Chapter 4: Chemical Bonding and Molecular Structure Assertion-Reason Questions | chemistry Class 11 CBSE

Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 1

Logic Check

Assertion (A): Noble gases exist as monoatomic molecules under normal conditions. Reason (R): Noble gases have a completely filled outer shell (ns2np6) which represents a highly stable electronic configuration.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 2

Logic Check

Assertion (A): The lattice enthalpy of NaCl is a better measure of its stability than the sum of ionization enthalpy and electron gain enthalpy. Reason (R): Even though the sum of ionization enthalpy and electron gain enthalpy for NaCl is positive (+147.1 kJ mol-1), the lattice formation enthalpy (-788 kJ mol-1) more than compensates for it.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 3

Logic Check

Assertion (A): BF3 is an example of an odd-electron molecule that violates the octet rule. Reason (R): Boron has only three valence electrons and forms only three bonds in BF3, leaving it with only six electrons around the central atom.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 4

Logic Check

Assertion (A): The bond length in O3 molecule is 128 pm, which is the same for both O-O bonds. Reason (R): O3 is best described as a resonance hybrid of two canonical structures, each with one O-O single bond and one O=O double bond.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 5

Logic Check

Assertion (A): CO2 has zero dipole moment. Reason (R): CO2 is a linear molecule in which the two C=O bond dipoles are equal and point in exactly opposite directions, cancelling each other.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 6

Logic Check

Assertion (A): The bond angle in H2O (104.5°) is less than that in NH3 (107°). Reason (R): H2O has two lone pairs on oxygen, whereas NH3 has only one lone pair on nitrogen; greater lone pair repulsion in H2O causes a larger reduction in bond angle.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 7

Logic Check

Assertion (A): Sigma bonds are stronger than pi bonds. Reason (R): In sigma bonds, the overlapping of atomic orbitals occurs in a sidewise (lateral) fashion, which allows greater overlap than the end-to-end overlap in pi bonds.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 8

Logic Check

Assertion (A): sp2 hybridised carbon atoms form trigonal planar structures with bond angles of 120°. Reason (R): In sp2 hybridisation, one s and two p orbitals mix to form three equivalent hybrid orbitals oriented in a plane at 120° to each other.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 9

Logic Check

Assertion (A): He2 molecule does not exist under normal conditions. Reason (R): The bond order of He2 calculated from MO theory is zero, indicating no net bonding between the two helium atoms.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 10

Logic Check

Assertion (A): O2 molecule is paramagnetic. Reason (R): The MO electronic configuration of O2 shows two unpaired electrons in the degenerate pi2px and pi2py antibonding orbitals.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 11

Logic Check

Assertion (A): The bond dissociation enthalpy of F2 (155 kJ mol-1) is much lower than that of Cl2 (243 kJ mol-1). Reason (R): Fluorine is less electronegative than chlorine, resulting in a weaker F-F bond.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 12

Logic Check

Assertion (A): Hydrogen bonding in water is responsible for its anomalously high boiling point compared to H2S. Reason (R): Oxygen is more electronegative than sulphur, enabling water to form strong intermolecular hydrogen bonds which require more energy to break.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 13

Logic Check

Assertion (A): PCl5 exists as a stable compound but NCl5 does not exist. Reason (R): Phosphorus has vacant 3d orbitals of comparable energy available for sp3d hybridisation, whereas nitrogen (second period) lacks accessible d orbitals.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 14

Logic Check

Assertion (A): The axial bonds in PCl5 are longer than the equatorial bonds. Reason (R): Axial bond pairs in PCl5 experience greater repulsion from the three equatorial bond pairs at 90° compared to the repulsion experienced by equatorial bond pairs.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 15

Logic Check

Assertion (A): The canonical forms of a resonance hybrid have real, independent existence. Reason (R): The molecule exists for certain fractions of time in each canonical form, switching between them.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 16

Logic Check

Assertion (A): With increasing bond order, bond length decreases and bond enthalpy increases. Reason (R): Higher bond order means more electron pairs are shared between atoms, increasing attractive force and reducing internuclear distance.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 17

Logic Check

Assertion (A): The dipole moment of NF3 is less than that of NH3. Reason (R): In NF3, the orbital dipole due to the lone pair on N acts in the opposite direction to the resultant N-F bond dipoles, decreasing the net dipole moment.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 18

Logic Check

Assertion (A): In the formation of a bonding molecular orbital, the electron density between the two nuclei increases. Reason (R): Bonding MO is formed by the constructive interference (addition) of wave functions of combining atomic orbitals.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 19

Logic Check

Assertion (A): The covalent radius is always smaller than the van der Waals radius for the same element. Reason (R): In a covalent bond, atoms share electrons and their nuclei are held closer together than in a non-bonded state where only van der Waals forces operate.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 20

Logic Check

Assertion (A): Ionic bonds are formed more easily between elements with high ionisation enthalpy. Reason (R): A high ionisation enthalpy means electrons are easily removed from an atom to form cations.

