Molecular Geometry: A Complete Overview

Molecular geometry can be defined as the arrangement of atoms of molecules in a 3-dimensional space. This gives a proper shape to molecules. There are some advanced techniques that help to understand the molecular geometry of molecules. For example, x-ray diffraction and spectroscopy. These techniques give data with great accuracy.

Basic knowledge about bonding in molecules helps to understand their geometries such as Lewis theory, VSEPR theory, and hybridization theory. These theories about bonding though give incomplete knowledge but are still very useful. They explain the electronic structure of molecules. The bonding sense is really important to figure out the reactivity, polarity, and physical properties of molecules.

Localized Bonds

The localized bond approach is a simple and sophisticated bond that is easy to draw on paper. Covalent bonds are explained with a combination of theories of bonding. To determine the molecular shapes or geometry, three theories are to be understood and are to be somehow simultaneously applied.

Lewis’s Concept of molecular geometry

G.N Lewis was an American chemist. He first recognized that bonding between two atoms is due to sharing of electrons. According to him, two electrons shared by each atom to form a bond form a bonding pair, while the electron pair on each atom not involved in bonding is called a lone pair. He was the first-ever scientist who presented the electronic structure of a molecule or a polyatomic ion.

Electron and molecular geometry provide a complete framework of molecular geometries.

Example: Lewis diagram of (NF3) molecule

The lewis diagram of a molecule (NH3) can be drawn by following the step below;

  • Count the total number of valence electrons in a molecule.

The valence electrons of nitrogen and the fluorine atoms are added and the number comes out is 26.

N + H + H + H

Valence electrons = 5 + 7 + 7 + 7 = 26

  • Choose the central atom

It is a crucial step to draw the best lewis structure. The central atom should be the least electronegative one. Oxygen and hydrogen are generally peripheral atoms. Hence, nitrogen in NF3 is the central atom here.

lewis structure of NF3 with nitrogen as central atom

  • Connect the central atom with surrounding atoms to draw the structure

The central atom is connected with other atoms through lines called bonds to give a skeleton to the molecule.

Nitrogen trifluoride Lewis structure

  • Complete the outermost electrons of peripheral atoms

Now complete the octet of all peripheral atoms.

distribution of electrons in lewis diagram of NF3

  • Place the remaining electrons on the central atom

After the connection and distribution of electrons, 2 electrons are left behind (from 26) which are placed on the central atom.

placement of remaning electrons on Nitrogen in lewis structure of NF3

  • Calculate the formal charge on each atom

Formal charge = Valence electrons – (Bonded electrons + Non-bonded electrons)

Formal charge on nitrogen = 5 – 3 – 2 = 0

Formal charge on fluorine-(1) =7- 6 -1 = 0 

Formal charge on flourine-(2) =7- 6 -1 = 0 

Formal charge on flourine-(3) = 7- 6 -1 = 0 

It may be noted that the best lewis diagram is the one with the least formal charge.

Best possible lewis diagram of NF3 with minimum charge distributionLewis structure of the resonating structures

Some molecular compounds have more than one lewis diagram. These possible structures are known as their canonical forms and are best described as hybrids of all the resonance forms.

For example, NO32-

Canonical forms of nitrate ion

The combined or hybrid structure deduced from the possible lewis structures is given below:

Representative lewis hybrid diagram of nitrate ion

Hybridization Theory

In a localized bond approach after drawing a lewis diagram of a molecule, hybridization or VSEPR model is applied to understand its best representation. Hybrid means a mixture of two which in molecular geometry states that two or more atomic orbitals of a central atom are mixed to form new and degenerate (same energy) hybridized orbitals. 

For example, in a methane molecule, carbon is bonded with 4 hydrogen atoms which means that carbon has four atomic orbitals involved in bonding. All four bonds of this molecule are sigma bonds and constitute sam lengths due to hybridization.

