MOLECULES Part 5: VALENCE BOND THEORY
- WACADEMY
- Jan 1, 2024
- 4 min read
Updated: Jan 2, 2024
MOLECULES Part 5: VALENCE BOND THEORY
So far we learned about Lewis Theory, which talks about the interaction of valence electrons to form chemical bonding using Lewis structures.
^above slide pretty much sums up Lewis Theory, and what Lewis structures look like.
Now, we have 2 more chemical bond theories which need to be discussed:
Valence bond (VB) theory - will be discussed in this post
Molecular orbital theory
VALENCE BOND THEORY
^What to catch from above slide:
VB theory describes a COVALENT BOND in terms of atomic orbital overlap
A covalent bond generally forms through overlap of half-filled orbitals from two atoms
Again, we had so far described covalent bonding as sharing of electrons, but we can also say covalent bonding occurs via overlap of orbitals. They essentially mean the same thing.
Just like Lewis structures, we are only concerned with valence electrons when it comes to chemical bonding
^This shows an example of atomic overlap, which represents covalent bonding between two atoms. We will discuss the different types of orbitals (s, p, d, and f) later on.
^What to catch from above slide:
Valence orbitals can be determined through our electron configurations: as a general rule of thumb, the orbitals (usually s and p) with the highest principal shell (or energy) level is counted for the number of valence electrons.
^Notice how Li for instance, the highest energy level is 2s, and it has one valence electron as indicated by its corresponding exponent. Take F for example, it has 2s and 2p who have the highest energy shells, so there are 7 valence electrons for F as indicated by the sum of the exponents for 2s and 2p (which is 2 +5).
^What to catch from above slide:
The reason carbon is able to form 4 bonds is because based on the atomic orbital drawn in the top right corner, there are 4 more valence electrons it can accommodate for bonding. For instance, it can form covalent bonds with 4 Hydrogen atoms with their half-filled 1s^1.
Ground state simply means it is a neutral atom and does not have a charge. Excited state means the atom is not neutral and there is a charge
^The boxes represent "atomic orbitals" and you fill these boxes with the number of electrons present in the atom.
^Notice the "s" orbital can fill up to maximum 2 electrons, and "p" orbital can fill up to 6 electrons.
REVIEW OF ELECTRON CONFIGURATION
^What to catch from above slide:
First off, the Aufbau principle requires us to fill the electrons from lowest energy orbital to highest energy orbital. So, we would fill 1s --> 2s --> 2p --> 3s --> 3p --> 4s --> 3d --> 4p --> 5s --> and so on and so forth.
Hund's rule: orbitals of the same energy (n) are filled with an electron singly before being paired
HYBRID ORBITALS (HYBRIDIZATION)
^What to catch from above slide:
hybrid orbitals are used for better representation of orbital overlap between bonded atoms
Largely, we have "sp3", "sp2" and "sp", which are determined based on the number of electron groups surrounding the atom
^Let's look at carbon's 2s and 2p orbitals, which contain the 4 valence electrons. They can be combined to form hybrid orbitals known as "sp3" hybrid orbitals. There are a total 4 "sp3" orbitals which can accommodate a total of 8 electrons.
^All of the above molecules are "sp3" hybridized and that is because they have a total of 4 electron groups surrounding the central atom.
General rule of thumb is this:
Rule of thumb: hybridization (sp3, sp2, sp) depends on the number of electron groups (as seen above)
^The immediate answer is NO. Not all "p" orbital needs to be hybridized. For instance, the "sp2" hybrid orbital has one of the "p" orbital unhybridized, and "sp" hybrid orbital has two "p" orbitals unhybridized!
^What to catch from above slide:
If there are 3 electron groups surrounding the central atom (e.g. BF3), we use the "sp2" hybrid orbitals to represent the bonded atoms
As mentioned earlier, one "p" orbital remains UNHYBRIDIZED
Both "sp3" and "sp2" hybrid orbitals have more "p" character as there are more "p" orbitals than the "s" orbital
^What to catch from above slide:
If there are 2 electron groups surrounding the central atom (e.g. BeCl2), we use the "sp" hybrid orbitals --> recall that Beryllium is an exception to the octet rule where it is satisfied with only 4 valence electrons rather than 8.
The "sp" hybrid orbital only uses up one of the three "p" orbitals, meaning there are two "p" orbitals that remain unhybridized
^What to catch from above slide:
Multiple covalent bonds simply refer to double and triple bonds
If you look at C2H4, there are 3 electron groups, meaning we have "sp2" hybrid orbitals and one unhybridized "p" orbital. The hybrid orbitals are used to bond with two Hydrogen atoms and one other Carbon atom, and the one remaining unhybridized "p" orbital is used to form the pi bond, which gives rise to a double bond between Carbon and Carbon.
Also remember that sigma bond represents DIRECT ORBITAL OVERLAP, while a pi bond represents INDIRECT ORBITAL OVERLAP
^What to catch from above slide:
It shows C2H2, which has a triple bond
the "sp" hybrid orbitals are used to bond with one Hydrogen and one the other Carbon atom, and the two unhybridized "p" orbitals are used to form 2 pi bonds, which gives rise to a triple bond
1 pi bond = double bond. 2 pi bonds = triple bond
^Now if we have more than 4 electron groups surrounding the central atom, we need to recruit the "d" orbitals into the hybrid orbitals!
PCl5 has 5 electron groups around Phosphorus, meaning we need to recruit one orbital from "d" orbitals to give rise to "sp3d" hybrid orbitals (only "sp3d" can accommodate 5 electron groups like in PCl5) --> see below diagram for confirmation
Likewise, SF6 has 6 electron groups around Sulphur. We need to recruit TWO MORE orbitals from "d" orbitals to give rise to "sp3d2" hybriod orbitals (only "sp3d2" can accommodate 6 electron groups like in SF6) --> again see below for confirmation
Refer to this credible website for more info on hybridization: https://general.chemistrysteps.com/hybridization-of-atomic-orbitals/
PRACTICE PROBLEMS ON HYBRIDIZATION & HYBRID ORBITALS
ANSWER KEY
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