In this section, we will explore the shapes of molecules as determined by the VSEPR Theory. VSEPR Theory is a powerful tool that helps us to guess the shape of a molecule. Such shapes are important as they determine the the structure and function of the compound. For example, the arrangement of carbon atoms in a diamond help establish its unique hardness and usefulness.
The VSEPR Theory - valence-shell electron-pair repulsion theory - provides us a way to predict the shapes of many molecules. To understand how this theory functions, lets look at a methane molecule. The image on the right is the methane molecule. The blue balls are the hydrogen 1s orbitals while the other yellow, red, green, and pink orbitals are the hybridized sp3 orbitals of the carbon atom.
You know that the carbon atom is bound to four hydrogen atoms. Each of the electron pairs repel each other since they have the same charge. Thus, the electrons try to stay away from each other as far as possible, maintaining the lowest energy position possible. The resulting shape is a tetrahedron, which has four faces, all being equilateral triangles. In this position, all four pairs of electrons and the hydrogen atoms bound by these electrons are equally spaced from each other. The bond angle, or the angle between any two outside atoms and the central atom, is 109.47 degrees. Look at the above diagram again and notice that all bond angles are 109.47 degrees, which is also the tetrahedral angle.
In an ammonia molecule, the nitrogen atom is the central atom, bonded to three hydrogen atoms. It is very similar to the methane molecule, except that ammonia has one unshared pair of electrons. This unshared pair of electrons always has a greater repulsive effect than the remaining shared pairs of electrons. Why? Because there is no positive force on the side of the unshared electron pair to "reduce" the negative charge of the electrons. The positive force is, for example, the hydrogen nucleus which are bonded to the other three pairs of electrons. Experiments conclude that the bond angle is 107 degrees, slightly less than 109.47 in methane. The resulting shape is a pyramid, as you can see in this diagram.
Now lets look at one final example, water. We know that the oxygen atom in water is the central atom and is bonded to two hydrogen atoms. However, it has two pairs of valence electrons. These two unshared pairs of electrons will have a greater repulsion effect than the one unshared pair of electrons in ammonia. The resulting effect is a bond angle of 105 degrees, smaller than that of methane (because of greater repulsion by unshared pairs). This shape is known as bent, since the shape of water seems quite bent.
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