Classification of Elements and Periodicity — Class 11 MCQs with Answers

Classification of Elements and Periodicity — important questions & MCQs with answers (Class 11 Chemistry)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   Modern periodic table is arranged by:

   AAtomic massBColourCDensityDAtomic number
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   Answer: Atomic number

   Explanation: Moseley's X-ray spectra showed properties repeat periodically with atomic number Z, not atomic mass — so the modern table is arranged by Z. Picking atomic mass repeats Mendeleev's error, which misordered Ar (40 u) after K (39 u).

2. Q2 of 6Easy

   Across a period (left → right), electronegativity:

   AZeroBDecreasesCConstantDIncreases
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   Answer: Increases

   Explanation: Across a period, protons increase while the outer shell stays the same, so effective nuclear charge Zeff rises and pulls bonding electrons in harder — electronegativity increases left to right, e.g. Li 0.98 to F 3.98 (Pauling scale).

3. Q3 of 6Medium

   Element with configuration [Ar] 3d¹⁰ 4s² 4p³ belongs to:

   Af-blockBd-block, group 5Cs-block, group 2Dp-block, group 15
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   Answer: p-block, group 15

   Explanation: For [Ar] 3d¹⁰ 4s² 4p³, the last electron enters a 4p orbital, so this is p-block; the group number for p-block equals 10 plus the s+p electron count, here 2 + 3 = 5, giving group 15. Stopping at the 3d¹⁰ count wrongly suggests d-block, group 5.

4. Q4 of 6Medium

   Down a group, atomic radius:

   ARandomBDecreasesCConstantDIncreases
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   Answer: Increases

   Explanation: Down a group, each successive period adds a whole new principal shell (n increases by 1), pushing the outer electrons farther out despite the rising nuclear charge — so atomic radius increases, e.g. Li 152 pm to Cs 265 pm.

5. Q5 of 6Hard

   Transition elements belong to:

   As-blockBd-blockCp-blockDf-block
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   Answer: d-block

   Explanation: Transition elements are defined by an incompletely filled (n−1)d subshell in the atom or a common ion, so their last electrons enter d orbitals, placing them squarely in the d-block, groups 3 through 12.

6. Q6 of 6Hard

   Most electronegative element:

   AChlorineBOxygenCFluorineDNitrogen
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   Answer: Fluorine

   Explanation: Fluorine has the highest electronegativity of any element, 3.98 on the Pauling scale, because its small 2p valence shell combined with high effective nuclear charge pulls bonding electrons in more strongly than oxygen (3.44), chlorine (3.16), or nitrogen (3.04) can manage.

Classification of Elements and Periodicity notes: key concepts (Class 11 Chemistry)

This chapter arranges elements by the modern periodic law — properties are a periodic function of atomic number. It shows how s, p, d and f blocks arise from the last subshell filled, and how effective nuclear charge drives trends in atomic and ionic radius, ionisation and electron gain enthalpy, electronegativity and valence.

Modern periodic law

Properties of elements are a periodic function of their atomic number Z, not atomic mass; this repositioned anomalous pairs like Ar–K and Co–Ni correctly.

Periods and groups

The table has 7 periods and 18 groups; the period number equals the highest principal quantum number n in the atom, and fixes how many elements it holds.

s-block elements

Groups 1 and 2, filling the ns orbital; reactive metals that lose 1 or 2 electrons, form ionic compounds, and give strongly basic oxides.

p-block elements

Groups 13–18, filling np orbitals; spans metals, metalloids and non-metals, includes the halogens and noble gases, and shows varied oxidation states.

d-block elements

Transition metals filling (n−1)d orbitals; hard, high-melting metals showing variable valency, coloured ions and catalytic and complex-forming behaviour.

f-block elements

Inner transition lanthanoids and actinoids filling (n−2)f orbitals; placed below the table, they show close similarity and, for actinoids, radioactivity.

Effective nuclear charge

Inner electrons screen the nucleus, so a valence electron feels a reduced Zₑff; across a period Zₑff rises while shells stay the same, tightening the atom.

Atomic radius

Radius falls across a period as Zₑff pulls electrons in, and rises down a group as new shells add; it is quoted as a covalent or metallic radius.

Ionic radius

Cations are smaller and anions larger than the parent atom; among isoelectronic species radius falls as nuclear charge rises (O²⁻ > F⁻ > Na⁺ > Mg²⁺).

Ionisation enthalpy

Energy to remove the most loosely bound electron from a gaseous atom; rises across a period, falls down a group, and IE₂ > IE₁ since the cation holds tighter.

Ionisation enthalpy anomalies

Be > B and N > O buck the trend: B loses an easily removed 2p electron, and N's stable half-filled 2p³ resists removal, so their neighbours dip below.

Electron gain enthalpy

Energy released when a gaseous atom gains an electron; most negative for halogens, while Cl is more negative than F due to F's small, crowded 2p shell.

Electronegativity

The tendency of a bonded atom to attract the shared electron pair; increases across a period and decreases down a group, peaking at fluorine (Pauling 4.0).

Metallic character and oxides

Metallic character falls across a period and rises down a group; metal oxides are basic, non-metal oxides acidic, and intermediate ones give amphoteric oxides.