Point Groups: Symmetry Classification of Molecules and Examples

Each molecule has a set of symmetry operations that define the molecule’s overall symmetry. These sets of symmetry operations are known as point groups.

To understand point groups, it is important to have a general idea of symmetry elements and the operations that can be applied through them. 

A symmetry element is a geometrical entity such as a point, a line, or a plane about which a symmetry operation is carried out to generate an equivalent orientation.

A symmetry operation produces a change in the orientation of a molecule, but the new orientation is indistinguishable from the original. Hence the two orientations are known to be equivalent to each other.

Repeating the same symmetry operation a specified number of times will yield the original orientation, which will now be called identical to the original.

Symmetry Elements and Operations

The first of 5 symmetry elements is identity, represented by E. Its symmetry operation can be described as “doing nothing”. All objects possess this symmetry element, regardless of whether they have other symmetry elements or not. 

In case a molecule possesses only identity, it will be classified into the groups of low symmetry and its point group would be C₁.

Another symmetry element is proper rotation, and its symmetry operation is rotation about an n-fold axis at 360°/n. Proper rotation is represented by the symbol C_n. If a molecule has multiple rotation axes, the one with a higher n value will be the principal axis.

An example is boron trifluoride, BF₃, which has a threefold principal axis, C₃. This will allow rotation at 360°/3 = 120° to yield an equivalent orientation. Performing this operation of rotation 3 times will result in an orientation identical to the original one. 

Also that this C₃ axis in BF₃ is associated with two rotation operations, one in the clockwise, and the other in the anti-clockwise direction, so it is written as 2C₃.

BF₃, being planar, also possesses 3C₂ axes that allow rotation at 180° each. The point group of BF₃ is therefore D_3h.

The “h” in D_3h represents σ_h, which is another symmetry operation: reflection through a plane. This “h” refers to a horizontal plane (perpendicular to the principal axis). In fact, the symmetry element plane of reflection also includes σ_v and σ_d (parallel to the principal axis), referring to vertical and dihedral planes, respectively.

The fourth symmetry element is improper rotation, represented by S_n. The operation involves rotation about an n-fold C_n axis followed by reflection through a σ_h plane.

The center of inversion is the final symmetry element and is denoted by i. The corresponding symmetry operation is an inversion in 3 dimensions through a point. Coordinates x, y, z will be transformed to -x, -y, -z.

Point Groups

Symmetry elements are collectively used to classify molecules according to their symmetries. The name of a point group is determined by the symmetry elements it possesses.

Groups C₁, C_i, C_s

If a molecule has an identity (E) as the only symmetry element, it belongs to the C₁ point group. 

If it possesses a center of inversion (i) in addition to identity, it will belong to the C_i point group.

Moreover, it belongs to the C_s point group if it has an identity and a plane of reflection (σ_h).

Groups C_n, C_nh, C_nv, C_∞v

A molecule will belong to the C_n group if it has an n-fold principal axis as well as identity.

On the other hand, a molecule with a C_n principal axis and σ_h planes of reflection in addition to identity, belongs to the C_nh group.

Furthermore, molecules having a C_n principal axis, n σ_v planes of reflection, and identity belong to the C_nv point group. Ammonia, NH₃, possesses a C₃ principal axis and 3 σ_v planes of reflection, so it belongs to the C_3v point group. 

C_∞v point contains the symmetry elements E, 2C_∞ (clockwise and anti-clockwise), and ∞σ_v.

Groups D_n, D_nd, D_nh, D_∞h

A molecule will belong to the D_n group if it has a C_n principal axis, nC₂ axes perpendicular to the principal axis, and identity. 

If it additionally contains σ_d planes of reflection, then D_nd group instead.

On the contrary, having n horizontal mirror planes σh along with a C_n axis, nC₂ perpendicular axes, and identity, will classify the molecule into D_nh group. An example is benzene, C₆H₆. It has the symmetry elements E, C₆, 3C₂, 3C’₂, and σ_h, so it belongs to the point group D_6h.

Moreover, the D_∞h group includes the symmetry elements E,  2C_∞, ∞σ_v, i, 2S_∞, ∞C’₂.

S_n Group

Molecules that are not classified into any of the groups given above but possess an S_n axis belong to the S_n group. Group S₂ is the same as C_i, so it will already have been classified. Also that molecules with S_n for n > 4 are rare.

The cubic groups: Tetrahedral (T, T_d, T_h), Octahedral (O, O_h), Icosahedral (I, I_h)

Molecules of high symmetry, possessing more than one principal axis, belong to one of the cubic groups. 

