Chapter 3: Classification of Elements and Periodicity in Properties Assertion-Reason Questions | chemistry Class 11 CBSE

Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 1

Logic Check

Assertion (A): The properties of elements are a periodic function of their atomic numbers and not atomic masses. Reason (R): Moseley showed that the plot of square root of X-ray frequency against atomic number gives a straight line, proving atomic number is more fundamental.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 2

Logic Check

Assertion (A): Atomic radius decreases across a period from left to right. Reason (R): Across a period, the principal quantum number increases, placing valence electrons farther from the nucleus.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 3

Logic Check

Assertion (A): Sodium ion (Na⁺) is smaller than the sodium atom (Na). Reason (R): Cation formation involves removal of electrons, so with fewer electrons but the same nuclear charge, effective nuclear charge per electron increases, contracting the ion.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 4

Logic Check

Assertion (A): The fluoride ion (F⁻) is larger than the fluorine atom (F). Reason (R): Addition of an electron to fluorine increases electron-electron repulsion and decreases effective nuclear charge per electron, expanding the electron cloud.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 5

Logic Check

Assertion (A): Ionization enthalpies are always positive. Reason (R): Energy is always required to remove an electron from an atom, regardless of the element or the shell from which it is removed.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 6

Logic Check

Assertion (A): Boron has a lower first ionization enthalpy than beryllium despite having a higher atomic number. Reason (R): The outermost electron in boron is in a 2p orbital which is more shielded from the nucleus than the 2s electron in beryllium.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 7

Logic Check

Assertion (A): Oxygen has a lower first ionization enthalpy than nitrogen. Reason (R): Nitrogen has a completely filled 2p subshell which is extra stable, making electron removal more difficult than in oxygen.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 8

Logic Check

Assertion (A): Chlorine has a more negative electron gain enthalpy than fluorine. Reason (R): The added electron in fluorine enters the compact 2p orbital and experiences greater repulsion from existing electrons than in chlorine's larger 3p orbital.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 9

Logic Check

Assertion (A): Noble gases have large positive electron gain enthalpies. Reason (R): In noble gases, the added electron must enter the next higher principal quantum level, creating a very unstable electronic configuration.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 10

Logic Check

Assertion (A): Electronegativity increases across a period from left to right. Reason (R): Atomic radius decreases across a period, so the nucleus is closer to bonding electrons and attracts them more strongly.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 11

Logic Check

Assertion (A): Helium is placed in Group 18 (p-block) in the periodic table. Reason (R): Helium has the electronic configuration 1s² and belongs to the s-block based on its last filled orbital.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 12

Logic Check

Assertion (A): Elements in the same group of the periodic table have similar chemical properties. Reason (R): Elements in the same group have the same number and distribution of electrons in their outermost orbitals.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 13

Logic Check

Assertion (A): Alkali metals are never found in free state in nature. Reason (R): Alkali metals have very low ionization enthalpies and react readily with water, oxygen, and other substances to form stable compounds.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 14

Logic Check

Assertion (A): The metallic radius of copper is taken as 128 pm. Reason (R): Metallic radius is half the internuclear distance between adjacent atoms in the metallic crystal; the distance between two adjacent copper atoms in solid copper is 256 pm.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 15

Logic Check

Assertion (A): The reactivity of halogens decreases from fluorine to iodine. Reason (R): As we descend Group 17, ionization enthalpy increases, making it harder for halogens to attract electrons.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 16

Logic Check

Assertion (A): Na₂O is a basic oxide while Cl₂O₇ is an acidic oxide. Reason (R): Elements on the extreme left of a period are metallic with low ionization enthalpy, while those on the extreme right are non-metallic with high negative electron gain enthalpy.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 17

Logic Check

Assertion (A): Lithium forms compounds with pronounced covalent character unlike other alkali metals. Reason (R): Lithium has a small atomic size and high charge-to-radius ratio, giving it high polarising power compared to other alkali metals.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 18

Logic Check

Assertion (A): The 3d transition series starts at scandium (Z=21) in the fourth period. Reason (R): After calcium fills the 4s orbital, filling of the 3d orbitals becomes energetically more favourable than filling 4p orbitals.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 19

Logic Check

Assertion (A): Phosphorus has a lower electron gain enthalpy than sulphur. Reason (R): Phosphorus has a half-filled 3p subshell (3p³) which is extra stable; adding an electron disrupts this stability.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 20

Logic Check

Assertion (A): Zn, Cd, and Hg do not show most properties of typical transition metals. Reason (R): These elements have (n-1)d¹⁰ns² configuration with completely filled d-subshells, so no unpaired d-electrons are available for characteristic transition metal behaviour.

