Orbital hybridization

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Orbital hybridization refers to the intermixing of atomic orbitals. It is an extension of the valence bond theory (VBT) of chemical bonding. Orbital hybridization plays a significant role in explaining the changes taking place during covalent chemical bonding.

How different types of atomic orbitals combine to form mixed orbitals that are on the same page to form a new chemical bond is an intriguing concept. Keeping that in mind, we will explain everything you need to know about orbital hybridization in detail in this article.

So, what are you waiting for? Dive into the article and continue reading!

What is orbital hybridization- Definition

The concept of orbital hybridization was introduced by Linus Pauling and Slater in 1931. Hybridization is defined as the linear combination of atomic orbitals (LCAO) to produce a set of new hybrid orbitals. Hybridization occurs during chemical bonding, i.e., it facilitates orbital overlap. Once a new chemical bond is formed, the atomic orbitals regain their individuality. N number of atomic orbitals combine to form N number of hybrid orbitals (where N is an integer). All the hybrid orbitals are unidirectional and possess the same shape and energy.

Image by Ammara W.

Basic rules of orbital hybridization

  • The number of hybrid orbitals formed in a set is equal to the total number of atomic orbitals hybridized.

For example, one s and one p-orbital combine to form two sp hybrid orbitals. Similarly, one s and three p atomic orbitals combine to form four sp3 hybrid orbitals.

  • The hybrid orbitals are named based on the number and kind of atomic orbitals hybridized.

For instance, an sp3 hybrid orbital is formed by the combination of an s and three p atomic orbitals thus, the name sp with superscript 3 is given.

  • Atomic orbitals of comparable energy mix to produce hybrid orbitals of equal energy.

For example, one s and one p atomic orbital of the same shell, let’s say 2s and 2p, combine to give two new same energy sp hybrid orbitals.

  • The hybrid orbitals possess the same shape.

For example, an s-orbital is spherical in shape, while a p-atomic orbital is dumbbell shaped. Two sp hybrid orbitals formed by the intermixing of an s and a p-orbital possess the same shape, which is neither spherical nor dumbbell shaped but a mixture of two in the ratio 1:1 (or 50% each).

  • The hybrid orbitals can possess the same or a different number of electrons.
  • Orbital hybridization is a bonding model. So hybridization only takes place during chemical bonding. Likewise, there is no existence of hybrid orbitals in the free atoms or once the molecule is formed.

Different types of orbital hybridization with examples

There are seven main types of orbital hybridization in covalent chemical bonding, namely:

  • sp hybridization
  • sp2 hybridization
  • sp3 hybridization
  • dsp2 hybridization
  • dsp3 hybridization
  • d2sp3 or sp3d2 hybridization
  • sp3d3 hybridization

Let’s talk about each of the above types, one at a time.

sp hybridization

sp hybridization refers to the intermixing of an s and a p atomic orbital to produce two sp hybrid orbitals. Each sp hybrid orbital exhibits a 50% s-character and a 50% p-character.

As per the VSEPR concept, covalent molecules in which the central atom is sp hybridized possess a linear shape. The bonded atoms form a mutual bond angle of 180° in such types of molecules. sp-hybridization is also known as diagonal hybridization.

An application of sp hybridization lies in understanding the formation of the BeF2 molecule. Beryllium (Be) lies in Group II A (or 2) of the Periodic Table. Its electronic configuration is 1s2 2s2. It has 2 valence electrons available for bonding. Both F-atoms are halogens that need one more valence electron to complete their octet.

During chemical bonding, the Be atom gets excited, and one 2s electron of beryllium shifts to an empty 2p orbital. The 2s and 2p atomic orbitals of beryllium then combine to produce two sp hybrid orbitals, each containing a single electron.

These equivalent sp hybrid orbitals fully overlap with the p orbitals of an adjacent F-atom to form the respective Be-F bond in a linear BeF2 molecule, as shown below.

sp2 hybridization

The intermixing of one s atomic orbital and two p atomic orbitals results in sp2 hybridization. Three equivalent sp2 hybrid orbitals are formed, each possessing a 33.3 % s-character and a 66.7 % p-character. It is also known as trigonal hybridization.

The molecule having an sp2 hybridized central atom adopts a trigonal planar electron geometry, while its molecular geometry or shape could either be trigonal planar or bent/ V-shaped.

Examples include BF3 and SO2.

The electronic configuration of boron (B) is 1s2 2s2 2p1. Upon excitation, one 2s electron of boron shifts to its empty 2p atomic orbital. One 2s and two half-filled 2p orbitals of boron then combine to produce three sp2 hybrid orbitals.

