In this article, we shall cover orbitals and orbital hybridisation. Hybridisation is a key concept within chemistry, as it underpins the dynamics of bonding as well as the shapes of molecules. Orbitals, on the other hand, are another key topic to consider regarding the arrangement of electrons within an atom. Ever since the shift was made from the concept of electrons being arranged in shells to orbitals, chemistry has developed immensely.
- We shall go over the definition of orbitals and hybrid orbitals, as well as the different types of orbitals.
- We'll then look at the different types of bonds created by the orbitals.
- We'll learn how to write out orbitals and the bonds they create.
- We'll look at the shape of hybrid orbitals, in the context of their molecular shape.
- Finally, we'll cover some key examples of orbitals creating bonds and how hybrid orbitals provide shape to molecules.
Meaning of Hybrid Orbitals
Here we will cover what orbitals are and the different types of orbitals. Then, we will cover how they come together and how hybrid orbitals are formed.
Atomic Orbitals
So what are orbitals?
An orbital is a mathematical function of the probability of the location of an electron within the atom.
The basic definition of the orbital tells us that it is associated with defining the parameters of electrons within an atom. There are several types of orbitals which we will cover here.
There are 4 main types of atomic orbitals, the shapes and properties of which we will describe below. It is key to remember that each orbital can house 2 electrons. Each electron must be in an opposing spin state due to Pauli's exclusion principle.
Shapes and Types of Orbitals
There are 4 types of orbitals as mentioned above, and each one has its own shape which is determined by the amount of lobes present in the orbital. Some orbitals have a specific shape, yet require multiple of them to come together to form an energy level.
| Orbital | Amount of lobes (and shape) | How many present |
| s | 1 (spherical) | 1 |
| p | 2 (dumbbell) | 3 |
| d | 3 or 4 | 5 |
| f | variable | 7 |
Below you will find an image of the first three types of orbitals: s, p, and d. You don't need to concern yourself with the shape and nature of f orbitals. The different lobes that orbitals can have are clearly shown in the diagram. Remember that for p and d orbitals, multiples of these orbitals come together at different axes.
Shapes of orbitals s, p, and d. Source: Toppr.com.
A key thing to point out is that s orbitals occur in all energy shells, yet p orbitals only occur from the second shell onwards. Likewise with higher orbital shapes, they are only added in higher shells. This is due to the fact that additional energy levels are able to contain more electrons and additionally, electrons in different configurations. For example, the d orbital is introduced only in the third shell, and the f orbital only in the fourth shell of the atom.
Orbitals in Bonding
Here we will cover the different types of bonds created by the overlap of orbitals. This comes from the theory of orbitals when they are applied to bonding. Instead of thinking of bonding in the instance of electrons, here we will consider bonding in terms of orbitals and how they can produce different types of bonds from their overlap.
Sigma bonds
Sigma (σ) bonds come from the overlap of single orbital lobes along the axis connecting the two bonding nuclei. Here the single electron lobes overlap and produce a single bond effectively. This can be either two s orbitals overlapping, an s and p orbital overlapping, or two p orbitals overlapping with a single lobe from each orbital. p orbital overlap that results in sigma bonds occurs only when both p orbitals are positioned horizontally on their axis.
Take a look the diagram below to visualise how sigma bonds are created by the overlap of orbitals.
Overlap of orbitals to produce sigma bonds. Source: Lawrie Ryan, Cambridge A Level Chemistry Coursebook, 2014
Localised Pi Bonds
Pi (π) orbitals form from the overlap of multiple lobes of two adjacent orbitals. Here this includes the overlap of p orbitals when they are in the vertical position axially. The top and bottom lobes overlap with each other perpendicular to the axis connecting the two bonding nuclei, so above and below the sigma bond. This is depicted in the diagram below:
Formation of pi bonds through the overlap of two vertical p orbitals. Source: Lawrie Ryan, Cambridge A Level Chemistry Coursebook, 2014
One key aspect of this type of bonding is that it creates a pi bond that is localised and fixed in shape. This means that the bonds created are stabilised in orientation in the molecule.
Delocalised Pi Bonds
There are pi bonds that are delocalised in terms of the electrons. Delocalisation of electrons through pi bonds occurs in large ring structures that allow for the overlap of multiple p orbitals. Take a look at the diagram below.
Delocalised electrons from pi bonds. Source: Lawrie Ryan, Cambridge A Level Chemistry Coursebook, 2014
It is important to remember that because of these large ring structures, the p orbitals are able to form large ring structures above and below the ring. These allow for the delocalisation of electrons, which creates clouds of electronegative density, and give specific chemical and physical properties to these ring structures.
Hybrid Orbitals
Hybrid orbitals form when different orbitals in a single atom combine together to produce a hybrid orbital for bonding purposes.
There are three types of hybrid orbitals which are sp, sp2, and sp3. These hybrid orbitals are comprised of different numbers of s and p orbitals combining in different ways. Crucially, these allow for the atom to bond differently depending on how the electrons are arranged in energy levels, which allows the resulting molecules to take on the shape that they do. These hybridised orbitals allow molecules to have distinct molecular geometries based on the VSEPR theory.
Equation and Formula for Orbital Hybridisation
Here we will discuss the different types of hybrid orbitals and how they form.
Hybrid orbitals form from the combination of s and p orbitals. For example, all rely on combining the existing s and p orbitals in an atom to create a hybrid orbital. Hybridisation of orbitals often occurs to bond to other elements in special orientations and geometries.
sp3 hybridsation
sp3 hybridisation relies on creating an atom which has 4 possible interacting electron domains for bonding. This is created by combining s and p orbitals which go through excitation and hybridisation.
