NEET UG Chemistry · Chapter 3

Classification of Elements and Periodicity

Modern periodic law, blocks, and trends in radius, ionisation enthalpy, electron gain enthalpy and electronegativity.

NCERT Class 11NEET: 1–2 Qs per paper 9 min read

1. Chapter at a Glance

The periodic table arranges more than 118 elements so that elements with similar properties fall into the same column. This chapter explains how the table developed, the modern periodic law, the s-, p-, d- and f-blocks, and the periodic trends in atomic radius, ionisation enthalpy, electron gain enthalpy, electronegativity and valence. NEET asks one or two questions every year, usually asking you to arrange elements in order of a property or to spot an exception.

What NEET asks most
  • Order of atomic and ionic radii, including isoelectronic species.
  • Ionisation enthalpy trends and exceptions (Be > B, N > O).
  • Electron gain enthalpy (Cl more negative than F).
  • Electronegativity, metallic character and oxide nature.
  • Locating an element (period, group, block) from its atomic number.

2. Development of the Periodic Table

  • Dobereiner's triads: groups of three elements in which the middle element's atomic mass is roughly the average of the other two (Li, Na, K).
  • Newlands' law of octaves: every eighth element had similar properties, but only up to calcium.
  • Mendeleev's periodic law: properties of elements are periodic functions of their atomic masses. He left gaps and predicted undiscovered elements (eka-aluminium = gallium, eka-silicon = germanium), and sometimes placed elements by properties rather than mass.
  • Modern periodic law (after Moseley): properties are periodic functions of atomic number.
  • The modern (long) form has 18 groups and 7 periods. The period number equals the number of the outermost shell.
  • Elements with Z > 100 get temporary IUPAC names from numerical roots: 0 nil, 1 un, 2 bi, 3 tri, 4 quad, 5 pent, 6 hex, 7 sept, 8 oct, 9 enn (e.g. Z = 119 is ununennium, Uue).

3. Blocks of the Periodic Table

BlockGroupsGeneral outer configurationNEET Note
s-block1, 2ns¹⁻²Reactive metals; low ionisation enthalpy
p-block13–18ns²np¹⁻⁶Metals, non-metals and metalloids; noble gases at group 18
d-block3–12(n−1)d¹⁻¹⁰ns⁰⁻²Transition metals; coloured ions, variable valency
f-blockSeparate rows(n−2)f¹⁻¹⁴(n−1)d⁰⁻¹ns²Lanthanoids and actinoids (inner transition)

Locating an element: write the configuration. The period is the highest n. The block is the subshell that receives the last electron. For p-block, group = 10 + (number of valence electrons); for d-block, group = (n−1)d electrons + ns electrons.

4. Atomic and Ionic Radius

  • Across a period (left to right): radius decreases, because the nuclear charge increases while electrons enter the same shell.
  • Down a group: radius increases, because a new shell is added each time.
  • A cation is smaller than its parent atom; an anion is larger.
  • Isoelectronic species: the one with more protons is smaller. Order: N³⁻ > O²⁻ > F⁻ > Na⁺ > Mg²⁺ > Al³⁺.
  • Noble gases are given van der Waals radii, which are larger than covalent radii, so they appear large at the end of a period.
IE, EN, non-metallic character increase radius, metallic character increase ↓ towards F: radius falls, IE and EN rise Cs F
General trends. Moving right across a period and up a group, radius falls while ionisation enthalpy and electronegativity rise. So caesium is the largest, most metallic atom here and fluorine the most electronegative.

5. Ionisation Enthalpy

  • Ionisation enthalpy (IE) is the energy needed to remove the outermost electron from a gaseous atom. IE₂ > IE₁ always.
  • It generally increases across a period and decreases down a group.
  • Factors: nuclear charge (↑ IE), atomic size (↓ IE), shielding by inner electrons (↓ IE), and stability of half-filled or filled subshells (↑ IE).
  • Exceptions in period 2: Be > B (removing a 2p electron is easier than a 2s electron) and N > O (N has a stable half-filled 2p³; O loses a paired electron). The order is Li < B < Be < C < O < N < F < Ne.
  • A sudden large jump between successive IEs shows that a noble-gas core has been reached, which tells you the number of valence electrons.

