MHT-CET Chemistry · Modern Periodic Table

Periodic Trends: Radius, Ionisation Enthalpy, Electron Gain and Electronegativity

Across a period the nuclear charge grows and the shell stays the same, so atoms shrink and hold their electrons harder; down a group a new shell is added, so atoms grow and hold them less — every trend in the chapter follows from those two facts.

All Modern Periodic Table notes MHT-CET Chemistry strategy

Why this matters

10 PYQs, two MODERATE. Six rank ionisation enthalpy, electron gain enthalpy or electronegativity; four rank atomic or ionic radius or name a diagonal pair. Two cards.

Concept 1 of 2: Atomic and Ionic Radius, and the Diagonal Relationship

Size grows down a group because each period adds a shell, and falls across a period because more protons pull the same shell in. An element and the one diagonally below-right of it end up with similar size and charge density, so they behave alike.

Definition

Down a group: radius increases — Li⁺ < Na⁺ < K⁺ < Rb⁺.
Across a period: radius decreases — Mg < Na in period 3.
Combining both: Mg < Na < K < Rb.
A period-4 element is bigger than its period-3 neighbour: [Ar]3d¹⁰4s²4p⁴ (Se) > [Ne]3s²3p⁴ (S); in the same period, p⁴ > p⁵.
Diagonal pairs: Li–Mg, Be–Al, B–Si.

Question	Answer
Largest of [Ne]3p⁴, [Ne]3p⁵, [Ar]4p⁴, [Ar]4p⁵	[Ar]3d¹⁰4s²4p⁴Q
Largest M⁺ of Rb, K, Na, Li	Rb⁺Q
Increasing radius of Na, K, Mg, Rb	Mg < Na < K < RbQ
Diagonal partner of Mg	LiQ

Practice this conceptself-check

The same idea in a real exam question:

MHT-CET · 2023 · 10th May Shift 2 · Q68Moderate

Example 1 · Modern Periodic Table · Periodic Trends

If four different elements A, B, C and D having electronic configurations as

A = [\text{Ne}]3s^23p^4

B = [\text{Ne}]3s^23p^5

C = [\text{Ar}]3d^{10}4s^24p^4

and

D = [\text{Ar}]3d^{10}4s^24p^5

. Identify the element with largest atomic radius.

Drill 3 more on atomic and ionic radius, and the diagonal relationship

Concept 2 of 2: Ionisation Enthalpy, Electron Gain and Electronegativity

All three measure how tightly an atom holds electrons, so all three rise across a period and fall down a group. The exceptions are about filled shells: a noble gas has the highest ionisation enthalpy in its period but will not accept an extra electron, so its electron gain enthalpy is positive.

Definition

Ionisation enthalpy: increases across, decreases down. He is the highest of all. Ne > Ar (down the group), Cl > S (across).
Electron gain enthalpy: noble gases positive (Ne, Ar …); halogens most negative.
Electronegativity: increases across, decreases down. In group 1 Li is highest; among O, S, F, Cl, S is lowest.

Question	Answer
Highest ΔᵢH₁ of He, Ar, Cl, I	HeQ
Decreasing ΔᵢH of Ne, Ar, Cl, S	Ne > Ar > Cl > SQ
Positive electron gain enthalpy	NeQ
Highest electronegativity in group 1	LiQ
Lowest electronegativity of O, S, F, Cl	SQ

Practice this conceptself-check · 1 quick reps

The same idea in a real exam question:

MHT-CET · 2024 · 13th May Shift 2 · Q100Moderate

Example 2 · Modern Periodic Table · Periodic Trends

Which from following is a correct decreasing order of ionization enthalpy for different elements?

Argon above neon

Both are noble gases, but ionisation enthalpy falls down the group. Neon's outer electrons are closer to the nucleus: Ne > Ar.

Drill 5 more on ionisation enthalpy, electron gain and electronegativity

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.

Reference tables (2)

Atomic and Ionic Radius, and the Diagonal Relationship4 rows

Question	Answer
Largest of [Ne]3p⁴, [Ne]3p⁵, [Ar]4p⁴, [Ar]4p⁵	[Ar]3d¹⁰4s²4p⁴Q
Largest M⁺ of Rb, K, Na, Li	Rb⁺Q
Increasing radius of Na, K, Mg, Rb	Mg < Na < K < RbQ
Diagonal partner of Mg	LiQ

Ionisation Enthalpy, Electron Gain and Electronegativity5 rows

Question	Answer
Highest ΔᵢH₁ of He, Ar, Cl, I	HeQ
Decreasing ΔᵢH of Ne, Ar, Cl, S	Ne > Ar > Cl > SQ
Positive electron gain enthalpy	NeQ
Highest electronegativity in group 1	LiQ
Lowest electronegativity of O, S, F, Cl	SQ

Watch out for (1)

Argon above neon
→ Ionisation Enthalpy, Electron Gain and Electronegativity

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