KEY2CHEM

Lewis Diagrams and VSEPR Theory

Lewis diagrams (also called Lewis structures) provide insight into the connections of atoms. From the Lewis diagram, the shape of the molecule can be determined using Valence Shell Electron Pair Repulsion (VSEPR) theory, which states that molecules will adopt shapes to minimize repulsions between electron pairs. The Lewis structure and shape of the molecule also provide insight into hybridization of atoms (which impacts their reactivity) as well as overall molecular polarity.

Example 1.

How many of these molecules are polar?         $$\require{mhchem}\ce{CO2}$$                   $$\require{mhchem}\ce{H2O}$$

A. $$0$$

B. $$1$$

C. $$2$$

Solution

B. $$1$$

$$\require{mhchem}\ce{H2O}$$ has polar bonds and a bent molecular shape; because the bond dipole moments do not cancel, $$\require{mhchem}\ce{H2O}$$ is a polar molecule. $$\require{mhchem}\ce{CO2}$$ has polar bonds and a linear molecular shape; its bond dipoles do cancel, and it is a nonpolar molecule.

Example 2.

What is the geometry in $$\require{mhchem}\ce{BF3}$$?

A. linear

B. trigonal planar

C. tetrahedral

Solution

B. trigonal planar

$$\require{mhchem}\ce{BF3}$$ has three groups around the central atom, making it trigonal planar.

Example 3.

How many of these molecules have $$sp^3$$ hybridized central atoms?          $$\require{mhchem}\ce{CF4}$$           $$\require{mhchem}\ce{NF3}$$

A. $$0$$

B. $$1$$

C.$$2$$

Solution

C. $$2$$

An atom with $$sp^3$$ hybridization is one that has a tetrahedral electron group arrangement ($$4$$ groups, regardless of whether they are bonding or nonbonding). $$\require{mhchem}\ce{CF4}$$ has $$4$$ bonding groups, and $$\require{mhchem}\ce{NF3}$$ has $$3$$ bonding and $$1$$ nonbonding group. Both are $$sp^3$$ hybridized.