6. Electron Gain Enthalpy and Electronegativity

  • Electron gain enthalpy (ΔegH) is the enthalpy change when a gaseous atom gains an electron. It is usually negative (energy released).
  • It becomes more negative across a period and less negative down a group.
  • Exceptions: Cl has the most negative ΔegH, more negative than F, because F is very small and its incoming electron is repelled by the crowded 2p electrons. Similarly S is more negative than O.
  • Noble gases, Be, Mg and N have positive or near-zero values (stable configurations).
  • Electronegativity (EN) is the tendency of an atom in a molecule to attract shared electrons. It is not measurable directly; the Pauling scale is common. F (4.0) is the most electronegative element, then O (3.5); Cs and Fr are the least.

7. Periodicity of Chemical Properties

  • Valence: for s- and p-block elements, equal to the number of valence electrons or 8 minus it.
  • Metallic character decreases across a period and increases down a group.
  • Nature of oxides: across period 3, oxides change from strongly basic (Na₂O) to amphoteric (Al₂O₃) to strongly acidic (Cl₂O₇).
  • Anomalous behaviour of period-2 elements is due to small size, high electronegativity and the absence of d orbitals (maximum covalency 4).
  • Diagonal relationship: Li resembles Mg, Be resembles Al, and B resembles Si, because their charge-to-size ratios are similar.

8. Formula and Trend Master Table

PropertyAcross a period (→)Down a group (↓)
Atomic radiusDecreasesIncreases
Ionisation enthalpyIncreasesDecreases
Electron gain enthalpy (magnitude)IncreasesDecreases
ElectronegativityIncreasesDecreases
Metallic characterDecreasesIncreases
Basic nature of oxidesDecreasesIncreases
Group of a p-block element10 + number of valence electrons
Effective nuclear chargeZeff = Z − σ (shielding constant)

9. Master Summary Table

TopicKey factNEET Note
Modern periodic lawBased on atomic numberMoseley's work
Largest atom (stable)CsSmallest: He (or H)
Highest IE₁HeLowest: Cs
Most negative ΔegHClNot F
Highest ENF (4.0)Then O
IE exceptionsBe > B; N > OSubshell stability
Isoelectronic sizeMore protons ⇒ smallerAl³⁺ smallest in its series
Diagonal pairsLi–Mg, Be–Al, B–SiSimilar charge/size

10. Mind Map

Periodicity
History
  • Triads, octaves
  • Mendeleev
  • Moseley
Structure
  • 18 groups, 7 periods
  • s, p, d, f blocks
  • IUPAC names
Physical trends
  • Radius
  • IE, ΔegH
  • EN
Chemical trends
  • Valence
  • Oxide nature
  • Diagonal relationship

11. Tips and Tricks

Shortcuts
  • Radius and IE are "opposites": whatever makes an atom bigger makes its electron easier to remove.
  • For isoelectronic ions, just compare nuclear charges.
  • Period-2 IE order: remember "Li B Be C O N F Ne".
  • Remember the "second-period anomaly": F, O and N have less negative ΔegH than Cl, S and P.
Common NEET traps
  • Choosing F instead of Cl for the most negative electron gain enthalpy.
  • Forgetting the Be/B and N/O ionisation enthalpy exceptions.
  • Saying anions are smaller than their atoms.
  • Confusing electronegativity (in a bond) with electron gain enthalpy (isolated atom).

