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7. Chemical Bonding & Molecular Geometry

Atoms bond to reach a stable electron arrangement — usually an octet. The type of bond, and the shape the bonds take, decide a molecule's properties.

Ionic vs Covalent Bonding

Ionic bonds transfer electrons (metal + nonmetal, e.g., NaCl). Covalent bonds share them (two nonmetals, e.g., H₂O). A large electronegativity difference favours ionic character; a small one favours covalent.

Lewis Structures and Formal Charge

Count valence electrons, draw single bonds, complete octets, and use multiple bonds where electrons remain. Formal charge checks the best structure: $$\text{Formal charge} = \text{valence e}^- - (\text{nonbonding e}^- + \tfrac{1}{2}\text{bonding e}^-)$$

Example: $\text{CO}_2$: 16 valence electrons, $\text{O=C=O}$, every atom formal charge 0 — the correct structure. In carbon monoxide, the triple-bonded structure gives C −1 and O +1, the best available.

VSEPR and Molecular Shape

Electron groups repel and spread out: 2 groups = linear, 3 = trigonal planar, 4 = tetrahedral. Lone pairs compress the angles and change the shape even when the electron geometry stays the same.

Example: $\text{NH}_3$ has 4 electron groups (3 bonds + 1 lone pair): tetrahedral electron geometry, trigonal pyramidal shape, bond angle ≈ 107°. $\text{H}_2\text{O}$ is bent (≈ 104.5°) and polar, while $\text{CO}_2$ is linear and nonpolar — shape decides polarity.

Hybridization

Orbitals mix to match the geometry: sp³ (tetrahedral, e.g., CH₄), sp² (trigonal planar, e.g., C₂H₄), sp (linear, e.g., C₂H₂).

Practice Problems

Draw the Lewis structure of NH₃ and name its molecular shape.
Which is polar: CO₂ or H₂O? Explain.
What is the hybridization of carbon in formaldehyde, CH₂O?
What is the bond angle in BF₃?