Reasoning Options

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Chapter sections

Chapter 3: Classification of Elements and Periodicity in Properties | Assertion-Reason

Assertion (A): The properties of elements are a periodic function of their atomic numbers and not atomic masses. Reason (R): Moseley showed that the plot of square root of X-ray frequency against atomic number gives a straight line, proving atomic number is more fundamental.

Assertion (A): Atomic radius decreases across a period from left to right. Reason (R): Across a period, the principal quantum number increases, placing valence electrons farther from the nucleus.

Assertion (A): Sodium ion (Na⁺) is smaller than the sodium atom (Na). Reason (R): Cation formation involves removal of electrons, so with fewer electrons but the same nuclear charge, effective nuclear charge per electron increases, contracting the ion.

Assertion (A): The fluoride ion (F⁻) is larger than the fluorine atom (F). Reason (R): Addition of an electron to fluorine increases electron-electron repulsion and decreases effective nuclear charge per electron, expanding the electron cloud.

Assertion (A): Ionization enthalpies are always positive. Reason (R): Energy is always required to remove an electron from an atom, regardless of the element or the shell from which it is removed.

Assertion (A): Boron has a lower first ionization enthalpy than beryllium despite having a higher atomic number. Reason (R): The outermost electron in boron is in a 2p orbital which is more shielded from the nucleus than the 2s electron in beryllium.

Assertion (A): Oxygen has a lower first ionization enthalpy than nitrogen. Reason (R): Nitrogen has a completely filled 2p subshell which is extra stable, making electron removal more difficult than in oxygen.

Assertion (A): Chlorine has a more negative electron gain enthalpy than fluorine. Reason (R): The added electron in fluorine enters the compact 2p orbital and experiences greater repulsion from existing electrons than in chlorine's larger 3p orbital.

Assertion (A): Noble gases have large positive electron gain enthalpies. Reason (R): In noble gases, the added electron must enter the next higher principal quantum level, creating a very unstable electronic configuration.

Assertion (A): Electronegativity increases across a period from left to right. Reason (R): Atomic radius decreases across a period, so the nucleus is closer to bonding electrons and attracts them more strongly.

Assertion (A): Helium is placed in Group 18 (p-block) in the periodic table. Reason (R): Helium has the electronic configuration 1s² and belongs to the s-block based on its last filled orbital.

Assertion (A): Elements in the same group of the periodic table have similar chemical properties. Reason (R): Elements in the same group have the same number and distribution of electrons in their outermost orbitals.

Assertion (A): Alkali metals are never found in free state in nature. Reason (R): Alkali metals have very low ionization enthalpies and react readily with water, oxygen, and other substances to form stable compounds.

Assertion (A): The metallic radius of copper is taken as 128 pm. Reason (R): Metallic radius is half the internuclear distance between adjacent atoms in the metallic crystal; the distance between two adjacent copper atoms in solid copper is 256 pm.

Assertion (A): The reactivity of halogens decreases from fluorine to iodine. Reason (R): As we descend Group 17, ionization enthalpy increases, making it harder for halogens to attract electrons.

Assertion (A): Na₂O is a basic oxide while Cl₂O₇ is an acidic oxide. Reason (R): Elements on the extreme left of a period are metallic with low ionization enthalpy, while those on the extreme right are non-metallic with high negative electron gain enthalpy.

Assertion (A): Lithium forms compounds with pronounced covalent character unlike other alkali metals. Reason (R): Lithium has a small atomic size and high charge-to-radius ratio, giving it high polarising power compared to other alkali metals.

Assertion (A): The 3d transition series starts at scandium (Z=21) in the fourth period. Reason (R): After calcium fills the 4s orbital, filling of the 3d orbitals becomes energetically more favourable than filling 4p orbitals.

Assertion (A): Phosphorus has a lower electron gain enthalpy than sulphur. Reason (R): Phosphorus has a half-filled 3p subshell (3p³) which is extra stable; adding an electron disrupts this stability.

Assertion (A): Zn, Cd, and Hg do not show most properties of typical transition metals. Reason (R): These elements have (n-1)d¹⁰ns² configuration with completely filled d-subshells, so no unpaired d-electrons are available for characteristic transition metal behaviour.

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