12. Important One-liners

  • Mendeleev arranged elements by atomic mass; the modern table uses atomic number.
  • Gallium was predicted by Mendeleev as eka-aluminium.
  • Fluorine is the most electronegative element.
  • Chlorine has the highest electron affinity (most negative ΔegH).
  • Helium has the highest first ionisation enthalpy.
  • The second ionisation enthalpy is always higher than the first.
  • Al₂O₃ is amphoteric; Na₂O is basic; SO₃ and Cl₂O₇ are acidic.
  • Lanthanoids and actinoids are placed in two separate rows at the bottom of the table.

13. NEET-pattern Questions with Detailed Answers

These questions are original and based on concepts that NEET UG asks again and again.

Q1Easy

The modern periodic law is based on:

  1. Atomic mass
  2. Atomic number
  3. Number of neutrons
  4. Atomic volume
Show solution

Moseley showed that properties depend on atomic number (the nuclear charge). Mendeleev's older law used atomic mass, which led to some wrong placements.

Answer: (B)

Q2Easy

Which element has the most negative electron gain enthalpy?

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

Cl has the most negative value. The small size of F causes electron–electron repulsion that lowers its value, and Br is larger than Cl. O is less negative than all three halogens.

Answer: (B)

Q3Medium

The correct order of ionic radii is:

  1. Na⁺ > Mg²⁺ > F⁻ > O²⁻
  2. O²⁻ > F⁻ > Na⁺ > Mg²⁺
  3. Mg²⁺ > Na⁺ > F⁻ > O²⁻
  4. F⁻ > O²⁻ > Mg²⁺ > Na⁺
Show solution

All four ions have 10 electrons. The nuclear charges are 8, 9, 11 and 12, and a larger nuclear charge pulls the electrons in more. So O²⁻ is the largest and Mg²⁺ the smallest.

Answer: (B)

Q4Medium

The first ionisation enthalpy of nitrogen is higher than that of oxygen because:

  1. N is larger than O
  2. N has a half-filled, stable 2p subshell
  3. O has a higher nuclear charge
  4. N is more electronegative
Show solution

N (2p³) has a symmetric half-filled subshell, which is extra stable. O (2p⁴) loses a paired electron that is repelled by its partner. Option (A) is false, because N is only slightly larger. (C) would predict the opposite order, and (D) is false because O is more electronegative.

Answer: (B)

Q5Medium

An element has the configuration [Ne]3s²3p³. Its period, group and block are:

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

The highest shell is n = 3, so the period is 3. The last electron enters a p orbital, so it is in the p-block. It has 5 valence electrons, so the group is 10 + 5 = 15. The element is phosphorus.

Answer: (A)

Q6Medium

Which oxide is amphoteric?

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

Al₂O₃ reacts with both acids and bases, so it is amphoteric. Na₂O and MgO are basic, and SO₃ is acidic.

Answer: (C)

Q7Hard

The successive ionisation enthalpies of an element (kJ mol⁻¹) are 738, 1451, 7733 and 10540. The element is most likely in group:

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

The big jump comes after the second electron (1451 → 7733). This means there are 2 valence electrons, so the element is in group 2 (these values are for Mg). A group-1 element would jump after the first IE.

Answer: (B)

Q8Hard

The IUPAC temporary symbol for the element with atomic number 114 is:

  1. Uuq
  2. Unq
  3. Uqu
  4. Unu
Show solution

The digits 1-1-4 give un-un-quad, so the name is ununquadium and the symbol Uuq (now officially flerovium, Fl). "Unq" would be element 104.

Answer: (A)

14. Quick Revision Summary

Remember before the exam
  • Modern periodic law: atomic number; 18 groups, 7 periods; s, p, d, f blocks.
  • Radius ↓ across, ↑ down; cation < atom < anion.
  • IE ↑ across, ↓ down; exceptions Be > B, N > O.
  • Most negative ΔegH: Cl; highest EN: F.
  • Oxides: basic → amphoteric → acidic across a period; diagonal pairs Li–Mg, Be–Al, B–Si.
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