Hybrid Orbitals/Hybridization
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Hybrid orbitals
What are they?
This is how we mix atomic orbitals on the same atom together to make hybrid orbitals. We mix together the outside, valence electrons. The number of atomic orbitals that mix together is the exact same number of hybrid orbitals that are created, always. So, if two atomic orbitals (2s and 2p) are mixed together, that will make two hybrid orbitals. Those hybrid orbitals would be called sp hybrid orbitals because they are made from one s and one p orbital. If three atomic orbitals (2s, 2p, 2p) are mixed together, that will make three hybrid orbitals. Those hybrid orbitals would be called sp2 hybrid orbitals because they are made from one s and two p orbitals. If four atom orbitals (2s, 2p, 2p, 2p) are mixed together, that will make four hybrid orbitals. Those hybrid orbitals would be called sp3 hybrid orbitals because they are made from one s and three p orbitals.
Their shapes
Like charges repel each other. Since these hybrid orbitals all contain negative electrons, they repel each other. This means the hybrid orbitals that are formed get as far away from each other as possible. If all four valence atomic orbitals (2s, 2p, 2p, 2p) are mixed, four sp3 hybrid orbitals are made. When these four orbitals get as far away from each other as they can, they form a tetrahedral shape with an angle of 109.5° between them. This is what happened in our methane, CH4, example earlier. If three of the four valence atomic orbitals are mixed (2s, 2p, 2p), then three sp2 hybrid orbitals are made. When these three orbitals try to get as far away from each other as they can, they form a trigonal planar shape with an angle of 120° between them. If only two of the four valence atomic orbitals mix (2s, 2p) then two sp hybrid orbitals are made. When these two hybrid orbitals try to get as far away from each other as they can, they form a linear shape with an angle of 180° between them.
Geometry of hybrid orbitals
The unhybridized p orbitals
We are almost there, but this is not the complete picture. Most of the important organic chemistry atoms (outside of hydrogen) start with one s and three p valence atomic orbitals—four total. So, if we start with four atomic orbitals, we must end up with four total orbitals in the end, always. We must not forget about the atomic orbitals that are not mixed together to make the hybrids. When we make sp2 hybrid orbitals, we mix three out of the four orbitals, the s, and two p orbitals to make three sp2 hybrid orbitals. There is a p atomic orbital left over, untouched, that is not mixed at all! It is unchanged. We call it an unhybridized orbital because, well, it’s not hybridized. It is still on the atom as a regular old p orbital. When we make sp hybrid orbitals, we mix two out of the four atomic orbitals, the s and p to make two sp hybrid orbitals. There are two p orbitals that remain untouched or unhybridized.
The unhybridized p orbitals with hybrid orbitals
What is the hybridization of an atom?
How does one determine what the hybridization (sp, sp2, or sp3) is for an atom? You need to count how many electron groups are on the atom. Lone pairs of electrons, even though they don’t point to an atom, count as electron groups since they do take up space. They must be in a hybrid orbital. Additionally, single bonds, double bonds and triple bonds each count as only one electron group since they each point to one atom. Alternatively, you can count the atoms at the ends of those single, double, or triple bonds.
We can determine the hybridization for an atom after we have counted the number of electron groups on it. If two electron groups are on an atom, it needs two hybrid orbitals – one for each electron group. Such an atom has sp hybridization because sp hybridization is when one s and one p orbital overlap to make two sp hybrid orbitals. If three electron groups are on an atom, it needs three hybrid orbitals. Such an atom has sp2 hybridization because sp2 hybridization is when one s and two p atomic orbitals mix to make three sp2 hybrid orbitals. If four electron groups are on an atom, the atom has sp3 hybridization because sp3 hybridization is when one s and three p atomic orbitals mix to make four sp3 hybrid orbitals.
Let’s look at an example, the hybridization of the carbon and oxygen atoms of formaldehyde. Let’s analyze the oxygen atom first. We notice that the oxygen atom has two lone pairs of electrons (1 and 2). We always need to remember that electron pairs take up space and need to go into a hybrid orbital. The oxygen atom also has one atom attached (the carbon on the double bond labeled 3). This is a total of three electron groups, so the oxygen atom needs three hybrid orbitals. It is therefore sp2 hybridized (one s and two p orbitals gives three sp2 hybrid orbitals). When we analyze the carbon atom’s hybridization, we notice there are no electron pairs of electrons on the carbon, but there are three atoms attached to it. These three atoms are the oxygen atom at the end of the double bond (1) and the two hydrogen atoms (2 and 3). With three electron groups on the carbon atom, it is also sp2 hybridized.
The unhybridized p orbitals with hybrid orbitals
Hybrid orbitals
Hybridization of atoms in some common bonding patterns
For neutral carbon atoms in most organic chemistry molecules, if it contains only single bonds, it is sp3 hybridized. If a carbon atom contains one double bond, it is sp2 hybridized. If a carbon atom contains one triple bond, it is sp hybridized. Below are common hybridizations and show how to count the electron groups.
Common hybridization bonding patterns
1. Identify the hybridization on the central atoms of the amino acid, histidine.
Answers
1. Identify the hybridization on the central atoms of the amino acid, histidine.
