell decides it. Elements in the same group have the same valency, so they form analogous compounds. And the single most fundamental property of an element is its atomic number, because that is what defines the element.

Definition

The valency facts the bank tests:

Valency depends on the number of electrons in the outermost (valence) shell.
Same group → same valency (Mg, Ca, Ba are all Group 2 → valency 2).
Standard valencies: noble gases (Ne) = 0; Group 2 (Mg) = 2; Group 15 (N) = 3; Group 14 (Si) = 4. Ordering them: Ne < Mg < N < Si.
A compound's formula reveals valency: XCl₂ means X has valency 2, so X is a Group-2 metal (same group as Mg).
The atomic number is the most fundamental characteristic of an element.

Standard valencies by group

\text{Ne}=0 \quad \text{Mg}=2 \quad \text{N}=3 \quad \text{Si}=4

Worked example

Arrange Ne, Si, N and Mg in increasing order of valency.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

NDA · Apr 2022 · Q63Hard

Example 1 · Atomic Structure and Periodic Classification · Periodic Trends, Valency and Atomicity

Which one of the following is the correct order of the valencies of elements Ne, Si, N and Mg?

Same group = same valency

Mg, Ca and Ba are all Group 2, so they share valency 2 and form MgCl₂, CaCl₂, BaCl₂. A set like (Na, Mg, Al) spans three groups and three valencies — not the answer.

Drill 4 more on valency, groups and the most fundamental property

Concept 2 of 4: Atomicity — atoms per molecule of a free element

Atomicity is how many atoms make up one molecule of an element in its free state. Noble gases travel alone (monatomic), most gases pair up (diatomic), and a few elements clump (phosphorus P₄, sulphur S₈). The bank asks you to match elements to their atomicity or to spot the polyatomic one.

Definition

The atomicities the bank tests:

Monatomic (1) — noble gases: He, Ne, Ar, Kr, Xe, Rn.
Diatomic (2) — H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂.
Tetra-atomic (4) — phosphorus P₄.
Octa-atomic (8) — sulphur S₈.

Polyatomic means more than two atoms per molecule — phosphorus (P₄) and sulphur (S₈) are the classic polyatomic elements.

Element	Molecule	Atomicity
Neon	Ne	1 (monatomic)
Nitrogen	N₂	2 (diatomic)
Chlorine	Cl₂	2 (diatomic)
Iodine	I₂	2 (diatomic)
Phosphorus	P₄	4 (tetra-atomic, polyatomic) NDA 2024 — phosphorus is the polyatomic element (P₄), unlike diatomic Cl₂ or metallic Al.
Sulphur	S₈	8 (octa-atomic, polyatomic)

Noble gases = 1; common gases = 2; phosphorus = 4; sulphur = 8.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

NDA · Sep 2025 · Q103Moderate

Example 2 · Atomic Structure and Periodic Classification · Periodic Trends, Valency and Atomicity

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

List I (Element)	List II (Atomicity)
A. Phosphorus	1. 2
B. Nitrogen	2. 1
C. Neon	3. 8
D. Sulphur	4. 4

Code: A B C D

Phosphorus is P₄, sulphur is S₈

The two polyatomic elements the bank loves are phosphorus (P₄, atomicity 4) and sulphur (S₈, atomicity 8). Don't call them diatomic.

Iodine is diatomic, not monatomic

Only the noble gases (He, Ne, Ar…) are monatomic. Iodine is I₂ (diatomic), so in a 'which is NOT monatomic' question, iodine is the answer.

Drill 2 more on atomicity — atoms per molecule of a free element

Concept 3 of 4: Reactivity and oxidising-power trends

Properties change smoothly along a group. For the halogens, oxidising power falls down the group (fluorine is the strongest). For the alkali metals, reactivity with water rises down the group (lithium is the least reactive). The bank asks you to order them or pick the extreme.

Definition

The two trends the bank tests:

Halogen oxidising power decreases down the group: F > Cl > Br > I. So increasing order = I < Br < Cl < F.
Alkali-metal reactivity with water increases down the group (Li < Na < K < Rb < Cs). So lithium is the least reactive of the alkali metals with water.

Reason: reactivity of metals rises and oxidising power of non-metals falls as atoms get larger down a group.

Trend	Direction down the group	Extreme the bank asks for
Halogen oxidising power	Decreases (F strongest, I weakest)	Increasing order: I < Br < Cl < F NDA 2023 — increasing oxidising order of halogens is I, Br, Cl, F.
Alkali-metal reactivity with water	Increases (Li least, Cs most)	Lithium is least reactive with water NDA 2017 — among alkali metals, lithium is the least reactive with water.