Take a look at the diagram below which shows how the s and 3 p orbitals of an atom come together to produce a hybrid orbital.
Hybridisation of an s and 3p orbitals into an sp3 hybrid orbital. Source: Opentextbc.ca
In the diagram above, can you see how two different types of orbital can hybridize into a different type of hybrid orbital having an energy level between the two original ones? Here the hybridised sp3 orbitals allow for four different atoms to bond with the orbital. In the unhybridised scenario, the half filled p orbitals do not allow for the appropriate geometry and bonding, yet in sp3 hybridisation, tetrahedral geometry of the atom is allowed.
sp and sp2 Hybridisation
Other types of hybridised orbitals include sp2 hybridised orbitals which allow for three atoms to bond, creating a trigonal planar molecular geometry, and the sp hybridised orbital which allows for two atoms to bond, usually with multiple bonds to satisfy the electronic configuration within the atom, ultimately creating a linear molecule. This way, combining the s and p orbitals allows for molecules to adopt the right shape, and for atoms to bond to each other.
This is due to the fact that sp hybridised orbitals combine a single s and a single p orbital, while sp2 hybridised orbitals combine one s orbital and 2 p orbitals (hence sp2).
Examples and shapes of these hybridized orbitals will be shown in the next section.
Chart for Hybrid Orbitals
Here we will go over the shape of hybrid orbitals and some common trends that arise.
Take a look at the chart below which shows the hybridisation of orbitals, as well as the number of bonding lobes present and the geometry that the molecule adopts. In the chart, only sp, sp2, and sp3 orbitals are depicted, yet there are more hybrid orbitals that can be present.
Hybrid Orbitals Chart. Source: courses.lumenlearning.com
The chart above shows the hybrid orbitals of s and p orbitals, which form sp, sp2, and sp3 hybridized orbitals. The molecular geometries that the atom adopts during bonding is shown, with sp being linear, sp2 being trigonal planar, and sp3 being tetrahedral.
These geometries are only possible by combining atomic orbitals, as without hybridisation, the distinct shapes of molecules would not be able to form. Crucially, hybridisation relies on a partial excitation, which allows for alternative bonding shapes to be formed.
Can you tell how with every increase in hybridisation another axis of possible bonding is added? Think of the amount of lobes available for bonding to correspond with the amount of letters present in the hybridised orbital.
Such as:
sp = 2 orbitals = 2 lobes
sp2 = 3 orbitals = 3 lobes
sp3 = 4 orbitals = 4 lobes
Examples of Hybrid Orbitals
Here we will cover some examples of orbitals and hybrid orbitals in molecules.
Examples of Molecular Bonds from Orbital Overlap
There are a few examples to clearly show how different orbitals overlap in different ways to produce different types of bonds. Below are some key examples:
- H2: The simplest bond in the hydrogen-hydrogen molecule is a single bond that forms from the overlap of two s orbitals, creating a sigma bond.
- C2H6: the carbon-carbon bond here is a single bond, which relies again on the overlap of sp3 orbitals, creating a sigma bond. The 6 carbon-hydrogen bonds are a result of overlap between carbon's sp3 orbital and hydrogen's 1s orbital.
- C2H4: the carbon bond is a double bond, which means it also uses p orbitals to create an additional pi bond. This is due to the fact that the carbons are sp2 hybridised and the sigma bond is formed as a result of overlap of 2 sp2 hybrid orbitals whilst the pi bond is formed as a result of overlap of the two perpendicular unhybridised p orbitals.
- C2H2: the carbon bond in ethyne is a triple bond that forms from two p orbitals bonding through two additional pi bonds along with a sigma bond. The triple bond comes from the fact that the carbons are sp hybridised and the sigma bonds between the carbon atoms are formed as a result of overlap of these 2 sp orbitals, with 2 pi bonds forming from overlap of the remaining 4 unhybridised p orbitals.
- N2: Here the triple bond between two nitrogens also relies on multiple orbital overlap to create two pi bonds and sigma bond. Nitrogen dimolecule is able to hybridise into sp hybrid orbitals to bond with itself through a sigma bond. This results in each Nitrogen atom being sp hybridised, which creates the right amount of unhybridised orbitals for the triple bond to arise.
- HCN: The bonding in hydrogen cyanide (HCN) occurs through formation of a triple bond between carbon and nitrogen. Both atoms in this case are sp hybridised, which allows for the linear shape of the molecule to form and also for the triple bond to arise. The Hydrogen atom in this molecule is not hybridised.
Examples of Hybrid Orbitals in Molecules
Some examples that concern hybrid orbitals are seen in the effects of 3D orientation of the molecule. Take carbon for example, which can hybridise into sp3 hybrid orbitals. This allows for the tetrahedral conformation of carbon to form, as seen in the example of CH4. Methane has a distinct tetrahedral molecular geometry which allows its bonds to be spread as far apart as possible, with a bond angle of 109.5°. This is due to the hybridisation of orbitals, as without it the tetrahedral bonding of carbon would not be possible.
Hybridisation is also seen in elements which are not specific to carbon or organic chemistry, such as nitrogen, as explained above.
Orbitals and Hybridisation - Key takeaways
- Orbitals are clouds of electron probability around the nucleus.
- The most common types of orbitals are s, p, d, and f
- Orbital overlap dictates the bond created.
- sigma bonds and pi bonds determine the structure of molecules.
- Hybrid orbitals arise from combining different orbitals (by excitation) for bonding purposes.
- Hybrid orbitals give shape to molecules.
- sp3 hybridised orbitals have a tetrahedral shape
- sp2 hybridised orbitals have a trigonal planar shape
- sp hybridised orbitals have a linear shape
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