Tetrahedral in Molecular Geometry: Definition, Structure & Examples

Instructor: Mia Primas

Mia has taught math and science and has a Master's Degree in Secondary Teaching.

In this lesson, we'll learn what a tetrahedral is in molecular geometry. We'll also look at features and examples of tetrahedral structures. At the end of the lesson, take a brief quiz to see what you learned.

Molecular Geometry

When we think of the structures of molecules in chemistry, we usually think of them as 2-dimensional shapes. They are usually drawn this way for simplicity; however, many molecules have a 3-dimensional structure. Molecular geometry is a type of geometry used to describe the shape of a molecule. There are several shapes in molecular geometry, but in this lesson, we'll focus on the tetrahedral.

A tetrahedral has a central atom surrounded by four other atoms. The central atom bonds with each of the surrounding atoms, which form bond angles of 109.5 degrees.

Structure of a Tetrahedral Molecule
tetrahedral1

Valence Shell Electron Repulsion Theory

When identifying the shape of a molecule, we need to identify the number of bonds and lone electron pairs of the molecule. The lone electron pairs are the electrons that surround the central atom but are not bonded to another atom. The total number of bonds and lone electron pairs determine the steric number of the molecule. Tetrahedral molecules have a steric number of four because they have four bonds and no lone electron pairs.

The presence of lone electron pairs affects the shape of the molecule. According to the valence shell electron repulsion theory (VSEPR), electron pairs repel each other whether they are bonded or in lone pairs. This repulsion is due to the negative charge of the electrons, which means like charges repel each other. VSEPR theory also states that the electrons and atoms of the molecule will arrange themselves to minimize the repulsion, which gives the molecule its geometric structure.

Not all molecules with a central atom and four surrounding atoms are tetrahedral. A square planar molecule also has a central atom bonded to four surrounding atoms. However, it has two lone electron pairs, shown in red in the image below, giving it a steric number of six. The lone electron pairs arrange themselves so that they are opposite from each other. This pushes the other bonds closer together so that they are on the same plane arranged 90 degrees from each other. This arrangement does not occur with the tetrahedral because there are no lone electron pairs; instead, the bonds are spread evenly throughout the 3-dimensional space.

Comparison of a Tetrahedral Molecule and Square Planar Molecule
tetrahedral2

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