Carbon has one (s) and three (p) orbitals which are hybridized to form four new degenerate orbitals. Thus an atom that has s and p orbitals in its valence shell can form 3 types of hybrid orbitals

  1. sp hybridization give a linear molecule. For example, Ethyne CH≡CH
  2. sp2 hybridization has a triangular planner geometry. For example, Ethene CH2=CH2
  3. sp3 hybridization gives a tetrahedral molecular geometry. For example, methane CH4

Note that, If d orbitals are also present in its valence shell, it may rise to the following hybrids and their corresponding shape.

d2sp3 hybridization: When two d orbitals d x2-y2 and dz2 are mixed with one “s” px, py, and pz orbitals, a set of 6 equivalent hybridized orbitals is formed. It gives octahedral geometry to a molecule or polyatomic ion. For example, SF6.

dsp3 hybridization: A dxy orbital, one s orbital, and px and py and pz orbitals combine to give five slightly different energy orbitals of dsp3. It has the shape of a trigonal bipyramidal vertex. For example, ClF4+.

sd3 hybridization: An s and dxy, dyz, dzx are combined to give rise to sd3 hybrid orbitals. It has tetrahedral vertices. For example, TeCl4.

dsp2 Hybridization: dx2-y2 orbital, one s orbital, px, and py orbitals combine to give four degenerate energy orbitals. It provides square planner geometry. For example, XeCl4.

VSEPR Model

The VSEPR theory provides a model to assign a shape to a molecule. It is the most simple and easiest approach to predicting the electron and molecular geometry of a molecule. This was first presented by Sidgwick and Powell and then further elaborated by Nyholm and Gillespie in the 1940s. This theory explains how the shape or geometry is controlled by repulsion between valence electrons of central and peripheral atoms. Its main postulates are summarized below:

  • All EXn bonds are significant 

In a molecule, each bond pair, when bonded or not is stereochemically unique. The repulsion between electron pairs actually determines the shape.

  • The sequence of repulsion of electron pairs 

Electronic pair repulsions follow the trend given below;

Lone pair – Lone pair > Lone pair – Bond pair > Bond pair – Bond pair

  •  Double and Triple bonds

Multi bonds such as double or triple bods are considered as one-electron density. The VSEPR model is best applied on the p-orbital as a valence shell e.g Halides, etc.

  • ABE notations and occupancy (x +y)

In this model, the central atom is represented by ‘A’, and bond pairs and lone pairs are shown with ‘B’ and ‘E’ respectively. For a given structure, ABxEy, ‘x’ is the number of bond pairs while ‘y’ is the number of lone pairs, surrounding the central atom. If the molecular model has no lone pairs its structure is ABxE0 type. The geometry that shows all electron pairs, whether lone pairs or bond pairs is called its prototype or electronic geometry.

It is to be noted that electron pairs around a central atom arrange themselves in such a way that they are at the maximum possible distance. Hence they experience minimum electrostatic repulsions.

ABxEy type geometries

There are two possible types of ABxEy geometries based on the availability of electron pairs. If the central atom has no lone pair it must have regular geometry. Whereas, in the presence of both bond pairs and lone pairs the electronic geometry deviates from the regular ones. This can be explained by the relatively greater repulsion of lone pairs.

Regular polyhedral geometries

 If a molecule has no lone pairs the geometry of the molecule is regular polyhedra. Regular polyhedra are classified as follows;

AB2

It is a regular prototype geometry. If the central atom is surrounded by 2 atoms, its bond pairs may experience an electrostatic repulsion. These pairs should be at 180° to minimize the repulsion.

  • Example: Carbon dioxide

Molecular geometry of carbon dioxide is linear with 180 angle

  • ABXEy: AB2
  • Bond angle(s): 180°
  • Molecular geometry: Linear

AB3

In this arrangement, central atom A is surrounded by three other atoms. The placement best suited for its minimum repulsions is 120 degrees. As they are in the same plane, the geometry is named trigonal planer geometry. Hence, a ceiling fan-like shape is observed.

