Steric Number Definition and Calculations

What the Steric Number Is and How To Determine It

Sulfur tetrafluoride has a steric number of 5.
Sulfur tetrafluoride has a steric number of 5. Ben Mills

Steric number is the number of atoms bonded to a central atom of a molecule the number of lone pairs attached to the central atom.

The steric number of a molecule is used in VSEPR (valence shell electron pair repulsion) theory to determine the molecular geometry of a molecule.

How to Find the Steric Number

Use the Lewis structure to determine the steric number. The steric number gives the electron-pair arrangement for the geometry that maximizes distance between valence electron pairs. When the distance between valence electrons is maximized, the energy of the molecule is at its lowest state and the molecule is in its most stable configuration. The steric number is calculated using the following formula:

Steric Number = (number of lone electron pairs on the central atom) + (number of atoms bonded to the central atom)

Here's a handy table that gives the bond angle that maximizes separation between electrons and gives the associated hybrid orbital. It's a good idea to learn the bond angle and orbitals, since these appear on many standardized exams.

S# bond angle hybrid orbital
4 109.5° sp3 hybrid orbital (4 total orbitals)
3 120° sp2 hybrid orbitals (3 total orbitals)
2 180° sp hybrid orbitals (2 total orbitals)
1 no angle s orbital (hydrogen has an S# of 1)
Steric Number and Hybrid Orbital

Steric Number Calculation Examples

  • Methane (CH4) - Methane consists of carbon bonded to 4 hydrogen atoms and 0 lone pairs. Steric numer is 4.
  • Water (H2O) - Water has two hydrogen atoms bonded to oxygen and also 2 lone pairs, so its steric number is 4.
  • Ammonia (NH3) - Ammonia also has a steric number of 4 because it has 3 hydrogen atoms bonded to nitrogen and 1 lone electron pair.
  • Ethylene (C2H4) - Ethylene has 3 bonded atoms and no lone pairs. Note the carbon double bond. Steric number = 3.
  • Acetylene (C2H2) - The carbons are bonded by a triple bond. There are 2 bonded atoms and no lone pairs. Steric number = 2.
  • Carbon Dioxide (CO2) - Carbon dioxide is an example of a compound that contains 2 sets of double bonds. There are 2 oxygen atoms bonded to carbon, with no lone pairs, so the steric number is 2.

Shape Versus Steric Number

Another way to look at molecular geometry is to assign the shape of the molecule according to steric number:

SN = 2 is linear

SN = 3 is trigonal planar

SN = 4 is tetrahedral

SN = 5 is trigonal bipyramidal

SN = 6 is octahedral

Steric Number Key Takeaways

  • In chemistry, a molecule's steric number is the number of atoms bonded to the central atom the number of lone electron pairs surrounding the central atom.
  • The steric number is used in VSEPR theory to predict molecular geometry.