Molecular Mass

The molecular mass of a molecule is simply the sum of the masses of the atoms in its formula. As with formula masses, it is important that you keep track of the number of atoms of each element in the molecular formula to obtain the correct molecular mass. (For more information about formula masses, see Chapter 3 “Ionic Bonding and Simple Ionic Compounds”, Section 3.5 “Formula Mass”.)

Molecular Shape: VSEPR Theory

Unlike ionic compounds, with their extended crystal lattices, covalent molecules are discrete units with specific three-dimensional shapes. The shape of a molecule is determined by the fact that covalent bonds, which are composed of negatively charged electrons, and any lone pairs of electrons tend to repel one another. This concept is called the valence shell electron pair repulsion (VSEPR) theory. For example, the two covalent bonds in BeCl₂ stay as far from each other as possible, ending up 180° apart from each other. The result is a linear molecule:

The three covalent bonds in BF₃ repel each other to form 120° angles in a plane, in a shape called trigonal planar:

The molecules BeCl₂ and BF₃ actually violate the octet rule; however, such exceptions will not be discussed in this text.

The four covalent bonds in CCl₄ arrange themselves three dimensionally, pointing toward the four corners of a tetrahedron and making bond angles of 109.5°:

The shape of a molecule with lone electron pair(s) around the central atom is influenced by both the bonds and the lone pairs.  For example, NH₃ has one lone electron pair and three bonded electron pairs. These four electron pairs repel each other and adopt a tetrahedral electron arrangement:

However, the shape of the molecule is described in terms of the positions of the atoms, not the lone electron pairs. Thus, NH₃ is said to have a trigonal pyramidal molecular shape.

Similarly, H₂O has two lone pairs of electrons around the central oxygen atom, in addition to the two bonded pairs:

Although the four electron pairs adopt a tetrahedral electron arrangement due to repulsion, the shape of the molecule is described by the positions of the atoms only. The shape of an H₂O molecule is bent.

In determining the shapes of molecules, it is useful to first determine the Lewis diagram for a molecule. The shapes of molecules with multiple bonds are determined by treating the multiple bonds as one bond. Thus, CH₂O has a shape similar to that of BF₃.  With three bonding groups around the central atom and no lone pairs, the CH₂O molecule has a trigonal planar shape.