Chapter 8: Problem 59

(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 $\left(\mathrm{B}_{3} \mathrm{~N}_{3} \mathrm{H}_{6},\right.$ which is analogous to benzene with alternating $\mathrm{B}$ and $\mathrm{N}$ in the ring $),$ or boron trichloride?

Short Answer

Expert verified

The compounds that are exceptions to the octet rule are arsenic pentafluoride (AsF5), borohydride ion (BH4-), and boron trichloride (BCl3).

Step by step solution

In carbon dioxide, the carbon atom forms a double bond with each oxygen atom. Thus, it has four shared electrons (two from each double bond) and fills its valence shell with eight electrons. Each oxygen atom also has six valence electrons and shares two electrons from the double bond, having eight electrons in their valence shell. Carbon dioxide follows the octet rule. #b. Water (H2O)#

In water, the oxygen atom forms two single bonds, one with each hydrogen atom. Oxygen has six valence electrons, and it shares one electron from each bond, giving it a total of eight electrons in its valence shell. Hydrogen shares one electron from each bond, having two electrons in its valence shell. Water follows the octet rule. #c. Ammonia (NH3)#

In ammonia, the nitrogen atom forms three single bonds, one with each of the three hydrogen atoms. Nitrogen has five valence electrons, and it shares three electrons from the bonds, filling its valence shell with eight electrons. Each hydrogen atom shares one electron from each bond, having two electrons in its valence shell. Ammonia follows the octet rule. #d. Phosphorus trifluoride (PF3)#

In phosphorus trifluoride, the phosphorus atom forms three single bonds, one with each of the three fluorine atoms. Phosphorus has five valence electrons, and it shares three electrons from the bonds, resulting in eight electrons in its valence shell. Each fluorine atom has seven valence electrons, sharing one electron from each bond, and having eight electrons in its valence shell. Phosphorus trifluoride follows the octet rule. #e. Arsenic pentafluoride (AsF5)#

In arsenic pentafluoride, the arsenic atom forms five single bonds, one with each of the five fluorine atoms. Arsenic has five valence electrons and shares five electrons from the bonds, which gives it a total of ten electrons in its valence shell. Each fluorine atom has seven valence electrons and shares one electron from each bond, having eight electrons in its valence shell. Arsenic pentafluoride is an exception to the octet rule. #a. Borohydride ion (BH4-)#

In the borohydride ion, the boron atom forms four single bonds, one with each of the four hydrogen atoms. Boron has three valence electrons, and it shares four electrons from the bonds, filling its valence shell with seven electrons (an odd number). Each hydrogen atom shares one electron from each bond, having two electrons in its valence shell. The borohydride ion is an exception to the octet rule. #b. Borazine (B3N3H6)#

In borazine, the three boron atoms each form three single bonds: one with a nitrogen atom and two with hydrogen atoms. The three nitrogen atoms also form three single bonds: one with a boron atom and two with hydrogen atoms. Boron and nitrogen both have three valence electrons and share three electrons from the bonds, which gives them six electrons in their valence shell. Each hydrogen atom shares one electron from each bond, having two electrons in its valence shell. Borazine follows the octet rule. #c. Boron trichloride (BCl3)#

In boron trichloride, the boron atom forms three single bonds, one with each of the three chlorine atoms. Boron has three valence electrons and shares three electrons from the bonds, filling its valence shell with six electrons (less than eight electrons). Each chlorine atom has seven valence electrons, sharing one electron from each bond, and having eight electrons in its valence shell. Boron trichloride is an exception to the octet rule. According to our analysis, the following compounds are exceptions to the octet rule: - Arsenic pentafluoride (AsF5) - Borohydride ion (BH4-) - Boron trichloride (BCl3)