Exercises: VSEPR Theory and Electron Dot Structures

Draw the electron dot (Lewis) structure of methane, $\text{CH}_4$. State how many bonding pairs and how many lone pairs surround the central carbon atom, and name the molecular geometry.

In your own words, state what VSEPR stands for and what the theory says about how electron pairs around a central atom arrange themselves. Then name the geometry of a molecule with two bonding pairs and no lone pairs on the central atom, such as $\text{CO}_2$.

Classify each molecule as linear, bent, trigonal planar, trigonal pyramidal, or tetrahedral: (a) $\text{H}_2\text{O}$, (b) $\text{NH}_3$, (c) $\text{BF}_3$.

Ammonia, $\text{NH}_3$, and methane, $\text{CH}_4$, both have four electron pairs around the central atom, yet their molecular shapes differ. Draw the electron dot structure of each, give the electron-pair geometry and the molecular geometry of each, and explain why the shapes differ.

Emma is studying for a chemistry quiz. She draws the structure of $\text{H}_2\text{O}$ as a straight line, H-O-H, with an angle of $180^\circ$. Explain what is wrong with her prediction, using the number of lone pairs on oxygen. Then state the approximate bond angle in real water and compare it to the angle in methane.

For $\text{CH}_2\text{O}$ (formaldehyde, with carbon as the central atom bonded to two H atoms and one O atom), count the total valence electrons, draw the electron dot structure including any double bond, and predict the molecular geometry. Explain how a double bond is treated as one region of electron density in VSEPR.

Compare $\text{BF}_3$, $\text{NF}_3$, and $\text{CF}_4$. For each one, count the valence electrons, draw the electron dot structure, and identify the number of bonding and lone pairs on the central atom. Then give each molecular geometry and approximate bond angle, and argue why $\text{BF}_3$ is planar while $\text{NF}_3$ is not, even though both have three fluorine atoms.

A student claims, "Any molecule with polar bonds must be a polar molecule." Test this claim using $\text{CO}_2$, $\text{H}_2\text{O}$, and $\text{CCl}_4$. For each molecule, draw the electron dot structure, give the geometry, describe the direction of the bond dipoles, and decide whether the molecule is polar. State whether the claim is correct and justify your answer.