JEE Main Chemistry · Chapter 9

Classification of Elements and Periodicity

Modern periodic law, s/p/d/f blocks, trends in radius, ionisation enthalpy, electron gain enthalpy and electronegativity.

NCERT Class 11JEE Main: 1 Q per shift 8 min read

1. Chapter at a Glance

The periodic table arranges elements in order of increasing atomic number so that elements with similar properties fall in the same group. Because the outer electronic configuration repeats periodically, so do properties such as atomic radius, ionisation enthalpy, electron gain enthalpy and electronegativity. This chapter is the base for all of inorganic chemistry. JEE Main commonly asks ordering questions (arrange by size, IE or EA) and exceptions to the trends.

What JEE asks most
  • Correct order of atomic/ionic radii, especially isoelectronic species.
  • Ionisation enthalpy trends with exceptions (Be > B, N > O, and similar).
  • Electron gain enthalpy: Cl more negative than F; noble gases positive.
  • Block, period and group from electronic configuration; acidic/basic oxides.

2. Development and the Modern Periodic Law

  • Döbereiner's triads: groups of three with the middle atomic mass near the average (Li, Na, K).
  • Newlands' law of octaves: every eighth element resembles the first (worked only up to Ca).
  • Mendeleev: arranged by atomic mass, left gaps and predicted properties of eka-aluminium (Ga) and eka-silicon (Ge).
  • Modern periodic law (Moseley): properties of elements are periodic functions of their atomic numbers. √ν ∝ Z for characteristic X-rays.
  • The long form has 7 periods and 18 groups. IUPAC names for new elements use roots: 0 nil, 1 un, 2 bi, 3 tri, 4 quad, 5 pent, 6 hex, 7 sept, 8 oct, 9 enn (e.g. Z = 120: unbinilium, Ubn).

3. s, p, d and f Blocks

BlockGroupsGeneral configurationNature
s1, 2ns1–2Reactive metals, fixed oxidation state, ionic compounds
p13–18ns² np1–6Metals, metalloids, non-metals, noble gases
d3–12(n−1)d1–10 ns0–2Transition metals, variable valency, coloured ions
fLanthanoids, actinoids(n−2)f1–14(n−1)d0–1ns²Inner-transition metals

Finding the position: period = highest principal quantum number n. For s-block, group = number of valence electrons; for p-block, group = 10 + valence electrons; for d-block, group = ns + (n−1)d electrons.

Atomic and ionic radius

  • Across a period: radius decreases (nuclear charge increases, same shell). Down a group: radius increases (new shells).
  • Cation is smaller than its parent atom; anion is larger.
  • Isoelectronic species: more protons → smaller size. O²⁻ > F⁻ > Na⁺ > Mg²⁺ > Al³⁺ (all 10 electrons).
  • Noble gases have van der Waals radii, so they appear larger than neighbours.
  • Due to the lanthanoid contraction, Zr and Hf have almost the same radius.

Ionisation enthalpy (IE)

Energy needed to remove the outermost electron from an isolated gaseous atom. It increases across a period and decreases down a group. IE₁ < IE₂ < IE₃ always.

Exceptions to remember
  • Be > B and Mg > Al: the 2p/3p electron is shielded by s electrons and easier to remove.
  • N > O and P > S: N has a stable half-filled 2p³; O loses a paired electron (repulsion helps).
  • Order of IE₁ in period 2: Li < B < Be < C < O < N < F < Ne.
  • Ga has nearly the same IE as Al because of poor shielding by 3d electrons.
LiBeBCNOFNe dipdipIE₁
Period 2 first ionisation enthalpies (drawn to scale, 520 to 2081 kJ mol−1): overall increase with dips at B and O.

Electron gain enthalpy (ΔegH)

  • Energy change when an electron is added to a gaseous atom. Generally becomes more negative across a period.
  • Cl has the most negative ΔegH, more than F, because F is small and its compact 2p shell repels the incoming electron. Similarly S > O in magnitude.
  • Noble gases, Be, Mg and N have positive or near-zero values (stable configurations).
  • Second electron gain enthalpy (O⁻ → O²⁻) is always positive.

Electronegativity, valence and metallic character

  • Electronegativity (Pauling): F (4.0) > O (3.5) > N ≈ Cl (3.0). Increases across, decreases down.
  • Metallic character decreases across a period and increases down a group.
  • Oxides: left side basic (Na₂O), middle amphoteric (Al₂O₃, ZnO), right side acidic (Cl₂O₇). CO, NO, N₂O are neutral.
  • Diagonal relationship: Li–Mg, Be–Al, B–Si show similar properties because of similar charge/size ratio.
  • Anomalous behaviour of second-period elements: small size, high electronegativity, no d orbitals (maximum covalency 4).

