What went wrong when the king rode under the arch?

It was too low and knocked off his crown

How does the chain of blame unfold in the poem?

Each accused person blames another, ending with the king's own changes

What is a chemical bond?

The attractive force that holds the constituent atoms or ions together in a chemical species.

Give the three main exceptions to the octet rule.

Incomplete octet (e.g. BeH\(_2\), BCl\(_3\)), odd-electron molecules (e.g. NO, NO\(_2\)) and the expanded octet (e.g. PF\(_5\), SF\(_6\)).

Define lattice enthalpy.

The energy needed to completely separate one mole of a solid ionic compound into gaseous ions; for NaCl it is about 788 kJ mol\(^{-1}\).

Why are all O-O bonds in ozone equal in length?

Ozone is a resonance hybrid of two canonical forms, so its bonds are intermediate between a single and a double bond.

State the order of repulsion in VSEPR theory.

Lone pair-lone pair > lone pair-bond pair > bond pair-bond pair.

Why is the bond angle in water 104.5 degrees?

Oxygen is sp\(^3\) hybridised with two lone pairs whose strong repulsion compresses the angle below the ideal tetrahedral value.

Distinguish a sigma bond from a pi bond.

A sigma bond forms by head-on (axial) overlap and is stronger; a pi bond forms by sidewise overlap of parallel p-orbitals and is weaker.

Why does the \(He_2\) molecule not exist?

Its bond order is \(\tfrac12(2-2)=0\), so no net bonding holds the atoms together.

Why is dioxygen paramagnetic?

Molecular orbital theory places two unpaired electrons in its degenerate \(\pi^{*}\) antibonding orbitals.

Define a hydrogen bond.

The attractive force linking a partially positive hydrogen atom of one molecule to a highly electronegative atom (F, O or N) of another.

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Chapter 4: Chemical Bonding and Molecular Structure — Class 11 Chemistry

Chemistry · 9 chapters

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Chapter 4: Chemical Bonding and Molecular Structure

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According to the octet rule, atoms combine to attain how many electrons in their valence shell?

Summary

This unit explains why atoms combine and how the shapes of molecules arise. The Kössel-Lewis approach relates bonding to the attainment of a stable noble-gas configuration: electron transfer gives ionic (electrovalent) bonds while electron sharing gives covalent bonds, both governed by the octet rule. Lewis dot structures, formal charge and the limitations of the octet rule (incomplete octet, odd-electron molecules and the expanded octet) are discussed. The stability of an ionic bond is measured by lattice enthalpy rather than by octet attainment alone. Bond parameters — bond length, bond angle, bond enthalpy and bond order — are defined alongside resonance (canonical structures and the resonance hybrid) and bond polarity expressed through dipole moment. The valence shell electron pair repulsion (VSEPR) theory predicts molecular geometry from electron-pair repulsions, where lone pair-lone pair repulsion exceeds lone pair-bond pair, which exceeds bond pair-bond pair. Valence bond theory explains bonding through orbital overlap, distinguishing sigma and pi bonds and introducing hybridisation (sp, sp\(^2\), sp\(^3\), sp\(^3\)d, sp\(^3\)d\(^2\)) to account for geometries such as those of methane, ammonia and water. Molecular orbital theory builds bonding and antibonding orbitals by the linear combination of atomic orbitals, giving bond order, stability and magnetic behaviour (explaining why dioxygen is paramagnetic). The unit closes with hydrogen bonding.

Kössel-Lewis approach and octet ruleIonic bond and lattice enthalpyBond parameters and resonanceDipole moment and bond polarityVSEPR theory and molecular shapesValence bond theory and hybridisationMolecular orbital theoryHydrogen bonding

Key terms

Octet rule

Atoms combine by transferring or sharing valence electrons to attain eight electrons in their valence shell, a stable noble-gas configuration.

Lattice enthalpy

The energy required to completely separate one mole of a solid ionic compound into its gaseous ions.

Formal charge

The difference between the valence electrons of a free atom and the electrons assigned to it in a Lewis structure.

Resonance

The representation of a molecule by several canonical structures of comparable energy whose hybrid describes the real, lower-energy molecule.

Hybridisation

The intermixing of atomic orbitals of slightly different energies to form an equal number of equivalent orbitals of identical energy and shape.

Bond order

In molecular orbital theory, one half the difference between the numbers of bonding and antibonding electrons, \(\tfrac12(N_b-N_a)\).

Important questions

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Octet rule

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Atoms combine by transferring or sharing valence electrons to attain eight electrons in their valence shell, a stable noble-gas configuration.

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Practice quiz · Chemical Bonding and Molecular Structure

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