Ch 10Chemical Bonding — Class 11 Chemistry, concept-first.

Why is diamond one of the hardest substances known while graphite, its own allotrope of pure carbon, is soft enough to write with? Why do hydrogen and oxygen exist as diatomic gas molecules while the…

Two everyday comparisons open this unit. Diamond (an allotrope of carbon) is very hard, while graphite, another allotrope of the very same element, is very soft.

The Kossel-Lewis approach. In 1916, Kossel and Lewis independently offered the first logical explanation for chemical bonding.

Having seen that atoms reach a stable, noble-gas-like electron count either by TRANSFERRING electrons completely or by SHARING them, this section classifies chemical bonds by exactly this distinction.…

Every element except the noble gases occurs in nature either as a compound or as a polyatomic/diatomic molecule -- never as isolated, uncombined atoms.

A Lewis structure (or Lewis dot structure) is a pictorial representation of the covalent bonding between the atoms of a molecule.

Sometimes the five-step recipe above yields MORE THAN ONE plausible Lewis structure for the same molecule.

The octet rule works well for molecules whose central atom is a second-period element, but it has three well-known families of exception.

When the electronegativity difference between two combining atoms is LARGE, rather than sharing electrons the less electronegative atom completely TRANSFERS one or more of its valence electrons to the…

In an ordinary covalent bond, BOTH combining atoms contribute one electron each, and the resulting pair is mutually shared.

A covalent bond is characterised by a set of measurable parameters: bond length, bond angle, bond order, and bond enthalpy.

Bond length is the distance between the nuclei of two covalently bonded atoms. For a covalent molecule A-B, the bond length equals the sum of the (covalent/bonded) radii of the two bonded atoms, (Fig…

Bond order is the number of bonds formed between two bonded atoms in a molecule. In simple Lewis theory, bond order equals the number of shared electron pairs between the two bonded atoms.

Bond angle is the fixed angle, usually expressed in degrees, between two covalent bonds meeting at a common atom.

Bond enthalpy is defined as the minimum amount of energy required to break one mole of a particular bond in molecules that are in their GASEOUS state. Its unit is kJ mol⁻¹.

Sometimes, applying the five-step Lewis-structure recipe honestly to a single molecule still yields MORE THAN ONE equally valid structure -- not because one is wrong, but because the true electronic s…

Partial ionic character in covalent bonds. When a covalent bond forms between two IDENTICAL atoms (as in H₂, O₂, Cl₂), both atoms have an equal tendency to attract the shared electron pair, so the sha…

Lewis theory tells us WHICH atoms are bonded to which (connectivity, via shared electron pairs), but it cannot by itself predict the three-dimensional SHAPE of a molecule.

Heitler and London gave the first theoretical, wave-mechanical treatment of covalent bond formation, working out the case of the hydrogen molecule; Pauling and Slater later developed this treatment fu…

VB theory's key working rules, drawn out from the H₂ picture above and generalised to any covalent bond:

When atoms combine to form a covalent molecule, it is specifically the ATOMIC ORBITALS of the combining atoms that overlap to form the covalent bond -- the shared (bond-pair) electrons then occupy thi…

Sigma (σ) bond. When two atomic orbitals overlap LINEARLY, along the axis joining the two nuclei, the resulting bond is a sigma bond -- also called 'head-on overlap' or 'axial overlap'.

Formation of hydrogen (H₂). Hydrogen's electronic configuration is 1s¹. When two hydrogen atoms combine, the 1s orbital of each (each containing one unpaired electron, of opposite spin to the other) o…

Simple orbital-overlap arguments (as in section 10.8.2) work well for small, symmetric diatomic molecules such as hydrogen and fluorine, but they fail for polyatomic molecules such as methane, ammonia…

sp hybridisation — BeCl₂ (linear). Beryllium's ground-state valence configuration is 2s² (2p empty). To form two equivalent Be-Cl bonds, one of the paired 2s electrons is promoted ('excited') into an…

Both Lewis theory and valence bond theory give useful, largely correct qualitative pictures of chemical bonding and molecular structure -- but both are built on the idea of electrons pairing up into B…

Molecular orbital wavefunctions can, in principle, be obtained by solving the Schrödinger wave equation for the whole molecule -- but this is far too mathematically complex to do exactly for any but t…

Applying LCAO and the Aufbau/Pauli/Hund filling rules, molecular orbital theory gives the electron configuration, bond order, and magnetic behaviour of the second-period homonuclear diatomic molecules…

Sample papers and previous-year board questions for this subject.