(b) Which of these compounds or ions is an exception to the octet rule: borohydride (BH4-), borazine (B3N3H6, which is analogous to benzene with alternating B and N in the ring), or boron trichloride?

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Identify the octet rule, which states that atoms tend to form compounds in ways that give them eight valence electrons, achieving a noble gas configuration.

Examine the structure of borohydride (BH4-): Boron typically forms three bonds, but in BH4-, it forms four bonds, achieving an octet by sharing electrons with hydrogen atoms.

Analyze borazine (B3N3H6): Boron and nitrogen alternate in a ring structure similar to benzene. Boron typically does not achieve a full octet in such structures, as it forms three bonds and has six valence electrons.

Consider boron trichloride (BCl3): Boron forms three bonds with chlorine atoms, resulting in only six valence electrons around boron, which is less than an octet.

Conclude that boron trichloride (BCl3) is an exception to the octet rule, as boron does not achieve a full octet in this compound.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Octet Rule

The octet rule is a chemical guideline stating that atoms tend to bond in such a way that they each have eight electrons in their valence shell, achieving a stable electron configuration similar to that of noble gases. This rule primarily applies to main group elements and helps predict the bonding behavior of atoms in molecules.

Electron Deficiency

Electron deficiency occurs when an atom does not have enough electrons to complete its octet, often seen in compounds with elements from group 13, such as boron. These compounds can form stable structures despite having fewer than eight electrons around the central atom, leading to exceptions to the octet rule.

Resonance Structures

Resonance structures are different ways of drawing the same molecule that illustrate the delocalization of electrons. In compounds like borazine, resonance can help explain the stability and bonding characteristics that deviate from the octet rule, as the electrons are shared among multiple atoms rather than localized.

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Related Practice

Textbook Question

Mothballs are composed of naphthalene, C₁₀H₈, a molecule that consists of two six-membered rings of carbon fused along an edge, as shown in this incomplete Lewis structure:

(b) Do you expect the C—C bond lengths in the molecule to be similar to those of C—C single bonds, C ═C double bonds, or intermediate between C—C single and C ═C double bonds?

Textbook Question

Mothballs are composed of naphthalene, C₁₀H₈, a molecule that consists of two six-membered rings of carbon fused along an edge, as shown in this incomplete Lewis structure:

(c) Not all of the C—C bond lengths in naphthalene are equivalent. Based on your resonance structures, how many C—C bonds in the molecule do you expect to be shorter than the others?

Textbook Question

(a) Which of these compounds is an exception to the octet rule: carbon dioxide, water, ammonia, phosphorus trifluoride, or arsenic pentafluoride?

Textbook Question

For Group 13–17 elements in the third row of the periodic tableand beyond, the octet rule is often not obeyed. A friendof yours says this is because these heavier elements are morelikely to make double or triple bonds. Another friend ofyours says that this is because the heavier elements are largerand can make bonds to more than four atoms at a time.Which friend is more correct?

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