The Lewis dot structure for Cl2, the chemical formula for chlorine gas, is written with two Cl symbols, each of which is surrounded by three pairs of dots, connected by a single line. Each dot represents a valence electron, each line represents a bonded pair of electrons and each Cl represents a chlorine atom. It is appropriate to write two dots in place of a line to show bonding.
Based on its position on the periodic table, each atom of chlorine contains seven valence electrons. The electron configuration for each atom of chlorine can be represented as [Ne]3s2 + 3p5, showing that in the third orbital level -- represented by the 3s in front of the s and p -- there are a total of seven electrons, shown by the 2 after the s and the 5 after the p. Pupils can check that the Lewis dot configuration is written correctly by adding the total number of electrons represented by lines and dots and checking that the sum is equal to the expected total based on the chemical formula. For chlorine gas, the total is 14 valence electrons. Sometimes, each pair of valence electrons can be written as a single line in a Lewis dot structure.