The steric number is one of the simplest and most reliable tools for predicting the hybridization and molecular geometry. It tells us how many atoms and lone pairs a given atom has, which in turn defines the geometry and the hybridization of this atom. For example, recall from General Chemistry how we determined the geometry of simple molecules:
We are looking at the central atom, marked in purple, and its steric number is the sum of the atoms and lone pairs it holds. Notice that multiple bonds do not matter – we only care about the number of atoms. For example, in HCN, the steric number of carbon is 2 even though it has a triple bond.
The good news in switching to Organic Chemistry is that we never exceed steric number 4, so geometries and hybridizations like trigonal bipyramidal, sp³d, and sp³d² are no longer needed. Therefore, let’s slightly modify the list of examples demonstrating how the steric number is determined:
Lone Pairs and Steric Number
As already mentioned, the steric number is the sum of the atoms and lone pairs of the given atom. What is important is that we do not count the number of electrons – it is the number of lone pairs. For example, the oxygen in water has two lone pairs and two hydrogens, therefore the steric number is 2 + 2 = 4, not 2 + 4.
Similarly, the steric number of nitrogen in ammonia is 4 (3 atoms + one lone pair), which indicates a tetrahedral electron geometry and trigonal pyramidal molecular geometry.
Remember, the difference between the electron geometry and the molecular geometry is that electron geometry includes all regions of electron density, while molecular geometry considers only the positions of the atoms. Therefore, in ammonia (NH₃), the steric number is 4, so the electron geometry is tetrahedral. However, one of those four regions is a lone pair. The lone pair is still there, and it occupies space even though it is not visible in the molecular shape. Because lone pairs repel more strongly than bonding pairs, it pushes the three N–H bonds downward, compressing the angles and giving NH₃ a trigonal-pyramidal molecular geometry.
If that lone pair were not present, the molecule would be trigonal planar, but the lone pair distorts the shape and defines the true geometry.
Why do we ignore the lone pair for naming the molecular geometry? One way to look at it is the fact that electrons are infinitely smaller and lighter than nuclei, and when looking at modern microscopes, we don’t see them.
Use this table to determine the electron and molecular geometry for all the combinations of atoms and lone pairs:
The Steric Number in Organic Molecules
We have mentioned that in organic molecules, we do not exceed a steric number of 4, so the list of electron and molecular geometries is shortened. However, the downside of this is that you will be working with larger molecules.
For example, there is no central atom in this molecule:
And the geometry is determined for each atom of interest. Let’s do it for the numbered atoms:
Oxygen 1 is connected to one atom and has two lone pairs, making it to SN = 3. Its electron geometry is trigonal planar, but it has a linear molecular geometry.
Carbon 2 has three atoms and no lone pairs, which is a steric number of 3. Therefore, its electron and molecular geometries are trigonal planar:
Oxygen 3 is connected to two atoms and has two lone pairs and just like in water, S.N. (O) = 2 atoms + 2 lone pairs = 4. Therefore, it has a tetrahedral electron geometry and a bent molecular geometry:
Carbon 4 is connected to three atoms, and has no lone pairs. SN = 3, which corresponds to a trigonal planar electron and molecular geometry.
Carbon 5 is connected to two atoms, no lone pairs, and this is SN=2, which is a linear electron and molecular geometry:
Notice again that we did not count the triple bond any differently than a single bond; it is atoms + lone pairs.
Steric Number and Hybridization
Steric number is a very reliable indicator for determining the hybridization of a given atom. The reason for this is that hybridization theory was developed to explain how atoms with a certain number of atomic orbitals can accommodate both bonding and nonbonding electrons, especially when there is a mismatch between the number of needed orbitals and the available atomic orbitals. Without going too much into the details, we simply need to recognize that the steric number tells us how many orbitals are required, and this directly determines the hybridization. Therefore, once you know how to determine the steric number from VSEPR theory, you only need to apply the following correlation:
If the steric number is 4, it is sp3
If the steric number is 3 – sp2
If the steric number is 2 – sp
So now, let’s go back to our molecule and determine the hybridization states for all the atoms.
There are, of course, some exceptions, mainly when the lone pair on the atom is delocalized over neighboring pi bonds, and you can read more about that in a separate post here.
Summarizing the Steric Number
The steric number is one of the simplest and most reliable tools for predicting the hybridization together with the electron and molecular geometry of an atom. It is calculated as the sum of the atoms bonded to the central atom and the lone pairs it possesses. Multiple bonds are counted the same as single bonds, and we never count individual electrons-only lone pairs matter.
- Electron geometry considers all regions of electron density around the central atom-this includes bonding pairs (atoms connected to the central atom) and lone pairs (nonbonding electrons). It tells us the overall arrangement of electrons in space.
- Molecular geometry, on the other hand, focuses only on the positions of the atoms themselves. For this, we “ignore” lone pairs when naming the shape, even though they still exist and influence the bond angles.
In organic molecules, the steric number rarely exceeds 4, which simplifies geometries and hybridizations to linear, trigonal planar, tetrahedral, bent, and trigonal pyramidal shapes.
