Question

Give three examples of compounds that do not satisfy the octet rule. Write a Lewis structure for each.

Short answer

BF₃, PCl₅, and NO are examples of compounds that do not satisfy the octet rule, with different reasons for each.

Step-by-step solution

Understanding the Octet Rule

The octet rule states that atoms tend to form compounds in ways that give them eight valence electrons and thus the same electron configuration as a noble gas. However, there are exceptions to this rule.

Defining Exceptions to the Octet Rule

Exceptions to the octet rule include molecules with an odd number of electrons, molecules where one or more atoms have more than eight electrons, and molecules where one or more atoms have fewer than eight electrons.

Example 1 - Boron Trifluoride (BF₃)

Boron trifluoride is an example where the central atom, boron, has only six valence electrons. In the Lewis structure of BF₃, boron forms three single bonds with fluorine atoms, resulting in only six electrons around boron.

Example 2 - Phosphorus Pentachloride (PCl₅)

Phosphorus pentachloride is an example of a molecule with an extended octet. In the Lewis structure of PCl₅, phosphorus forms five bonds with chlorine atoms, which results in 10 electrons around the phosphorus atom.

Example 3 - Nitric Oxide (NO)

Nitric oxide is an example of a molecule with an odd number of electrons. In the Lewis structure of NO, nitrogen has a total of 11 electrons, forming one double bond with oxygen and having one unpaired electron.

Key concepts

Understanding Lewis Structures

Lewis structures, also known as electron-dot structures, are diagrams that show the bonding between atoms of a molecule and the lone pairs of electrons that may exist in the molecule. The primary purpose of a Lewis structure is to reveal how atoms in a molecule bond and fulfill the octet rule where possible, although exceptions exist.
For each atom, valence electrons are represented as dots, and bonds between atoms are shown as lines, with each line representing a pair of shared electrons. Lewis structures allow us to visualize the location of these electrons and understand the molecular geometry and bonding.

• The first step to drawing a Lewis structure is to determine the total number of valence electrons that must be included.
• Then, arrange the atoms to reveal relationships among them based on chemical intuition or known geometry.
• After that, distribute the valence electrons among the atoms, giving priority to filling octets around atoms where possible.

Ultimately, these structures are crucial tools used to depict molecular structures and gain insight into the chemical bonding and potential reactivity of molecules.

Exploring Boron Trifluoride (BF₃)

Boron trifluoride is a classic example where the octet rule is not fully satisfied. In the molecule BF₃, boron, which is the central atom, is surrounded by three fluorine atoms. Each fluorine is able to fulfill its octet requirement by forming a single bond with boron.
However, boron ends up with only six valence electrons around it, which means it does not satisfy the octet rule. This makes boron trifluoride a prime example of a molecule that has an electron deficiency.

• BF₃ is a planar molecule with a trigonal planar shape. This geometry is predicted because boron in BF₃ has three regions of electron density around it.
• Despite its electron deficiency, BF₃ is stable and commonly used in numerous chemical applications, including as a catalyst.

The resonance within BF₃ due to its ability to accept electron pairs from donors further displays its unique chemistry.

Delving into Phosphorus Pentachloride (PCl₅)

Phosphorus pentachloride is an example of a molecule that breaks the octet rule by having more than eight electrons. Unlike typical atoms in molecules that satisfy the octet rule, phosphorus in PCl₅ is surrounded by ten electrons.
This is because phosphorus, which can expand its valence shell using d orbitals, forms five bonding pairs with five chlorine atoms. The result is a stable compound with an extended octet about the phosphorus atom.

• The molecule exhibits a trigonal bipyramidal geometry, with the three equatorial chlorine atoms bonding at 120 degrees to each other, and two axial chlorine atoms bonding at 90 degrees to the equatorial plane.
• PCl₅ is used industrially as a chlorinating reagent and is one of the most important phosphorus chlorides.

The ability of some atoms like phosphorus to exceed the octet is due to available d orbitals that are energetically accessible, providing them with the flexibility to form compounds like PCl₅.

Analyzing Nitric Oxide (NO)

Nitric oxide is a notable molecule that exemplifies an exception to the octet rule by having an odd number of electrons. NO contains 11 electrons in total, leaving one unpaired electron, which makes it a free radical.
In the Lewis structure of nitric oxide, nitrogen and oxygen are connected by a double bond, but an unpaired electron remains on the nitrogen atom.

• This presence of a free radical gives NO unique properties like high reactivity, which is significant in biological systems where it functions as a signaling molecule.
• The molecular geometry of NO contributes to its role in vasodilation and neurotransmission processes in living organisms.

The reactive nature of nitric oxide and its biological relevance further highlight its importance despite the non-fulfillment of the octet rule in its structure.