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19.14: Hybrid Orbitals - sp and sp²

Last updated

Sep 18, 2024
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- 19.13: Hybrid Orbitals - sp³
- 19.15: Sigma and Pi Bonds

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Figure

$\PageIndex{1}$ (Public Domain; Frank Dicksee via Wikipedia)

How do you open the closed circle?

Romeo and Juliet were two of the greatest-known fictional lovers of all time. Their embrace allowed no other person to be a part of it—they only wanted to be with each other. It took outside intervention to get them away from one another. Paired electrons are similar to Romeo and Juliet. They do not bond covalently until they are unpaired; then, they can become a part of a larger chemical structure.

sp Hybridization

A beryllium hydride $\left( \ce{BeH_2} \right)$ molecule is predicted to be linear by VSEPR. The beryllium atom contains all paired electrons and so must also undergo hybridization. One of the $2s$ electrons is first promoted to the empty $2p_x$ orbital (see figure below).

Diagram showing the transition of an electron from a 2p orbital to a 2s orbital. The electron, originally paired, moves to an upward position in the 2s orbital.

Figure

$\PageIndex{2}$ : Promotion of

$\ce{Be}$

$2s$ electron. (CC BY-NC 3.0; Joy Sheng via CK-12 Foundation)

Now the hybridization takes place only with occupied orbitals, and the result is a pair of $sp$ hybrid orbitals. The two remaining $p$ orbitals ( $p_y$ and $p_z$ ) do not hybridize and remain unoccupied (see figure below).

Diagram showing two orbitals: 2sp with two upward arrows, and two separate boxes labeled 2py and 2pz, both empty.

Figure

$\PageIndex{3}$ :

$\ce{Be}$ hybrid orbitals. (CC BY-NC 3.0; Joy Sheng via CK-12 Foundation)

The geometry of the $sp$ hybrid orbitals is linear, with the lobes of the orbitals pointing in opposite directions along one axis, arbitrarily defined as the $x$ -axis (see figure below). Each can bond with a $1s$ orbital from a hydrogen atom to form the linear $\ce{BeH_2}$ molecule.

Diagram illustrating molecular orbital theory: shows combination of atomic orbitals, with overlapping p orbitals forming bonding and antibonding interactions, and a linear molecular structure.

Figure

$\PageIndex{4}$ : The process of

$sp$ hybridization is the mixing of an

$s$ orbital with a single

$p$ orbital (the

$p_x$ orbital by convention), to form a set of two

$sp$ hybrids. The two lobes of the

$sp$ hybrids point opposite one another to produce a linear molecule. (CC BY-NC 3.0 (3D molecule available under public domain; Jodi So, using 3D molecule by Ben Mills (User: Benjah-bmm27/Wikimedia Commons via CK-12 Foundation; 3D molecule: http://commons.wikimedia.org/wiki/File:Beryllium-hydride-molecule-IR-3D-balls.png))

Other molecules whose electron domain geometry is linear and for whom hybridization is necessary also form $sp$ hybrid orbitals. Examples include $\ce{CO_2}$ and $\ce{C_2H_2}$ , which will be discussed in further detail later.

sp² Hybridization

Boron trifluoride $\left( \ce{BF_3} \right)$ is predicted to have a trigonal planar geometry by VSEPR. First, a paired $2s$ electron is promoted to the empty $2p_y$ orbital (see figure below).

Diagram showing the electron configuration change: a 2s orbital with an up and down arrow transitions to one up arrow, while a 2p orbital with one up arrow gains a second up arrow.

Figure

$\PageIndex{5}$ : Promotion of

$2s$ electron. (CC BY-NC 3.0; Joy Sheng via CK-12 Foundation)

This is followed by hybridization of the three occupied orbitals to form a set of three $sp^2$ hybrids, leaving the $2p_z$ orbital unhybridized (see figure below).

Diagram showing orbital hybridization with three electrons in 2sp² orbitals, each with an upward arrow. A separate unoccupied 2p orbital is to the right.

Figure

$\PageIndex{6}$ : Formation of

$sp^2$ orbital. (CC BY-NC 3.0; Joy Sheng via CK-12 Foundation)

The geometry of the $sp^2$ hybrid orbitals is trigonal planar, with the lobes of the orbitals pointing towards the corners of a triangle (see figure below). The angle between any two of the hybrid orbital lobes is $120^\text{o}$ . Each can bond with a $2p$ orbital from a fluorine atom to form the trigonal planar $\ce{BF_3}$ molecule.

A set of molecular orbital diagrams showing different hybridizations and shapes, alongside a 3D molecular model with spherical and lobed shapes connected by rods.

Figure

$\PageIndex{7}$ : The process of

$sp^2$ hybridization is the mixing of an

$s$ orbital with a set of two

$p$ orbitals (

$p_x$ and

$p_y$ ) to form a set of three

$sp^2$ hybrid orbitals. Each large lobe of the hybrid orbital points to one corner of a planar triangle. (CC-BY-NC 3.0 (3D molecule available under the public domain; CK-12 Foundation - Jodi So, using 3D molecular structure by Ben Mills (Wikimedia: Benjah-bmm27) via 3D molecule: http://commons.wikimedia.org/wiki/File:Boron-trifluoride-3D-balls.png))

Other molecules with a trigonal planar electron domain geometry form $sp^2$ hybrid orbitals. Ozone $\left( \ce{O_3} \right)$ is an example of a molecule whose electron domain geometry is trigonal planar, though the presence of a lone pair on the central oxygen makes the molecular geometry bent. The hybridization of the central $\ce{O}$ atom of ozone is $sp^2$ .

Summary

Paired electrons can be hybridized and then participate in covalent bonding.

Review

Does the ground state beryllium atom contain any unpaired electrons?
Why does one 2s electron in Be get promoted to a 2p orbital?
What is the geometry of the two sp orbitals?

How do you open the closed circle?

sp Hybridization

sp² Hybridization

Summary

Review

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