The steric number of a molecule is a tool used to illustrate a molecule's shape or geometric representation. The grading system for the numbers relates the different geometric shapes. For example, if the steric number is 1 then the geometry of the molecule is linear. The steric number is calculated by using the electrons of the central atom, or the atom around which the other atoms rotate, and the number of bonds of those rotating atoms. Learning to calculate the steric number correctly can help you better visualize the molecule.
Count the number of atoms bound to the central atom. For example, H20, or water, has two hydrogen atoms bound to the central oxygen atom.
Count the number of lone pairs of electrons of the central atom. These pairs of electrons orbit the central atom but are bonded to each other and are illustrated in atomic representations. For the example, the oxygen atom has two lone electron pairs.
Add the number of bonds and electron pairs together. For the example, an H2O molecule has two bonds and two pairs, resulting in four. The steric number of a molecule of water is 4.