(a) Which of these compounds is an exception to the octet rule: carbon dioxide, water, ammonia, phosphorus trifluoride, or arsenic pentafluoride? (b) Which of these compounds or ions is an exception to the octet rule: borohydride \(\left(\mathrm{BH}_{4}^{-}\right),\) borazine \(\left(\mathrm{B}_{3} \mathrm{N}_{3} \mathrm{H}_{6},\) which is analogous \right. to benzene with alternating \(\mathrm{B}\) and \(\mathrm{N}\) in the ring \(),\) or boron trichloride?

First, let's examine the electron configurations for each compound: 1. Carbon dioxide (CO₂) - carbon has 4 valence electrons and each oxygen atom has 6 valence electrons. Carbon shares double bonds with both oxygen atoms, which results in 8 valence electrons for each atom. 2. Water (H₂O) - oxygen has 6 valence electrons and shares a single bond with two hydrogen atoms, resulting in 8 valence electrons for oxygen and 2 for each hydrogen. 3. Ammonia (NH₃) - nitrogen has 5 valence electrons and shares a single bond with three hydrogen atoms, resulting in 8 valence electrons for nitrogen and 2 for each hydrogen. 4. Phosphorus trifluoride (PF₃) - phosphorus has 5 valence electrons and fluorine has 7 valence electrons, so phosphorus shares a single bond with each of the three fluorine atoms. This gives phosphorus 8 electrons and each fluorine atom also has 8 electrons. 5. Arsenic pentafluoride (AsF₅) - arsenic has 5 valence electrons and fluorine has 7 valence electrons. Arsenic forms a single bond with each of the five fluorine atoms, resulting in 10 valence electrons for arsenic and 8 for each fluorine atom. From this analysis, we find that arsenic pentafluoride (AsF₅) is the exception to the octet rule in this list because arsenic has 10 valence electrons instead of the typical 8.

Now let's analyze the electron configurations for the remaining compounds: 1. Borohydride (BH₄⁻) - boron has 3 valence electrons and each hydrogen atom has 1 valence electron. Boron forms a single bond with each of the four hydrogen atoms, resulting in 7 valence electrons for boron and 2 for each hydrogen. 2. Borazine (B₃N₃H₆) - this compound is analogous to benzene, with alternating boron and nitrogen atoms in a hexagonal ring. Each boron atom has 3 valence electrons, nitrogen has 5 valence electrons, and each hydrogen atom has 1 valence electron. Each nitrogen atom shares a single bond with three hydrogen atoms and the boron atoms form double bonds, resulting in 8 valence electrons for nitrogen and 6 for boron. 3. Boron trichloride (BCl₃) - boron has 3 valence electrons and chlorine has 7 valence electrons. Boron shares a single bond with each chlorine atom, resulting in 6 valence electrons for boron and 8 for each chlorine atom. By analyzing these compounds, we can see that borohydride (BH₄⁻) and boron trichloride (BCl₃) are exceptions to the octet rule, as boron has 7 valence electrons in BH₄⁻ and 6 valence electrons in BCl₃.