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Classification of Elements: one-page cheat sheet

NEET Chemistry · Unit 9 · Inorganic Chemistry

ScoreChemby CSJ Sir
Weightage: 2–3 questions · 8–12 marks

1 · Classification & nomenclature

Scientist	Basis
Döbereiner	triads (middle mass = mean)
Newlands	law of octaves
Lothar Meyer	atomic volume curve
Mendeleev	atomic mass
Moseley	atomic number (modern law)

IUPAC roots0 nil, 1 un, 2 bi, 3 tri, 4 quad, 5 pent, 6 hex, 7 sept, 8 oct, 9 enn

Z = 119

un + un + enn + ium = ununennium (Uue)

2 · Blocks

Block	Valence configuration
s	$n s^{1 - 2}$
p	$n s^{2} n p^{1 - 6}$
d	$(n - 1) d^{1 - 10} n s^{0 - 2}$
f	$(n - 2) f^{1 - 14} (n - 1) d^{0 - 1} n s^{2}$

Period no.

=

outermost shell

n

Group (p)

=

valence electrons

+ 10

3 · Size

Periodradius decreases (rising

Z_{eff}

)

Groupradius increases (new shells)

Cation / anioncation

<

atom

<

anion

N^{3 -} > O^{2 -} > F^{-} > Na^{+} > Mg^{2 +} > Al^{3 +}

Isoelectronic ions: more nuclear charge, smaller ion.

Ga vs AlGa (135 pm)

<

Al (143 pm): poor

d

shielding

Zr ≈ Hflanthanoid contraction (also Nb ≈ Ta)

4 · Ionisation enthalpy

Trendrises across a period, falls down a group

AnomaliesBe

>

B, Mg

>

Al (

s

p

); N

>

O, P

>

S (half-filled

p^{3}

)

Li < B < Be < C < O < N < F < Ne

Successive

Δ_{i} H_{1} < Δ_{i} H_{2} < Δ_{i} H_{3}

; big jump after the valence shell is empty

5 · Electron gain enthalpy

Element	$Δ_{e g} H$ (kJ mol $^{- 1}$ )
Cl	$- 349$ (most negative)
F	$- 328$
Br	$- 325$
I	$- 295$
S	$- 200$
O	$- 141$

Cl $>$ F and S $>$ O: the compact 2p shell repels the incoming electron. Noble gases are positive.

6 · Electronegativity

PaulingF 4.0, O 3.5, N 3.0, Cl 3.0, C 2.5, Si 1.8

Mulliken

χ = \frac{1}{2} (IE + EA)

Trendup across a period, down a group

Linkmore electronegative, less metallic

P < Si < Mg < Na (metallic character)

7 · Nature of oxides

Nature	Examples
Basic	Na $_{2}$ O, MgO, BaO
Amphoteric	Al $_{2}$ O $_{3}$ , ZnO, SnO $_{2}$
Acidic	SiO $_{2}$ , P $_{4}$ O $_{10}$ , SO $_{3}$ , Cl $_{2}$ O $_{7}$
Neutral	CO, NO, N $_{2}$ O

Across a period: basic to acidic. Higher oxidation state of the same element: more acidic.

8 · Second-period anomaly

Causesmall size, high electronegativity, no $d$ -orbitals

Resultmaximum covalency 4 (BF

_{4}^{-}

, not BF

_{6}^{3 -}

)

DiagonalLi–Mg, Be–Al, B–Si

Seen in NEET 2024

Order the electronegativity of N, O, F, C, Si.

Si (1.8) < C (2.5) < N (3.0) < O (3.5) < F (4.0)

Seen in NEET 2023

Largest ion with a neon configuration: N $^{3 -}$ , O $^{2 -}$ , F $^{-}$ , Na $^{+}$ ?

All have 10 electrons. N $^{3 -}$ has the least nuclear charge (+7), so it is the largest.

9 · Quick orders

Period 3, first ionisation enthalpy:

Na < Al < Mg < Si < S < P < Cl < Ar

Period 2, atomic radius:

Li > Be > B > C > N > O > F

Electron gainCl

>

>

>

I; S

>

Seen in NEET 2025

Which are main-group configurations?

[Ne]3 $s^{1}$ (Na) and [Kr]4 $d^{10}$ 5 $s^{2}$ 5 $p^{5}$ (I) are $s$ and $p$ block. [Ar]3 $d^{3}$ 4 $s^{2}$ (V) is a $d$ -block element.

Seen in NEET 2020

Oxide	Nature
CO	neutral
BaO	basic
Al $_{2}$ O $_{3}$	amphoteric
Cl $_{2}$ O $_{7}$	acidic

Seen in NEET 2021

Why do Zr (40) and Hf (72) have almost identical radii?

14 poorly shielding $4 f$ electrons raise $Z_{eff}$ : lanthanoid contraction.

Six traps to remember

Be $>$ B and N $>$ O in first ionisation enthalpy, against the general rise.
Cl, not F, has the most negative electron gain enthalpy.
In an isoelectronic series, Al $^{3 +}$ is the smallest, N $^{3 -}$ the largest.
Zr and Hf have almost equal radii because of lanthanoid contraction.
Ga is smaller than Al, because the 3 $d$ electrons shield poorly.
Al $_{2}$ O $_{3}$ is amphoteric, CO is neutral, and Cl $_{2}$ O $_{7}$ is acidic.

Classification of Elements and Periodicity: free one-page cheat sheet