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Lesson: Chapter 10 — Chemical Bonds

10.2. Covalent bonds 4 of 8

Two more molecules

In water and methane the outer atoms were hydrogen, which has no electrons to spare. These two molecules show what happens when the outer atoms carry lone pairs as well.

Hydrogen chloride

Chlorine is 2, 8, 7 and needs one electron; hydrogen has one to share. They share a single pair, and the chlorine atom keeps its remaining three pairs untouched.

The hydrogen chloride molecule: a chlorine atom sharing one pair of electrons with a hydrogen atom, and keeping three pairs of its own.

Figure to be added

A chlorine atom and a hydrogen atom drawn as overlapping shells, forming the hydrogen chloride molecule (figure 10.13).

The chlorine atom now counts eight: six in its own three pairs and two in the shared pair. The hydrogen atom counts two, which is all it can hold.

Carbon tetrachloride

Carbon is four electrons short and chlorine one. Four chlorine atoms therefore satisfy one carbon atom, each sharing a single pair with it.

The carbon tetrachloride molecule: a carbon atom joined by a single bond to each of four chlorine atoms, every chlorine atom keeping three lone pairs.

Figure to be added

A carbon atom and four chlorine atoms drawn as overlapping shells, forming the carbon tetrachloride molecule (figure 10.14).

Twelve lone pairs sit around the outside of this molecule and take no part in holding it together. Only the four shared pairs are bonds.

Two questions, asked separately

For every atom in a molecule, ask how many bonds it forms and how many lone pairs it keeps. The first follows from how far it is short of eight; the second is whatever valence electrons are left over.