In this lesson, you will learn about trigonal bipyramidal molecules. We will discuss the angles formed within the molecules and look at a few examples. You can then take a brief quiz to see what you learned.
When we think of the structures of molecules in chemistry, we usually think of them as two-dimensional shapes. They are usually drawn this way for simplicity. However, many molecules have a three-dimensional structure. Molecular geometry is a type of geometry used to describe the shape of a molecule.
Valence Shell Electron Repulsion Theory
When identifying the shape of a molecule, we need to first know the number of bonds and lone electron pairs with 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 determines the steric number of the molecule. Trigonal bipyramidal molecules have a steric number of five - there are five bonds and no lone electron pairs (note: some definitions allow for lone electrons pairs, as we will see later in the lesson).
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 - like charges repel each other.
VSEPR theory also states that the electrons and atoms of the molecule will arrange themselves to minimize the repulsion. The arrangement that takes place is what gives the molecule its geometric structure. There are several such shapes in molecular geometry, but in this lesson we will focus on trigonal bipyramidal molecules.
Structure of a trigonal bipyramidal molecule.
Trigonal Bipyramidal Defined
A trigonal bipyramidal molecule has a central atom surrounded by five other atoms. Understanding the meaning of the terms trigonal and bipyramid will help us to visualize the three-dimensional arrangement of the atoms. A bipyramid consists of two pyramids (three-dimensional triangles) that share a base. The term trigonal tells us that the overall shape is three-sided, like a triangle. Putting the two terms together, we can see that a trigonal bipyramidal molecule has a three-sided shape with each side being a bipyramid.
In the image labeled 'Side view and three-dimensional view of a trigonal bipyramidal' below the light grey circle represents the central atom, and the dark grey circles are the surrounding atoms. The lines drawn do not represent the bonds between the atoms; they are shown to help us see the pyramidal shape that is formed. Remember that the only bonds in the molecule are those between the central atom and each surrounding atom.
Side view and three-dimensional view of a trigonal bipyramidal.
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The atoms of trigonal bipyramidal molecules are arranged on two planes that intersect at the central atom. The two planes form a 90 degree angle. The axial plane consists of the central atom and two of the surrounding atoms; they have a linear arrangement, meaning that they form a straight line. The second plane is the equatorial plane, which consists of the central atom and three surrounding atoms; the three bonds are spaced evenly, forming 120 degree bond angles.
Axial and equatorial planes of a trigonal bipyramidal.
It is important to note that the definition that we are using is this lesson is a strict definition of trigonal bipyramidal, with five bonds and no lone electron pairs. However, there are more flexible definitions. With a more flexible definition the steric number is still five, but there may be lone electron pairs.
For example, sulfur tetrafluoride has four bonds and one lone electron pair. The overall shape of the molecule is similar to a molecule with five bonds and no lone electron pairs, such as phosphorous pentachloride. Both molecules have an axial plane and an equatorial plane that form a 90 degree angle. Yet, the bond angles of phosphorous pentachloride are slightly smaller due to the repulsivity of the lone electron pair.
Examples of trigonal bipyramidal molecules.
In molecular geometry, the number of bonds and lone electron pairs determines the shape of a molecule. Using a strict definition of trigonal bipyramidal, there is a central atom bonded to five surrounding atoms and no lone electron pairs. However, some definitions allow for lone electron pairs. The shape is formed by atoms that lie on an axial plane and an equatorial plane.
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