We will use the chart below to predict the correct shape of a molecule.
First we must start with a correct Lewis dot structure of the molecule. This is of outmost importance. If the Lewis dot structure is incorrect, you will not be able to predict the shape correctly.
Next, we must identify the central atom and ask how many other atoms it is connected to and how many lone pairs of electrons (two dots) are on it. Then we have to look at the chart to identify the correct shape.
The purple dot in the middle is the central atom in a molecule. The blue dots are atoms the central atom is connected to and two dots represent a lone pair. Thus, for example, a molecule that has a central atom that is connected to two other atoms and has one lone pair must be bent.
The chart also shows angles for different shapes.
Let's take a look at the Lewis structure of H2SO4 shown below. The central atom, sulfur, is attached to four other atoms (oxygens) and has no lone pairs of electrons on it. The shape must be tetrahedral with a 109.5 angle.
In the example above, XeF4, Xenon is attached to four other atoms and has two lone pairs of electrons. The shape must be square planar.