Each sp2 hybrid orbital has one electron. It overlaps with a p atomic orbital of fluorine (F) to form the B-F sigma bonds in the trigonal planar BF3 molecule.

Contrarily, in SO2, two sp2 hybrid orbitals of sulfur overlap with the p-orbitals of oxygen atoms to form the S=O sigma bonds. However, the third sp2 hybrid orbital contains a lone pair of electrons. Therefore, the molecule has a bent or V-shape.

sp3 hybridization

Four sp3 hybrid orbitals are formed by the intermixing of one s atomic orbital with three p orbitals. Each sp3 hybrid orbital possesses a 25 % s-character and a 75 % p-character, respectively. It is also referred to as tetrahedral hybridization.

Examples include CH4, NH3, H2O, etc.

In all the above examples, the central atom has sp3 hybridization. The electronic configuration of a carbon (C) atom is 1s2 2s2 2p2. One 2s electron of carbon shifts to an empty 2p orbital. The 2s and three 2p orbitals mix to produce four sp3 hybrid orbitals. These sp3 hybrid orbitals overlap to form the sp3-s C-H sigma bonds on each side of CH4.  All four C-H bond lengths are equal. Conversely, each H-C-H bond angle is equal to 109.5° in CH4.

If one of the four sp3 hybrid orbitals contains a lone pair of electrons, then the molecule adopts a trigonal pyramidal shape such as NH3.

If two of the four sp3 hybrid orbitals contain unbonded or lone pairs of electrons, no orbital overlapping occurs with these two. The molecule consequently possesses a bent shape, such as H2O. In either case, there should be a total of 4 orbitals or 4 regions of electron density around the central atom for it to be sp3 hybridized.

dsp2 hybridization

One s atomic orbital, and two p atomic orbitals (px and py) of the outermost shell (n) combine with a d (dx2-y2) atomic orbital of the (n-1) shell to form four dsp2 hybrid orbitals. It is also known as square planar hybridization.

dsp2 is specifically important in transition metal complexes such as [Ni(CN)4]2+. Learn more about that in our article, crystal field theory (CFT).

dsp3 hybridization

dsp3 hybridization occurs when an s orbital, three p orbitals and one d atomic orbital (dz2) of the outermost shell combine to form five dsp3 hybrid orbitals. The five dsp3 hybrid orbitals are subdivided into two groups. One group consists of two equivalent oppositely directed orbitals that occupy axial positions. In contrast, the three equivalent orbitals of the second group attain equatorial positions in a trigonal bipyramidal molecule.

In PCl5, the central P-atom is dsp3 hybridized and it has a trigonal bipyramidal shape.

SF4, ClF3 and XeF2 are also formed by the dsp3 hybridization of the central atom. However, these three possess different shapes due to a differing number of lone pairs present on the central atom, as per the VSEPR concept.

sp3d2 hybridization

One s, three p and two d orbitals (dz2 and dx2-y2) of an atom combine to form six d2sp3 or sp3d2 hybrid orbitals. The six sp3d2 hybrid orbitals are directed such that they lie at six corners of a regular octahedron to form the required sigma bonds by overlapping with the concerning orbitals of other atoms.

An example of d2sp3 hybridization is SF6. It has an octahedral shape, and the central S-atom is sp3d2 hybridized in it.

sp3d3 hybridization

Seven sp3d3 hybrid orbitals are formed by the intermixing of one s atomic orbital with three p orbitals and three d orbitals (dxy, dyz and dxz) of an atom. The hybrid orbitals overlap with the concerning orbitals such that the molecule adopts a pentagonal bipyramidal shape.

An example of sp3d3 hybridization is IF7. The central I-atom is sp3d3 hybridized. It forms the I-F sigma bonds by overlapping with the p-orbitals of seven fluorine atoms in IF7.

The discussion above tells us that hybridization is also important in determining the shapes of molecules.

Another point to remember before we proceed forward is that the orbital hybridization concept only applies to sigma bond formation. Pi bonds are always formed by the side-by-side overlapping of unhybridized orbitals.

What is the difference between hybridization and bond formation

Hybridization is a model that proposes the changes taking place at an atomic level during chemical bonding. In addition to hybridization, several different models can be used to understand the vast topic of chemical bond formation. The other models include VSEPR theory, VBT and MOT.

Orbital hybridization practice problems

Let’s find out how much you have learned through this article and whether you can determine the hybridization present in a compound through these practice problems.

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