  • Example: Boron trifluoride (BF3)

boron trifluoride

  • ABXEy: AB3
  • Bond angle(s): 120°
  • Molecular geometry: Trigonal planar

AB4

It has four atoms that encapsulated the central atom ‘A’. As the number of outer atoms or bond pairs are four, the two of the bonded pair are forced to go outside the plane. It gives a tetrahedral geometry.

  • Example: Dichloromethane (CH2Cl2)

Dichloromethane AB4 type tetetrahedral geometry

  • ABXEy: AB4
  • Bond angle(s): 109°
  • Molecular geometry: Tetrahedron

AB

Here A is surrounded by five bonded pairs. The bonded pairs have two groups. Three of them are present in the same plane while the other two are perpendicular to them.

  • Example: Phosphorus Pentachloride

Molecular geometry of phosphorus pentachloride PCl5, trigonal bipyramidal

    • ABXEy: AB5
    • Bond angle(s): 180°, 117°
    • Molecular geometry: Trigonal bipyramid

AB6

When an atom is surrounded by 6 atoms or bond pairs, each of them is adjusted at a position to form an octahedron. 

  • Example:  Sulphur Hexafluoridemolecular geometry of Sulfur hexafluoride SF6: octahedral
  • ABXEy: AB6
  • Bond angle(s): 180°, 90°
  • Molecular geometry: Octahedron

Irregular Geometries 

When any of the bond pairs are replaced by a lone pair, the geometry formed is known as irregular polyhedra. Lone pairs are under the influence of one nucleus hence have more energy and occupy more space.

AB1E1  

This type of geometry has and bond pair and lone pair. It’s still a linear geometry.

  • Example: Carbon monoxide

molecular geometry of carbon monoxide CO, ABE linear geometry

  • ABXEy: ABE
  • Bond angle(s): 180°
  • Molecular geometry: Linear

AB2

If two bonded pairs and one lone pair are present in a molecule its geometry is V or bent shape and its angle is less than120°  (the expected angle of occupancy for 3). It is because one of the electron pairs is lone pair hence, occupies more space.

  • Example: Nitrogen dioxide NO2

Molecular geometry of Nitrogen dioxide AB2E bent or v-shaped

  • ABXEy: AB3E
  • Bond angle: 117°
  • Molecular geometry: Bent or V-shaped

AB3

This prototype or electron geometry is near tetrahedral but is actually pyramidal with less the 109°angle between atoms. In case of ammonia, the bond angle is 107°, while for phosphorus trichloride, it is 104°. This decrease in the bond angle for PCl3 is due to the electronegative chlorine atoms that attract each other.

  • Example: Phosphorus trichloride

Molecular geometry of Phosphorus trichloride PCl3

    • ABXEy: AB3E
    • Bond angle: 104°
    • Molecular geometry: Pyramid

AB2E2

The central atom is surrounded by 2 boned pairs as well 2 lone pairs. It also has V or bent-shaped geometry. The bond angle for this geometry is less than 107°.

Example: Water H2water

  • ABXEy: AB2E2
  • Bond angle: 104°
  • Molecular geometry: Bent or V-shaped

AB4

It has 4 bonded pairs and one lone pair having a sea-saw like geometry.

  • Example: ClX4+Molecular geometry of chlorine tetrafluoride ClF4+ AB4E see-saw geometry
  • ABXEy: AB4E1
  • Bond angle: 184°
  • Molecular geometry: See-saw

AB4E2

It has 4 bond pairs and 2 lone pairs that have a square planer geometry. 

Molecular geometry of Xenon trifluoride XeF4, AB4E2, square planer geometry

Example: Xenon tetrafluoride (XeF4)

  • ABXEy: AB4E2
  • Bond angle(s): 90°, 180°
  • Molecular geometry: Square Planar

How to determine the molecular geometry

Molecular geometry is best elucidated by using all the three theories discussed above. The unison of these three approaches is utilized to deduce and predict the best possible geometry. Some common examples are given belowin whoch Lewis, VSEPR, and hybridization theories have been used to explain the molecular geometry.

