Core Concepts
In this tutorial, you will learn how to identify the molecular geometry and bond angles of a molecule. You will learn about the more common molecular geometries: tetrahedral, linear, bent, trigonal pyramidal, and trigonal planar – along with their bond angles.
If you enjoy this tutorial, feel free to check out our other tutorials on bonding listed below.
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Vocabulary
- Electron Geometry: Describes the arrangement of bonds and lone pairs around a central atom.
- Molecular Geometry: Describes the arrangement of atoms around the central atom with acknowledgment to only bonding electrons.
- Hybridization: Orbitals are combined in order to spread out electrons.
- Bond angles: The angle between adjacent bonds of an atom.
What is molecular geometry?
Molecular geometry refers to the three-dimensional structure, or arrangement, of the atoms that make up a molecule. It is determined by the bonds between the atoms and any lone pairs of electrons that are present in the molecule. The geometry of a molecule can have a big impact on its chemical and physical properties, such as its reactivity and solubility.
For example, the shape of a water molecule (H2O) is bent, which gives it a high surface tension and allows it to dissolve many other substances. Molecular geometry is usually studied using the VSEPR (valence shell electron pair repulsion) model, which predicts the shape of a molecule based on the repulsion between the electrons in the outermost shell of the atoms.
Chemists are able to predict the arrangement of atoms and chemical bonds using the valence-shell electron-pair repulsion theory or VSEPR. This theory revolves around the idea that electrons repel each other and therefore will bond accordingly.
Types of configurations and angles
There are three main types of configurations: linear, trigonal, and tetrahedral. Below is a table demonstrating the relationship between the number of bonding partners and these configurations.
| Configuration | Bonding Attachments | Bond Angle |
| Linear | 2 | 180 |
| Trigonal | 3 | 120 |
| Tetrahedral | 4 | 109.5 |
Determining molecular geometry and bond angles
To determine the molecular geometry of a structure we need to know two things. Firstly, we must know how many total attachments there are. additionally, we need to know how many of these attachments are bonds and lone pairs. Notice in the table below how if there are no lone pairs, the molecular geometry and electron geometry will be the same.
In the table below, you will see the coordination between the number and type of attachments in relation to the bond angles. For the most part, this information will have to be memorized.
| Attachments | Molecular Geometry | Electron Geometry | Hybridization | Bond Angles |
| 2 | linear | linear | sp | 180 |
| 3 | trigonal planar | trigonal planar | sp2 | 120 |
| 3 (2 bonds and 1 lone pair) | bent | trigonal planar | sp2 | about 118 |
| 4 | tetrahedral | tetrahedral | sp3 | 109.5 |
| 4 (3 bonds and 1 lone pair) | trigonal pyramidal | tetrahedral | sp3 | <109.5 |
| 4 (2 bonds and 2 lone pairs) | bent | tetrahedral | sp3 | about 105 |
Practice Example:
What is the molecular geometry and bond angle of water (H2O)?
The answer is the molecular geometry of water would be bent. Notice there are 4 attachments, or, electron groups surrounding oxygen. This would make the electron geometry tetrahedral. However, this is not the molecular geometry. Two of these attachments are bonds and the other two are lone pairs. Therefore, the resulting molecular geometry is a a bent geometry. Now that we know the molecular geometry, we can determine the bond angle to be about 105 degrees from our chart.
What is the molecular geometry of BF3, boron trifluoride?
If we drew the electron dot structure for BF3, boron trifluoride, we will notice that there are three attachment point, and 3 bonds, to the central atom, boron. Based on the chart, the molecular geometry for BF3 would be trigonal planar, with an angle of 120 degrees between the bonds.