Molecular and Structural Formulas

A molecular formula gives the types and the count of atoms for each element in a compound. An example of a molecular formula is ethane, C2H6. Here the formula indicates carbon and hydrogen are combined in ethane. The subscripts tell us that there are 2 carbon atoms and 6 hydrogen atoms in a formula unit. Both of the carbon atoms are considered to be "central atoms" for constructing Lewis diagrams.

The structural formula shows the atoms in a formula unit and the bonds between atoms as lines. Single bonds are one line, Double bonds are two lines. Triple bonds are three lines. The Lewis dot structure shows the number of valence electrons and types of bonds in the molecule.


Lewis dot structure, C2H6

ethane Lewis dot

Ball and stick mode, C2H6ethane ball and stick model

Lewis dot structure for water H2O

Electron pairs that are shared are physically between the symbols for the atoms.

Electron pairs that are unshared are called lone pairs.

Lone pairs are electron pairs not shared between atom symbols.


water dot formula