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Classification of Elements and Periodicity in Properties – Class 11 Chemistry notes, important questions and MCQs

NCERT Class 11 Chemistry, chapter 3. Notes, key terms, important questions with answers and MCQs.

Chapter overview

The modern periodic table arranges elements by atomic number. Properties like atomic radius, ionisation enthalpy, electron gain enthalpy and electronegativity change regularly across periods and down groups.

Periodic table

Modern periodic law (Moseley): properties are periodic functions of atomic number.
18 groups, 7 periods; s-, p-, d-, f-blocks.
Elements with Z > 100 get IUPAC temporary names (e.g. 101 = Unnilunium).

Trends

Atomic radius decreases across a period and increases down a group.
Ionisation enthalpy and electronegativity generally increase across a period and decrease down a group (F is most electronegative).
Exceptions: IE of N > O and Be > B (stable half-filled/filled subshells); Cl has the most negative electron gain enthalpy.

Key terms

Ionisation enthalpy: energy needed to remove the outermost electron from a gaseous atom
Electronegativity: tendency of a bonded atom to attract shared electrons
Electron gain enthalpy: enthalpy change when an electron is added to a gaseous atom; usually negative (energy released)

Important questions with answers

Why does ionisation enthalpy decrease down a group?2 marks

Atomic size increases and inner electrons shield the outer electron more, so the outer electron is held less tightly and is easier to remove.

Why is the second ionisation enthalpy of sodium much higher than the first?3 marks

After losing one electron, Na⁺ has the stable noble-gas configuration [Ne]. Removing another electron means breaking a stable completely filled shell, which needs much more energy.

Explain the trends in metallic character, atomic radius and electronegativity across period 3 (Na to Cl), and explain why the oxides change from basic to acidic.5 marks

Across the period, nuclear charge increases while electrons enter the same shell, so atomic radius decreases (Na largest, Cl smallest among Na–Cl). Electronegativity and ionisation enthalpy increase, so metallic character decreases: Na, Mg, Al are metals; Si is a metalloid; P, S, Cl are non-metals. Oxides follow the same trend: Na₂O and MgO are basic, Al₂O₃ is amphoteric, and SiO₂, P₄O₁₀, SO₃ and Cl₂O₇ are acidic, because metals lose electrons to form ionic, basic oxides while non-metals form covalent oxides that give acids in water.

Quick revision

Fluorine is the most electronegative.
Size increases down a group.
Cl has the most negative electron gain enthalpy.

Exam tips

Isoelectronic ions: more protons → smaller size (Al³⁺ < Mg²⁺ < Na⁺ < F⁻ < O²⁻).
Noble gases have positive electron gain enthalpy, and N has a less negative value than expected due to its half-filled 2p subshell.

Practice MCQs

Modern periodic table is based on
1. atomic mass
2. valency
3. density
4. atomic number
Show answer
Answer: (d) atomic number. Moseley.
Number of groups in the modern periodic table is
1. 18
2. 8
3. 7
4. 16
Show answer
Answer: (a) 18. 18 groups.
Most electronegative element is
1. oxygen
2. chlorine
3. fluorine
4. nitrogen
Show answer
Answer: (c) fluorine. 4.0 on Pauling scale.
Across a period from left to right, atomic radius
1. increases
2. first increases then decreases
3. stays same
4. decreases
Show answer
Answer: (d) decreases. Nuclear charge increases.
Element with configuration [Ne] 3s² 3p⁴ belongs to
1. group 14, period 3
2. group 4, period 3
3. group 16, period 2
4. group 16, period 3
Show answer
Answer: (d) group 16, period 3. Sulphur.
Which has higher first ionisation enthalpy
1. O
2. N
3. both equal
4. cannot say
Show answer
Answer: (b) N. Half-filled 2p in N.
Among isoelectronic Na⁺, Mg²⁺, Al³⁺, F⁻, the smallest is
1. Na⁺
2. Mg²⁺
3. F⁻
4. Al³⁺
Show answer
Answer: (d) Al³⁺. Most protons.
Which has the largest atomic radius
1. K
2. Mg
3. Na
4. Ca
Show answer
Answer: (a) K. Down the group and left in the period.

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Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.