Question

(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?

Short answer

(a) Arsenic pentafluoride (AsF₅) is the exception to the octet rule in the given compounds, as arsenic has 10 valence electrons instead of the typical 8. (b) 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.

Step-by-step solution

(a) Identify the exception to the octet rule in the given compounds

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.

(b) Identify the exception to the octet rule in the given compounds

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.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and they are important because they determine how an atom interacts with others to form chemical bonds. Every element has a specific number of these electrons based on its position in the periodic table. For example, carbon, in group 14, has four valence electrons, allowing it to form up to four bonds with other elements.
Knowing the number of valence electrons helps predict the chemical properties and reactivity of the element, particularly how it will bond in molecules. Helium and neon, for instance, are stable gases partly because they have a full outer shell of electrons, which satisfies the octet rule (or duet for helium).
In molecules, elements often try to reach an octet of valence electrons through sharing, losing, or gaining electrons, to achieve a stable electronic configuration similar to noble gases.

Electron Configuration

Electron configuration is a description of the distribution of electrons in an atom's shells and subshells. It follows the principles of quantum mechanics and provides insight into how atoms bond to form compounds.
For example, carbon's electron configuration is \(1s^2 2s^2 2p^2\), indicating four electrons in its outer shell. This configuration hints at its ability to form four covalent bonds, such as in methane (CH₄).
Understanding electron configurations is crucial because it can predict how an atom will interact with others. It explains why some elements are more likely to donate or accept electrons during reactions, and why certain molecules, like water (H₂O), form the structures they do. The arrangement of electrons determines molecule shapes, bond angles, and physical and chemical properties.

Exceptions to the Octet Rule

While the octet rule suggests atoms strive to have eight valence electrons to achieve stability, certain compounds do not adhere to this rule. There are three main types of exceptions.

• Some molecules or ions house an odd number of electrons, leading to unavoidable unpaired electrons, such as with nitric oxide (NO).
• Some elements can form stable compounds with fewer than eight electrons. Boron, in compounds like boron trifluoride (BF₃), is a classic example of forming stable structures with less than an octet of electrons.
• Conversely, some elements can hold more than eight electrons, participating in what's called expanded octets. This happens in elements like phosphorus in phosphorus pentachloride (PCl₅) and arsenic in arsenic pentafluoride (AsF₅), as observed above.

These exceptions occur because atoms either do not have enough electrons for a full octet or are large enough to accommodate more than eight electrons using their d orbitals.

Lewis Structures

Lewis structures are diagrams that represent the valence electrons of atoms within a molecule. These structures help visualize how electrons are shared in chemical bonds, particularly in covalent bonds.
In drawing a Lewis structure, start by determining the total number of valence electrons for the molecule or ion. Next, arrange the atoms to show how they share electrons. Lines between atoms represent shared electron pairs, also known as covalent bonds, while dots represent lone (unshared) electron pairs.
Lewis structures are useful tools to predict molecular geometry, bond orders, and resonance by illustrating which atoms are connected and how electrons are distributed in a molecule. They can clarify misunderstandings about molecular composition and suggest formal charges and resonance structures for better chemical understanding.