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# Steric number

One way to determine the hybridization of an atom is to calculate its steric number, which is equal to the number of sigma bonds surrounding the atom plus the number of lone pairs on the atoms. In this video, we focus on atoms with a steric number of 4, which corresponds to sp³ hybridization. Created by Jay.

## Want to join the conversation?

• when do we have a pi bond?
• a pi bond is when there is a double or triple bond between atoms.
single bond = 1 sigma bond
double bond = 1 sigma bond + 1 pi bond
triple bond = 1 sigma bond + 2 pi bonds
• since lone pairs of electrons don't contribute in the bonding, why do they form hybrid orbitals?
• I think by thinking of the lone pairs as being in the hybrid orbitals, you are able to come up with the correct molecular geometry that is close to the bond angles that have been experimentally observed.
• So when the Steric number of an element equals 4, then the element MUST BE sp3 hybridized?
• have you considered the halogens commonly known as the group 17 elements? those are not hybridised, yet they have an SN of 4 due to their 3 lone pairs ofelectrons
• Shouldn't the bond angle of water be approximately 104.5?
• Yup! And for anyone wondering how to get this:
A tetrahedral structure (e.g. methane) will have bond angles of 109.5 degrees. For every lone pair of electrons that exists in the central atom, you subtract 2.5 degrees. The oxygen in water has two lone pair of electrons, and so it will have 109.5 - 2*(2.5) = 104.5 degree bond angle.
• What is the meaning of steric?
• The word steric comes from the Greek word stereos, meaning solid.
In chemistry, it refers to the directions in which the bonds in a molecule are pointing in space.
• I am confused why water is sp3 hybridised. I thought there are only 1 sigma bond and 1 pi bond, so shouldn't it be a sp hybridised bond?
• Good guess, but as Jay said in the video, the oxygen in water actually has two sigma bonds (one with each attached hydrogen) and then two lone pairs of electrons.

If it had a pi bond, we would find a double bond in water, which would change its chemistry quite drastically!

I hope this helps :)
• Why Nitrogen needs sp3 Hybridization to form ammonia? Why doesn't the 2s orbital have 2 electrons and the 2p bond with those 3 hydrogens
• If the 2s orbital bonds with a hydrogen the bond will be shorter than the bonds formed by 3 2p orbitals with hydrogen. If this was the case, ammonia would have 3 long bonds and a short bond. However, when the shape of ammonia was discovered, it was clear that all N-H bond lengths were equal, hence the idea of 2s and 2p orbitals forming individual bonds with the hydrogen was discarded and hybridization was able to explain the shape of ammonia.
• Why does nitrogen in ammonia have to be hybridized? It has three halfway filled p orbitals, couldn't it simply form bonds with them without hybridization?
• This is a great question that goes into the details of molecular orbital theory and chemical bonding.

When a nitrogen atom forms an ammonia molecule, it doesn't use its three half-filled 2p orbitals separately to form bonds. Instead, it "hybridizes" them with its 2s orbital to form four "sp3" hybrid orbitals.

Here's why this happens:

- *Geometry*: The geometry of an ammonia molecule (NH3) is trigonal pyramidal, not trigonal planar. If nitrogen used its three 2p orbitals as they are, we would expect the bonds to be 120 degrees apart, resulting in a planar molecule, which is not the case. However, when nitrogen uses sp3 hybrid orbitals to bond, it can achieve the tetrahedral electron-pair geometry (with bond angles about 109.5 degrees) that matches the observed geometry.

- *Overlapping*: When it comes to forming covalent bonds, the efficiency of orbital overlap is crucial. Hybrid orbitals (sp3 in this case) can form stronger, more stable overlapping (and thus bonds) than the pure s and p orbitals.

- *Energy*: In order to have the most stable (lowest energy) configuration, atoms will hybridize their orbitals to allow for maximum bonding. In the case of nitrogen in ammonia, this means hybridizing to have four equivalent sp3 orbitals, three of which are used for bonding with hydrogen atoms, and one of which holds the lone pair of electrons.

So, while it may seem as if nitrogen could use its unhybridized orbitals to form bonds, the reality is that hybridization allows for stronger bonds, a more stable molecule, and a geometry that fits with what we observe in reality.
• What is VSCPR theory at
• It is VSEPR theory(Valance Shell Electron Pair Repulsion Theory). It is used to understand the geometry of molecules.
Check This Out!!
http://en.wikipedia.org/wiki/VSEPR_theory
• This may sound silly.
Don't electrons in a bond repel each other the way they repel other electron bond pairs together?

What keeps them together?
• Yes, electrons in a bond do repel each other, but the attractions of the positive nuclei are strong enough to hold them both in the bond.