VSEPR Applied (3 things)

The next simplest structure is that which involves 3 things. 

An easy example of this would be BH3. 

In this case, imagine that your feet and hands hate each other (but that your feet are tied together and therefore stay together as one). You hands would no longer end up straight out (as this would make them closer than they should be to your feet). Instead, you would end up like this:

In a similar manner, the three bonds in BH3 repel each other and take the shape shown here:

This structure is called trigonal planar, and the angles are 120^o. The shape of the molecule is the same, trigonal planar.

A slightly different circumstance would be found with the molecule HNO. This molecule also has three things around the central atom (the oxygen, the hydrogen, and the lone pair of electrons). Just as before, the structure is therefore trigonal planar. However the shape is different.

To understand the difference between structure and shape requires us to understand how shape is measured. In order to determine what the actual shapes of molecules are we use something called X-ray diffraction. This involves bouncing X-rays off of a crystal of a molecule and analyzing the reflected pattern. The method of analysis is well above the level of this text, but what is important to understand is that X-rays do not bounce off of electrons (they're too small). The X-rays only bounce off of atoms. In other words, the X-ray beam only shows where the atoms are.

Thus, when we “look” at a molecule, we see only the location of the atoms. In other words, for HNO, the structure may be trigonal planar, but we only see the atoms arranged at an angle.

The term shape is used to indicate this “what we see” version of the molecule. In this case the shape is labeled bent, or more correctly bent (120) (implying that we will encounter another bent shape with different angles).

A note about angles
Four Things