Reasoning Options

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Chapter sections

Chapter 4: Chemical Bonding and Molecular Structure | Assertion-Reason

Assertion (A): Noble gases exist as monoatomic molecules under normal conditions. Reason (R): Noble gases have a completely filled outer shell (ns2np6) which represents a highly stable electronic configuration.

Assertion (A): BF3 is an example of an odd-electron molecule that violates the octet rule. Reason (R): Boron has only three valence electrons and forms only three bonds in BF3, leaving it with only six electrons around the central atom.

Assertion (A): The bond length in O3 molecule is 128 pm, which is the same for both O-O bonds. Reason (R): O3 is best described as a resonance hybrid of two canonical structures, each with one O-O single bond and one O=O double bond.

Assertion (A): CO2 has zero dipole moment. Reason (R): CO2 is a linear molecule in which the two C=O bond dipoles are equal and point in exactly opposite directions, cancelling each other.

Assertion (A): The bond angle in H2O (104.5°) is less than that in NH3 (107°). Reason (R): H2O has two lone pairs on oxygen, whereas NH3 has only one lone pair on nitrogen; greater lone pair repulsion in H2O causes a larger reduction in bond angle.

Assertion (A): Sigma bonds are stronger than pi bonds. Reason (R): In sigma bonds, the overlapping of atomic orbitals occurs in a sidewise (lateral) fashion, which allows greater overlap than the end-to-end overlap in pi bonds.

Assertion (A): sp2 hybridised carbon atoms form trigonal planar structures with bond angles of 120°. Reason (R): In sp2 hybridisation, one s and two p orbitals mix to form three equivalent hybrid orbitals oriented in a plane at 120° to each other.

Assertion (A): He2 molecule does not exist under normal conditions. Reason (R): The bond order of He2 calculated from MO theory is zero, indicating no net bonding between the two helium atoms.

Assertion (A): O2 molecule is paramagnetic. Reason (R): The MO electronic configuration of O2 shows two unpaired electrons in the degenerate pi2px and pi2py antibonding orbitals.

Assertion (A): The bond dissociation enthalpy of F2 (155 kJ mol-1) is much lower than that of Cl2 (243 kJ mol-1). Reason (R): Fluorine is less electronegative than chlorine, resulting in a weaker F-F bond.

Assertion (A): Hydrogen bonding in water is responsible for its anomalously high boiling point compared to H2S. Reason (R): Oxygen is more electronegative than sulphur, enabling water to form strong intermolecular hydrogen bonds which require more energy to break.

Assertion (A): PCl5 exists as a stable compound but NCl5 does not exist. Reason (R): Phosphorus has vacant 3d orbitals of comparable energy available for sp3d hybridisation, whereas nitrogen (second period) lacks accessible d orbitals.

Assertion (A): The axial bonds in PCl5 are longer than the equatorial bonds. Reason (R): Axial bond pairs in PCl5 experience greater repulsion from the three equatorial bond pairs at 90° compared to the repulsion experienced by equatorial bond pairs.

Assertion (A): The canonical forms of a resonance hybrid have real, independent existence. Reason (R): The molecule exists for certain fractions of time in each canonical form, switching between them.

Assertion (A): With increasing bond order, bond length decreases and bond enthalpy increases. Reason (R): Higher bond order means more electron pairs are shared between atoms, increasing attractive force and reducing internuclear distance.

Assertion (A): The dipole moment of NF3 is less than that of NH3. Reason (R): In NF3, the orbital dipole due to the lone pair on N acts in the opposite direction to the resultant N-F bond dipoles, decreasing the net dipole moment.

Assertion (A): In the formation of a bonding molecular orbital, the electron density between the two nuclei increases. Reason (R): Bonding MO is formed by the constructive interference (addition) of wave functions of combining atomic orbitals.

Assertion (A): The covalent radius is always smaller than the van der Waals radius for the same element. Reason (R): In a covalent bond, atoms share electrons and their nuclei are held closer together than in a non-bonded state where only van der Waals forces operate.

