Determining Molecular Shape using VSEPR

So far we have been drawing molecules in just two dimensions, without any thought to the actual shape, especially in the third dimension. Let's think about what would influence the shape of a molecule. It is the relationship among the electrons that are bound to the central atom that determines the shape. As we know, electrons repel each other. So, each electron cloud will get as far apart from another as possible, while still being held by the central atom. The theory that helps predict the exact shape of a molecule is called the VSEPR theory. This stands for valence shell electron pair repulsion theory. The theory is based on the fact that the valence electrons of the central atom stay as far apart from each other as possible due to their repulsions.Instead of showing you the different molecular shapes that are determined by the VSEPR method, you will do the following lab in order to determine these for yourself.

Linear

image of a linear with 180° between planes Trigonal Planar has the shape of a triangle with a central atom in the center of the triangle. The triangle is planar, meaning that all four of the atoms lie in one plane, or they would lie flat on a tabletop. The bond angles of a planar triangular molecule are 120°.

Trigonal Planar

image of a triangular planar with 120° measured between planes Trigonal Planar has the shape of a triangle with a central atom in the center of the triangle. The triangle is planar, meaning that all four of the atoms lie in one plane, or they would lie flat on a tabletop. The bond angles of a planar triangular molecule are 120°.

Tetrahedrons

Tetrahedrons have four atoms bound to a center atom. It is a pyramid made from four triangular sides. The bond angles present in a tetrahedron are 109.5°. A tetrahedron is a three-dimensional molecule. When you look at it in a static position, such as the picture of CH₄ below, it appears as though the bond angles are different. However, this is a completely symmetrical molecule. You can turn it so that any one of the H atoms shown below is in the top position, and it will look identical to this picture. All of these pictures show a representation of a tetrahedron. In the center image, the central atom is hidden within the pyramid. Methane (CH₄) and carbon tetrachloride (CCl₄) are some common tetrahedrons.

image of a TetraHedron

Trigonal Bipyramid

trigonal by pyramidal image with 120° measured between planes Trigonal Bipyramid has the shape of two trigonal pyramids sharing a base. The central atom is in the center of the common base. Three of the atoms are bonded to the central atom to form the planar triangular base. These bond angles are 120°. Two additional atoms are bonded at a 90° angle to the center atom. There are a total of six atoms in a trigonal bipyramidal molecule.

Octahedron

image of Octahedral with 90° measured between planes Octahedron refers to a molecule that is eight-sided. It is formed by two square pyramids that share a base. All of the bonds are equivalent, 90°. Contrary to its name, an octahedral molecule only has seven atoms, the center atom, which is found in the center of the common square base, the four atoms that create the square base corners, and the two atoms that form the top of each pyramid.

In your lab, did you notice the patterns in the molecular geometries? Molecules that have the same number of clouds have the same base shape. Then, if an electron cloud is replaced with a lone pair of electrons, the electron geometry remains the same but the molecular geometry has a different name. Look at the chart below. It is a great summary of the VSEPR model. Look closely at trigonal planar for example. See how it has the same overall shape as the tetrahedron? They both have four electron clouds, however the trigonal planar has one less one that is replaced with a lone pair of electrons. So, if you will study these in order, it will be easier to remember!

VSEPR - Molecular Shapes
Bond Angle with the following indicated:
# of Electron Clouds
Basic Geometry (0 lone pair)
1 lone pair
2 lone pairs
3 lone pairs

To review and practice the VSEPR method go through the following presentation. The last several slides have practice problems that allow you to predict the shape of the molecules using the VSEPR theory.

Remember to work on the module practice problems as you complete each section of content.

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