Question

Which of these molecules have Lewis structures that involve exceptions to the octet rule? Classify each exception. (a) \(\mathrm{PCl}_{3}\) (b) \(\mathrm{SnF}_{4}\) (c) \(\mathrm{BCl}_{3}\) (d) \(\mathrm{NO}\)

Short answer

\(\mathrm{SnF}_4\), \(\mathrm{BCl}_3\), and \(\mathrm{NO}\) have exceptions to the octet rule due to expansion, incomplete octet, and odd electrons, respectively.

Step-by-step solution

Explain the Octet Rule

The octet rule states that atoms tend to form bonds until they are surrounded by eight valence electrons. This is a common rule for atoms of main-group elements to achieve a stable electron configuration.

Analyze Molecule (a) \(\mathrm{PCl}_3\)

\(\mathrm{PCl}_3\) (phosphorus trichloride) is a molecule where phosphorus is the central atom surrounded by three chlorine atoms. Each chlorine atom has a full octet, and phosphorus also satisfies the octet rule with five valence electrons and three additional electrons from the chlorine atoms' shared bonds, making a total of eight electrons. Thus, it follows the octet rule.

Analyze Molecule (b) \(\mathrm{SnF}_4\)

\(\mathrm{SnF}_4\) (tin(IV) fluoride) has tin as the central atom, bonded to four fluorine atoms. Tin (Sn) is in group 14 and can expand its octet due to its d-orbital availability, accommodating more than eight electrons around it. This molecule thus involves an exception to the octet rule, specifically an expansion.

Analyze Molecule (c) \(\mathrm{BCl}_3\)

\(\mathrm{BCl}_3\) (boron trichloride) has boron as the central atom bonded to three chlorine atoms. Boron, being in group 13, tends to form compounds where it only has six electrons around it because it lacks enough valence electrons to achieve an octet. This is an exception to the octet rule, known as an incomplete octet.

Analyze Molecule (d) \(\mathrm{NO}\)

\(\mathrm{NO}\) (nitric oxide) contains nitrogen and oxygen. Nitrogen has five valence electrons, and with its bond to the oxygen atom (providing one shared electron), it has seven electrons, not throughout the octet. This molecule features an exception to the octet rule due to having an odd number of electrons, resulting in a free radical.

Key concepts

Lewis Structures

Lewis structures are a simplified way to represent molecules and ion systems. They show how valence electrons, which are the outer shell electrons, are organized among atoms in a molecule. Typically, Lewis structures aim to depict molecules in keeping with the octet rule. The octet rule suggests that atoms are most stable when surrounded by eight electrons.
This visual approach helps understand the bonding of molecules:

• Bonds are drawn as lines between atoms.
• Lone pairs (non-bonded valence electrons) are represented as dots.
• The central atom is usually one that can form multiple connections, like phosphorus in PCl₃.

By mastering Lewis structures, you can easily predict the shape, size, and reactivity of molecules.

Exceptions to the Octet Rule

While the octet rule is a useful guideline, several molecules do not follow it. These include exceptions such as molecules with

• an incomplete octet,
• expanded octet,
• free radicals.

For example, in SnF₄, the tin atom has more than eight electrons due to available d-orbitals, demonstrating an expanded octet.
Understanding these exceptions is crucial as many common molecules exhibit them, indicating alternative paths to stability beyond completing an octet.

Valence Electrons

Valence electrons play a vital role in chemical bonding and determine an atom's ability to bond with others. These electrons are in the outermost shell of an atom. Knowing the number of valence electrons allows predictions on how an atom might react.

• Atoms with a full valence shell are less likely to react, as seen with noble gases.
• Atoms strive to reach a stable octet configuration through bonding. For example, chlorine in Cl in PCl₃ has seven valence electrons and forms a bond to achieve an octet.

Thus, counting valence electrons is a critical first step in understanding molecule instability and reactivity.

Incomplete Octet

Some atoms, like boron in BCl₃, are content with less than eight electrons around them. Boron commonly forms stable compounds with only six electrons, which results in what we call an incomplete octet. This situation arises because boron lacks enough valence electrons to reach an octet.

Incomplete octets are typically seen in molecules with elements from groups having fewer available electrons for bonding, making these compounds significant exceptions to the octet rule.

Expanded Octet

An expanded octet happens when atoms hold more than eight electrons in their valence shells. This is possible for atoms that have available d-orbitals, typically starting from the third period on the periodic table. For instance, in SnF₄, the tin atom can accommodate extra electrons due to its d-sublevel.

Understanding expanded octets helps explain the behavior and formation of some molecules that apparently defy the octet rule. Furthermore, it highlights the flexibility of electron accommodation beyond simpler molecules.

Free Radicals

Free radicals are molecules with unpaired electrons. These unpaired electrons make the molecule highly reactive and unstable.

One example is the nitric oxide molecule (NO), which has an odd number of electrons, leaving one unpaired.

• This unpaired electron creates a free radical.
• It has implications in various chemical reactions and even biological processes.

Being reactive and short-lived, free radicals play important roles in both chemistry and our atmosphere.