5. Master Table of Trends

PropertyAcross period (→)Down group (↓)
Atomic radiusDecreasesIncreases
Ionisation enthalpyIncreases (exceptions)Decreases
Electron gain enthalpy (magnitude)IncreasesDecreases (except F < Cl)
ElectronegativityIncreasesDecreases
Metallic characterDecreasesIncreases
Basic nature of oxidesDecreasesIncreases
Effective nuclear chargeIncreasesSlight increase
Zeff = Z − σ    (σ = shielding constant)

6. Mind Map

Periodic Classification
History
  • Döbereiner, Newlands
  • Mendeleev
  • Moseley: atomic number
Blocks
  • s, p, d, f
  • Position from configuration
  • IUPAC naming
Trends
  • Radius, IE, EA
  • Electronegativity
  • Oxide nature
Special cases
  • Be>B, N>O
  • Cl more negative than F
  • Diagonal relationship

7. Tips and Tricks

Shortcuts
  • For isoelectronic species, compare the Z/e ratio: larger ratio → smaller ion.
  • A huge jump between successive IEs tells the number of valence electrons (e.g. a big jump after IE₂ means group 2).
  • Half-filled and fully filled subshells are extra stable: they have high IE and low (less negative) electron gain enthalpy.
  • Down group 13, IE does not fall smoothly: B > Tl > Ga > Al > In (poor shielding by d and f electrons).
Mistakes students make
  • Saying F has the most negative electron gain enthalpy — it is Cl.
  • Confusing electronegativity (tendency in a bond) with electron gain enthalpy (isolated atom energy).
  • Forgetting that noble gases have the highest IE in their period.

8. Important One-liners

  • Moseley showed that atomic number, not atomic mass, is the basis of periodicity.
  • Helium has the highest first ionisation enthalpy of all elements.
  • Caesium has the lowest ionisation enthalpy among stable elements.
  • Fluorine is the most electronegative element.
  • Al₂O₃ and ZnO are amphoteric; they react with both acids and bases.
  • Lanthanoid contraction makes 4d and 5d elements of a group nearly equal in size.

9. PYQ-pattern Questions with Solutions

These questions are original and modelled on concepts frequently tested in JEE Main.

Q1Easy

The correct order of ionic radii is:

  1. Al³⁺ > Mg²⁺ > Na⁺ > F⁻
  2. F⁻ > Na⁺ > Mg²⁺ > Al³⁺
  3. Na⁺ > F⁻ > Mg²⁺ > Al³⁺
  4. Mg²⁺ > Na⁺ > F⁻ > Al³⁺
Show solution

All four have 10 electrons. Nuclear charge: F (9) < Na (11) < Mg (12) < Al (13). More protons pull the same electrons closer, so size decreases: F⁻ > Na⁺ > Mg²⁺ > Al³⁺.

Answer: (B)

Q2Medium

The correct order of first ionisation enthalpy is:

  1. B < Be < N < O
  2. Be < B < O < N
  3. B < Be < O < N
  4. Be < B < N < O
Show solution

Be (2s²) is more stable than B (2p¹), so Be > B. N (half-filled 2p³) is more stable than O, so N > O. Overall order B < Be < O < N.

Answer: (C)

Q3Easy

The element with the most negative electron gain enthalpy is:

  1. F
  2. Cl
  3. Br
  4. O
Show solution

Chlorine. In fluorine, the very small 2p shell causes strong repulsion to the incoming electron, so its value is less negative than chlorine's.

Answer: (B)

Q4Medium

An element has successive ionisation enthalpies (kJ mol−1) 738, 1451, 7733, 10540. It most likely belongs to group:

  1. 1
  2. 2
  3. 13
  4. 14
Show solution

The big jump is between IE₂ (1451) and IE₃ (7733). So two electrons are easy to remove: two valence electrons → group 2 (this is magnesium).

Answer: (B)

Q5Medium

Which of the following oxides is amphoteric?

  1. Na₂O
  2. SO₃
  3. Al₂O₃
  4. CO₂
Show solution

Al₂O₃ reacts with acids (forming Al³⁺ salts) and with bases (forming aluminates). Na₂O is basic; SO₃ and CO₂ are acidic.

Answer: (C)

Q6Hard

An element has the configuration [Ar] 3d10 4s² 4p³. Its period and group are:

  1. 4, 15
  2. 4, 5
  3. 3, 15
  4. 4, 13
Show solution

Highest n = 4, so period 4. It is a p-block element with 2 + 3 = 5 valence electrons: group = 10 + 5 = 15 (arsenic).

Answer: (A)

Q7Hard

The IUPAC symbol for the element with atomic number 119 is:

  1. Uue
  2. Uun
  3. Une
  4. Ubn
Show solution

1 = un, 1 = un, 9 = enn → "ununennium", symbol Uue.

Answer: (A)

Q8Hard

Which pair of elements has almost identical atomic radii?

  1. Ti and Zr
  2. Zr and Hf
  3. Na and K
  4. Cu and Ag
Show solution

Between Zr (4d) and Hf (5d) come the 14 lanthanoids. Their poorly shielding 4f electrons cause the lanthanoid contraction, which cancels the expected increase in size. So Zr (160 pm) and Hf (159 pm) are almost equal.

Answer: (B)

10. Quick Revision Summary

Remember before the exam
  • Modern periodic law is based on atomic number.
  • Radius: decreases across, increases down; isoelectronic — more protons, smaller ion.
  • IE: increases across; exceptions Be > B and N > O.
  • Most negative electron gain enthalpy: Cl; most electronegative: F.
  • Oxides go from basic to amphoteric to acidic across a period.
  • Diagonal pairs: Li–Mg, Be–Al, B–Si; lanthanoid contraction: Zr ≈ Hf.
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