A Lewis symbol represents an atom's valence electrons as dots around its element symbol. It deliberately ignores inner electrons because ordinary chemical bonding depends mainly on the valence shell.

For main-group elements, periodic-table position gives a useful valence-electron count. Carbon has four valence electrons, oxygen six, and chlorine seven, so their Lewis symbols contain four, six and seven dots respectively.

Dots are placed singly around the four sides of the element symbol before pairing them. This convention makes unpaired and paired valence electrons visible at a glance.

For a monatomic ion, electrons are added or removed according to the charge. A chloride ion has one more electron than neutral chlorine and is written with eight valence dots inside brackets with a negative charge. A sodium ion has lost its single valence electron and is written with no valence dots and a positive charge.

Lewis symbols are bookkeeping models rather than pictures of electron positions. Their purpose is to make valence-electron counts explicit before those electrons are allocated to bonds and lone pairs in molecular Lewis structures.