1. Molecular geometry of CLF4+

  • Step 1: Lewis structure

Lewis diagram of ClF4

Step 2: Electron Geometry

Electron geometry of ClF4+

  • Bond angle(s): 90°, 180°
  • Electron geometry: Square pyramid
  • Hybridization: dsp3
  • Step 3: Molecular geometry

clf4 molecular geometry

  • ABXEy: AB4E1
  • Occupancy: 4+1 =5
  • Bond angle(s): 184°
  • Molecular geometry: See-saw

2. Molecular geometry of XeCl4

  • Step 1: Lewis structure

 

Lewis diagram of Xenon Tetrachloride

  • Step 2: Electron Geometry

Electron Geometry of XeCl4

 

  • Bond angle(s): 90°, 180°
  • Electron geometry: Octahedron
  • Hybridization: dsp2
  • Step 3: Molecular Geometry

Molecular geometry of XeCl4

  • ABXEy: AB4E2
  • Occupancy: 4+2 =6
  • Bond angle(s): 90°, 180°
  • Molecular geometry: Square Planar

3. Molecular geometry of SF5

  • Step 1: Lewis structure

Lewis diagram of Sulfur pentafluoride ion SF5-

  • Step 2: Electron Geometry

Electron geometry of SF5-

  • Bond angle(s): 90°, 180°
  • Electron geometry: Octahedron
  • Hybridization: dsp3
  • Step 3: Molecular Geometry

sulfur pentafluoride ion

  • ABXEy: AB4E2
  • Occupancy (x+y): 5 + 1 =6
  • Bond angle(s): 90°, 180°
  • Molecular geometry: Square pyramid

4. Molecular geometry of POF3

  • Step 1: Lewis diagram

Lewis diagram of Phosphorus oxy trifluoride (POF3)

  • Step 2: Electron Geometry

Electron geometry of POF3 AB4 tetrahedral geometry

  • Bond angle(s): 107°
  • Electron geometry: Octahedron
  • Hybridization: sp3
  • Step 3: Molecular Geometry

molecualr geometry

  • ABXEy: AB4E2
  • Occupancy (x+y): 4+0 =4
  • Bond angle(s): 108°, 35°
  • Molecular geometry: Tetrahedron

Key Takeaways

Molecular geometries of some common chemical substances

Concepts Berg

What is the molecular geometry of BF3 boron trifluoride?

Boron trifluoride has a trigonal planar molecular geometry with a 120° bond angle.

How to determine, how many dots are on an element’s lewis dot structure?

The dots on Lewis structures of elements can be determined by the group number. The valence electrons are the dots on the symbol.

How do you find the molecular geometry?

Molecular geometry can be determined by applying bonding theories such as Lewis, VSEPR, and hybridization. Moreover, the predicted geometry by these theories can be verified by using instrumental techniques.

How does molecular geometry affect properties?

The molecular geometry can be related to physical and chemical properties. If the molecular attain the shape with the minimum torsional and angular strain it is considered most stable and therefore less reactive.

What is the molecular geometry of CH3NH2?

The molecular geometry of methylamine is a pyramid. ABE type is AB3E, and occupancy is 3 + 1= 4.

What is the molecular geometry of SeF6?

It has the shape of an octahedron, with bond angles of 90° and 180°.

What is the molecular geometry of NCl3?

The geometry of NCl3 is pyramidal in shape.

What is the molecular geometry of ethanol?

Ethanol has oxygen as the central atom with two lone pairs and two bond pairs present on it. The ethanol molecule is bent or v-shaped.

Do double bonds matter when trying to predict molecular geometry?

According to VSEPR theory, double bond is considered a one-electron density equivalent. However, the predicted angle of molecular geometry may be affected by two bond pairs.

Reference Books

  • Basic Inorganic Chemistry By Cotton and Winkilson
  • Inorganic Chemistry fifth edition By Shriver and Atkins

Reference links