Methane, CH₄, belongs to the tetrahedral point group T_d while sulfur hexafluoride, SF₆, belongs to the octahedral point group O_h. Some boranes and buckminsterfullerene, C₆₀ belong to the icosahedral (20-faced) group.

In fact, a flowchart can help assign the point groups to various molecules depending on the symmetry elements they possess.

Drawing a Point Group

A point group is drawn by first identifying all of the symmetry elements and operations that a molecule possesses.

Then these operations are applied to the mathematical functions i.e. orbitals and rotations, which results in either a +1 (symmetric) or a -1 (antisymmetric) value for each of the symmetry operations.

This allows us to write down the all the distinct irreducible representations, followed by allotting them Mulliken’s symbols:

1-dimensional irreducible representation is denoted by A or B.

2-dimensional irreducible representation is denoted by E.

3-dimensional irreducible representation is represented by T.

“A” refers to symmetric rotation about the principal axis, while “B” refers to antisymmetric rotation about the principal axis.

Symmetric or antisymmetric with respect to the secondary axis is denoted by subscripts “1” and “2” respectively. In case of the absence of secondary axes, σ_v planes are used instead.

Single and double prime (‘ and “) are used to denote the symmetric and antisymmetric representations of the horizontal plane, σ_h.

Finally, in groups with a center of symmetry, symmetric or antisymmetric with respect to the inversion center is indicated with the subscript “g” or “u”.

After assigning Mulliken’s symbols, reducible representations are determined.

Concepts Berg

What are point groups and examples?

There are five types of symmetry elements and their corresponding operations that result in similar orientations. Classifying objects using their set of symmetry elements and operations results in point groups.

There are various point groups: C₁, C_s, C_i are groups of low symmetry whereas O, T, and I are examples of high symmetry point groups. Some examples of intermediate symmetry groups include C_n, C_nv, C_nh, D_n, D_nd, D_nh.

How do you identify point groups?

Point groups are identified using the set of symmetry elements they each possess.

Let us take an example of the point group D_4h. The subscript “4” implies that there is a C₄ principal axis of rotation. Moreover, the “D” refers to the presence of a further 4 perpendicular C₂ axes of rotation (to the C₄ principal axis). Finally, the subscript “h” indicates a σ_h plane of reflection.

What are the two uses of point groups?

From the point groups of substances,  we can predict some of their physical properties in polarity and chirality.

How do you define a point group?

A point group is defined as the set of symmetry elements and their corresponding operations that define the molecule’s overall symmetry.

What is the point group of S₈?

The point group of octasulfur, S₈ is D_4d. This is because it has the symmetry elements and operations E, a C₄ principal axis (associated with 2 rotations: clockwise and counter-clockwise), a C₂ axis (parallel to principal C₄ axis), 4C₂‘ axes (perpendicular to principal C₄ axis), 4 σ_d planes of reflection, and finally one S₈ rotary-reflection axis.

What is the point group for CIS PT?

Cisplatin belongs to the point group C_2v. It contains the symmetry elements E, C₂, σ_v, and σ_v‘.

What is the CN^– point group?

Cyanide anion possesses the symmetry elements/operations E, 2 C_∞, and ∞ σ_v, so it belongs to the point group C_∞v.

What point groups are chiral / optically active?

A chiral molecule is one that cannot be superimposed upon its mirror image. As chiral molecules rotate plane-polarized light, they are said to be optically active.

The theory of optical activity states that a molecule may be chiral if it does not possess a rotary-reflection axis, S_n. However, it may not be directly identified.

For example, C_nh point groups contain C_n and σ_h, which are the components of S_n. Any molecule with a center of inversion also possesses S₂ (as i is equivalent to the combination of C₂ and σ_h). Hence all molecules with centers of inversion are also achiral.

So the point groups that do not contain the symmetry elements mentioned above are bound to be chiral.

What is the point group of BF₃?

The point group of BF₃ is D_3h. This is because BF₃ possesses a C₃ principal axis, 3 C₂ axes perpendicular to the principal axis, and a σ_h plane of reflection, in addition to identity.

What is the point group of H₂O₂?

H₂O₂ belongs to the point group C₂, as it possesses only a C₂ axis other than identity.

What is the point group of PdCl₄^-2?

PdCl₄^-2 has the symmetry elements E, 2C₄, C₂, 2C₂‘, 2C₂“, i, 2S₄, σ_h, 2σ_v, 2σ_d, so its point group is D_4h.

Reference Books

• Physical Chemistry | Fifth Edition, by P. W. Atkins (University of Oxford, Oxford, UK)

Reference links

• Symmetry (chemtube3d.com)