Lewis structures

Once the point about which atom supplied which electron has been made, the crosses can be dropped. Draw every valence electron as a dot and the result is a Lewis dot structure.

Lewis dot structure

A diagram in which the valence electrons of each atom in a molecule are shown as dots, so that the covalent bonds are represented.

Two kinds of pair

In any of these diagrams the electron pairs fall into two groups, and the distinction runs through the rest of the chapter.

Bonding pairs

Pairs shared between two atoms. These are the bonds.

Lone pairs

Pairs belonging to one atom alone, taking no part in bonding.

A Lewis structure makes the distinction visible: each bonding pair is replaced by a short line, and only the lone pairs stay as dots.

Central and terminal atoms

Take ammonia. The nitrogen atom sits at the centre and is called the central atom; the three hydrogen atoms at the edges are terminal atoms. Around the nitrogen there are three bonding pairs and one lone pair.

The three notations side by side

Molecule	Dot and cross	Lewis dot structure	Lewis structure
Cl₂	Chlorine, dot and cross: one shared pair, three lone pairs on each atom.	Chlorine, Lewis dot structure.	Chlorine, Lewis structure: a single line between the atoms.
H₂	Hydrogen, dot and cross: one shared pair and nothing else.	Hydrogen, Lewis dot structure.	Hydrogen, Lewis structure: a single line between the atoms.
H₂O	Water, dot and cross: two shared pairs and two lone pairs on the oxygen atom.	Water, Lewis dot structure.	Water, Lewis structure: two lines and two lone pairs on oxygen.
NH₃	Ammonia, dot and cross: three shared pairs and one lone pair on nitrogen.	Ammonia, Lewis dot structure.	Ammonia, Lewis structure: three lines and one lone pair on nitrogen.
CH₄	Methane, dot and cross: four shared pairs and no lone pairs.	Methane, Lewis dot structure.	Methane, Lewis structure: four lines and no lone pairs.
O₂	Oxygen, dot and cross: two shared pairs forming a double bond.	Oxygen, Lewis dot structure.	Oxygen, Lewis structure: a double line between the atoms.
N₂	Nitrogen, dot and cross: three shared pairs forming a triple bond.	Nitrogen, Lewis dot structure.	Nitrogen, Lewis structure: a triple line between the atoms.
CO₂	Carbon dioxide, dot and cross: two double bonds, one to each oxygen atom.	Carbon dioxide, Lewis dot structure.	Carbon dioxide, Lewis structure: a double line on each side of the carbon atom.

Every atom reaches eight

In all eight molecules above, each central and terminal atom finishes with eight electrons in its valence shell once the shared pairs are counted. Compounds like these are said to have a complete octet. Hydrogen is the standing exception, finishing at two.