Representing a Covalent Bond — Lewis Structure (Lewis Dot Structure)

A Lewis structure (or Lewis dot structure) is a pictorial representation of the covalent bonding between the atoms of a molecule. In it, each shared pair of valence electrons (a bond pair) is drawn as a pair of dots BETWEEN the two bonded atoms, while each atom's unshared valence electrons (lone pairs) are drawn as pairs of dots on that individual atom.

The five-step recipe, illustrated for water (H₂O):

Step 1 — Draw the skeletal structure. Place the most electronegative atom at the centre (with the general exception that hydrogen and fluorine, which form only one bond each, always sit at terminal positions, never centrally). For water: two H atoms flank a central O.

Step 2 — Count total valence electrons. Sum the valence electrons of every atom in the molecule. For a polyatomic ANION, add the magnitude of the negative charge to this sum (the extra electrons the ion carries); for a CATION, subtract the magnitude of the positive charge. For water: [2 × 1 (H)] + [1 × 6 (O)] = 2 + 6 = 8 valence electrons.

Step 3 — Draw single bonds. Connect the skeletal atoms with single bonds first; each single bond accounts for two of the total valence electrons (one bond pair). For water, the two O-H bonds use up 4 of the 8 electrons.

Step 4 — Distribute the remaining electrons as lone pairs. Place the leftover valence electrons as lone pairs, giving each atom an octet (a duet for hydrogen), starting with the MOST electronegative atom and then working outward. For water, the remaining 4 electrons (two lone pairs) go onto the central, most-electronegative oxygen, which now has a complete octet (2 bond pairs + 2 lone pairs = 8 electrons around it).

Step 5 — Verify the octet (or duet) and adjust if needed. Check every atom. If any atom is short of an octet, convert one of a neighbour's lone pairs into an additional bond (forming a double or triple bond) to make up the shortfall. For water, oxygen already has a complete octet and both hydrogens have duets, so no adjustment is needed -- the final Lewis structure is as in Fig 10.5.

A second worked example — nitric acid (HNO₃):

1. Skeletal structure: H bonded to one O, which is bonded to N, which is further bonded to two more O atoms (Fig 10.6's frame).
2. Total valence electrons = [1×1 (H)] + [1×5 (N)] + [3×6 (O)] = 1 + 5 + 18 = 24.
3. Four single bonds can be drawn (H-O, O-N, N-O, N-O), using 8 electrons (4 bond pairs).
4. The remaining 16 electrons (24 − 8) are distributed as 8 lone pairs, starting with the most electronegative atom, oxygen: six lone pairs (three each) go to the two terminal (non-hydrogen-bearing) oxygens to complete THEIR octets, and two lone pairs go to the oxygen bonded to hydrogen.
5. Checking octets: nitrogen is one electron pair short. So one lone pair is moved from a terminal oxygen to form a second N-O bond (making it N=O), completing nitrogen's octet. The final Lewis structure of HNO₃ (Fig 10.6) has one N=O double bond, one N-O single bond, and one N-O-H linkage. …