Structural details are often crucial to the activity of the compound. For example, the compounds dextrophane and levorphane differ in a very subtle way. In Figure 2, the same molecules are shown as computer generated molecular models. When you compare Figure 1 and Figure 2 you will find that the lines in the structural formulas indicate the attachment of atoms to one another. These lines are called bonds.
Some bonds are single bonds, some are double bonds, others are triple bonds.
Generally the greater the number of bonds between two atoms, the stronger the attachment of the two atoms. Notice also that in the molecular model, the atoms have different colors and sizes. The colors are obviously used to distinguish one type of atom from another. The space-filling model is designed to give a somewhat more accurate representation of the molecule by portraying the space filled by the electrons around the atoms. Most chemists prefer to see the ball and stick models, but they use space-filling representations when they are interested in the spatial requirements of certain parts of a molecule.