Lewis structures and the octet rule

Why do atoms join together? Kössel and Lewis explained it with the stability of the noble gases: atoms bond to reach a full outer shell of eight electrons (an octet). They can do this in two ways:

• Transferring electrons: one atom has electrons to spare and the other is short, so one gives and the other takes. This makes an ionic (electrovalent) bond.
• Sharing electrons: both atoms are short, so they share pairs. This makes a covalent bond (the Lewis-Langmuir idea). One shared pair is a single bond, two pairs a double bond, three pairs a triple bond.

A Lewis symbol shows an element's valence electrons as dots round its symbol. A Lewis structure shows how these electrons are arranged in a molecule. To draw one:

• Count all the valence electrons. For a negative ion, add one electron per negative charge; for a positive ion, subtract one per positive charge.
• Write the atoms, usually with the least electronegative one in the centre, and join them by single bonds.
• Use the remaining electrons as lone pairs to complete the octets of the outer atoms, then the central atom.
• If the central atom is still short, turn lone pairs into double or triple bonds.

For example, the sulphate ion SO₄²⁻ has 6 (S) + 4 × 6 (O) + 2 (charge) = 32 valence electrons to place. Carbon monoxide needs a triple bond: :C≡O:.

When several structures are possible, formal charge helps choose. For each atom: formal charge = valence electrons – non-bonding electrons – ½ × bonding electrons. The best structure has the smallest formal charges on its atoms.

The octet rule has exceptions:

• Incomplete octet: the central atom has fewer than eight, as in BeH₂ and BCl₃.
• Odd-electron molecules: an odd total means at least one atom cannot have an octet, as in NO and NO₂.
• Expanded octet: elements of period 3 and beyond can use d orbitals to hold more than eight, as in PF₅ and SF₆.

The octet rule also cannot explain why noble gases like xenon form compounds, or the shapes and energies of molecules. Later theories deal with these.