The Lewis structure: covalent versus ionic

What the Lewis structure is for

The Lewis structure is a representation that shows, around the symbol of each atom, all the valence electrons: those that are shared (bonding pairs) and those that are not (lone pairs, or sometimes a single unpaired electron). It lets you check at a glance that the octet rule is satisfied.

Method

1. Count the total number of valence electrons in the molecule.
2. Connect the atoms with bonding pairs (lines).
3. Complete each atom with lone pairs until the octet (or the duet for H) is reached.

Example: water (H2O)

     ..
H -- O -- H
     ..

(two O-H bonding pairs, and two lone pairs on the oxygen: 2+2+2+2 = 8 electrons around the oxygen)

Example: carbon dioxide (CO2)

..           ..
:O = C = O:
..           ..

(two double bonds, no lone pair on the carbon, two lone pairs on each oxygen: the carbon and each oxygen do have 8 electrons)

Example: sodium chloride (NaCl)

Here, there is no shared bonding pair: the Lewis structure shows a complete transfer, with a bracket around each ion to indicate the charge.

[Na]+   [ :Cl: ]-
         ..

The essential difference

	Covalent bond	Ionic bond
Electrons	shared (bonding pair)	fully transferred
Between	non-metal and non-metal	metal and non-metal
Result	a neutral molecule	two ions of opposite charge
Example	H2O, CO2	NaCl

Classic mistake

Don't mix up the two types of lines: in a covalent Lewis structure, a line between two atoms represents a shared pair. In an ionic compound, there is no line between the ions, only brackets and charges, because the electrons are no longer shared at all.