Practice this conceptself-check · 3 quick reps

The same idea in a real exam question:

NDA · Apr 2023 · Q112Moderate

Example 3 · Atomic Structure and Periodic Classification · Periodic Trends, Valency and Atomicity

Which among the following is the correct arrangement of halogens in the increasing order of their oxidizing nature?

Oxidising power falls down the group, reactivity rises

For halogens (non-metals), oxidising power decreases down the group (F is best). For alkali metals, reactivity with water increases down the group (Li is least, Cs most). Don't mix the two directions.

Drill 1 more on reactivity and oxidising-power trends

Concept 4 of 4: Noble gases — inertness and uses

Noble gases (Group 18) have full outer shells, so they are chemically inert and travel as single atoms. Each has a signature use the bank tests — argon shields tungsten filaments, neon glows in advertising signs, xenon flashes in cameras. Radon, the heaviest, is also an inert gas (and radioactive).

Definition

Noble-gas facts and uses:

The noble gases (He, Ne, Ar, Kr, Xe, Rn) are inert (chemically unreactive) and monatomic.
Radon is an inert (noble) gas — and radioactive.
Argon — filled into bulbs so the tungsten filament lasts longer.
Neon — advertising / neon signs (red glow).
Krypton — airport landing lights and lighthouses.
Xenon — photographer's flash guns.

Noble gas	Signature use
Argon	Fills bulbs so the tungsten filament lasts longer
Neon	Advertising / neon signs
Krypton	Airport landing lights and lighthouses
Xenon	Photographer's flash gun
Radon	An inert (noble) gas; also radioactive NDA 2017 — radon is an inert gas (Group 18).

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

NDA · Apr 2017 · Q94Moderate

Example 4 · Atomic Structure and Periodic Classification · Periodic Trends, Valency and Atomicity

Match List I with List II and select the correct answer using the code given below the Lists :

List I (Noble gas)	List II (Use)
A. Argon	1. In lights for advertising display
B. Neon	2. Airport landing lights and in light houses
C. Krypton	3. Light in photographer's flash gun
D. Xenon	4. In tungsten filament to last longer

Code : A B C D

Argon protects the filament; neon makes the glow

Argon fills the bulb (inert, prevents the tungsten filament burning out); neon is the one used in glowing advertising signs. The bank swaps these in match-the-list questions.

Drill 1 more on noble gases — inertness and uses

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)

Valency, groups and the most fundamental property
Standard valencies by group
$\text{Ne}=0 \quad \text{Mg}=2 \quad \text{N}=3 \quad \text{Si}=4$

Reference tables (3)

Atomicity — atoms per molecule of a free element6 rows

Element	Molecule	Atomicity
Neon	Ne	1 (monatomic)
Nitrogen	N₂	2 (diatomic)
Chlorine	Cl₂	2 (diatomic)
Iodine	I₂	2 (diatomic)
Phosphorus	P₄	4 (tetra-atomic, polyatomic) NDA 2024 — phosphorus is the polyatomic element (P₄), unlike diatomic Cl₂ or metallic Al.
Sulphur	S₈	8 (octa-atomic, polyatomic)

Noble gases = 1; common gases = 2; phosphorus = 4; sulphur = 8.

Reactivity and oxidising-power trends2 rows

Trend	Direction down the group	Extreme the bank asks for
Halogen oxidising power	Decreases (F strongest, I weakest)	Increasing order: I < Br < Cl < F NDA 2023 — increasing oxidising order of halogens is I, Br, Cl, F.
Alkali-metal reactivity with water	Increases (Li least, Cs most)	Lithium is least reactive with water NDA 2017 — among alkali metals, lithium is the least reactive with water.

Noble gases — inertness and uses5 rows

Noble gas	Signature use
Argon	Fills bulbs so the tungsten filament lasts longer
Neon	Advertising / neon signs
Krypton	Airport landing lights and lighthouses
Xenon	Photographer's flash gun
Radon	An inert (noble) gas; also radioactive NDA 2017 — radon is an inert gas (Group 18).

Watch out for (5)

Same group = same valency
→ Valency, groups and the most fundamental property
Phosphorus is P₄, sulphur is S₈
→ Atomicity — atoms per molecule of a free element
Iodine is diatomic, not monatomic
→ Atomicity — atoms per molecule of a free element
Oxidising power falls down the group, reactivity rises
→ Reactivity and oxidising-power trends
Argon protects the filament; neon makes the glow
→ Noble gases — inertness and uses

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