Covalent structures, molecular shape and polarity

What you’ll learn

• Explain covalent and coordinate bonding.
• Predict molecular shapes from electron-pair repulsion.
• Distinguish bond polarity from overall molecular polarity.

Understand the idea

A covalent bond is the attraction between two nuclei and a shared pair of electrons. In a coordinate bond, both electrons originally come from the same donor atom; after formation it is a covalent bond. Electron-dot diagrams must include lone pairs, charges and the correct total number of valence electrons.

Electron regions around a central atom arrange to minimise repulsion. Two regions give a linear arrangement, three trigonal planar and four tetrahedral. Lone pairs repel more strongly than bonding pairs, reducing the bond angles in ammonia and water. Five regions give trigonal bipyramidal arrangements and six octahedral arrangements; a multiple bond counts as one region when predicting the basic shape.

An electronegativity difference makes a bond polar. Molecular polarity depends on the vector sum of bond dipoles: symmetric arrangements can cancel them. Carbon dioxide has polar bonds but no permanent molecular dipole, whereas bent water has a net dipole.

Giant covalent networks contain bonds throughout the solid. Diamond has four bonds per carbon and no mobile electrons; graphite and graphene have three bonds per carbon with delocalised electrons. Graphite's layers are weakly attracted to one another, while the bonds within a layer are strong. Simple molecular substances instead consist of discrete molecules.

Key formulae

$$\mathrm{NH_3+H^+\rightarrow NH_4^+}$$

$$\theta(\mathrm{CH_4})=109.5^\circ,\quad\theta(\mathrm{NH_3})\approx107^\circ$$

$$\theta(\mathrm{H_2O})\approx104.5^\circ,\quad\theta(\mathrm{CO_2})=180^\circ$$

$$\boldsymbol\mu_{\mathrm{molecule}}=\sum\boldsymbol\mu_{\mathrm{bond}}$$

Worked example

Predict the shape and polarity of $\mathrm{BCl_3}$.

1. Boron forms three bonds and has no lone pair in its simple valence-shell description.
2. Three bonding regions arrange trigonal planar with angles of 120 degrees.
3. The three equal polar bonds are symmetrically arranged, so their dipoles cancel.

Answer: Trigonal planar, 120° bond angles, and no permanent molecular dipole.