Assertion (A): Ionic bonds are formed more easily between elements with high ionisation enthalpy. Reason (R): A high ionisation enthalpy means electrons are easily removed from an atom to form cations.

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Chapter sections

Chapter 4: Chemical Bonding and Molecular Structure | Assertion-Reason

Assertion (A): Noble gases exist as monoatomic molecules under normal conditions. Reason (R): Noble gases have a completely filled outer shell (ns2np6) which represents a highly stable electronic configuration.

Assertion (A): BF3 is an example of an odd-electron molecule that violates the octet rule. Reason (R): Boron has only three valence electrons and forms only three bonds in BF3, leaving it with only six electrons around the central atom.

Assertion (A): The bond length in O3 molecule is 128 pm, which is the same for both O-O bonds. Reason (R): O3 is best described as a resonance hybrid of two canonical structures, each with one O-O single bond and one O=O double bond.

Assertion (A): CO2 has zero dipole moment. Reason (R): CO2 is a linear molecule in which the two C=O bond dipoles are equal and point in exactly opposite directions, cancelling each other.

Assertion (A): The bond angle in H2O (104.5°) is less than that in NH3 (107°). Reason (R): H2O has two lone pairs on oxygen, whereas NH3 has only one lone pair on nitrogen; greater lone pair repulsion in H2O causes a larger reduction in bond angle.

Assertion (A): Sigma bonds are stronger than pi bonds. Reason (R): In sigma bonds, the overlapping of atomic orbitals occurs in a sidewise (lateral) fashion, which allows greater overlap than the end-to-end overlap in pi bonds.

Assertion (A): sp2 hybridised carbon atoms form trigonal planar structures with bond angles of 120°. Reason (R): In sp2 hybridisation, one s and two p orbitals mix to form three equivalent hybrid orbitals oriented in a plane at 120° to each other.

Assertion (A): He2 molecule does not exist under normal conditions. Reason (R): The bond order of He2 calculated from MO theory is zero, indicating no net bonding between the two helium atoms.

Assertion (A): O2 molecule is paramagnetic. Reason (R): The MO electronic configuration of O2 shows two unpaired electrons in the degenerate pi*2px and pi*2py antibonding orbitals.

Assertion (A): The bond dissociation enthalpy of F2 (155 kJ mol-1) is much lower than that of Cl2 (243 kJ mol-1). Reason (R): Fluorine is less electronegative than chlorine, resulting in a weaker F-F bond.

Assertion (A): Hydrogen bonding in water is responsible for its anomalously high boiling point compared to H2S. Reason (R): Oxygen is more electronegative than sulphur, enabling water to form strong intermolecular hydrogen bonds which require more energy to break.

Assertion (A): PCl5 exists as a stable compound but NCl5 does not exist. Reason (R): Phosphorus has vacant 3d orbitals of comparable energy available for sp3d hybridisation, whereas nitrogen (second period) lacks accessible d orbitals.

Assertion (A): The axial bonds in PCl5 are longer than the equatorial bonds. Reason (R): Axial bond pairs in PCl5 experience greater repulsion from the three equatorial bond pairs at 90° compared to the repulsion experienced by equatorial bond pairs.

Assertion (A): The canonical forms of a resonance hybrid have real, independent existence. Reason (R): The molecule exists for certain fractions of time in each canonical form, switching between them.

Assertion (A): With increasing bond order, bond length decreases and bond enthalpy increases. Reason (R): Higher bond order means more electron pairs are shared between atoms, increasing attractive force and reducing internuclear distance.

Assertion (A): The dipole moment of NF3 is less than that of NH3. Reason (R): In NF3, the orbital dipole due to the lone pair on N acts in the opposite direction to the resultant N-F bond dipoles, decreasing the net dipole moment.

Assertion (A): In the formation of a bonding molecular orbital, the electron density between the two nuclei increases. Reason (R): Bonding MO is formed by the constructive interference (addition) of wave functions of combining atomic orbitals.

Assertion (A): The covalent radius is always smaller than the van der Waals radius for the same element. Reason (R): In a covalent bond, atoms share electrons and their nuclei are held closer together than in a non-bonded state where only van der Waals forces operate.

Assertion (A): Ionic bonds are formed more easily between elements with high ionisation enthalpy. Reason (R): A high ionisation enthalpy means electrons are easily removed from an atom to form cations.

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Assertion (A): O2 molecule is paramagnetic. Reason (R): The MO electronic configuration of O2 shows two unpaired electrons in the degenerate pi2px and pi2py antibonding orbitals.