CONNECTIVITY OR BONDING SEQUENCE
The term connectivity, or bonding sequence, describes the way atoms are connected together, or their bonding relationships to one another, in covalent compounds. For example, in the methane molecule one carbon is connected to four hydrogen atoms simultaneously, while each hydrogen atom is connected to only one carbon. No hydrogen atoms are connected together. In complex molecules the complete connectivity map is given by structural formulas (see below).
TYPES OF FORMULAS
The simplest type of formula for a compound indicates the types of atoms that make it up and their numbers. This is called a molecular formula. Examples of molecular formulas are BH3, C6H6, or C3H5ClO. Chemical catalogs such as the Aldrich catalog, scientific manuals, and databases such as Chemical Abstracts typically contain molecular formula indices to help locate substances whose elemental makeup is known.
Condensed structural formulas give some idea of the connectivity, but are still largely abbreviated. For example the ethane molecule, which has molecular formula C2H6 can be represented by the condensed formula CH3CH3, This at least tell us that each carbon is connected to three hydrogen atoms, and that two carbon atoms are connected together.
Lewis formulas are a second type of structural formulas. They give the most complete representation of the connectivity that is possible in two dimensions. The three types of formulas mentioned so far are shown below for the ethane molecule.