Question

The most common type of exception to the octet rule are compounds or ions with central atoms having more than eight electrons around them. \(\mathrm{PF}_{5}, \mathrm{SF}_{4}, \mathrm{ClF}_{3}\) and \(\mathrm{Br}_{3}^{-}\) are examples of this type of exception. Draw the Lewis structure for these compounds or ions. Which elements, when they have to, can have more than eight electrons around them? How is this rationalized?

Short answer

The Lewis structures for the given compounds or ions are as follows: PF5 has Phosphorus as the central atom bonded to five Fluorine atoms; SF4 has Sulfur as the central atom bonded to four Fluorine atoms; ClF3 has Chlorine as the central atom bonded to three Fluorine atoms; and Br3- has a central Bromine atom bonded to two other Bromine atoms. Elements like Phosphorus, Sulfur, Chlorine, and Bromine, which are in the third period and beyond, can have more than eight electrons around them due to the availability of d-orbitals. This allows them to form stronger and more stable molecules by expanding their octet using d-orbitals, in addition to the s and p orbitals.

Step-by-step solution

1. Drawing the Lewis structure for PF5 (phosphorus pentafluoride)

: 1. Count valence electrons: Phosphorus has 5 valence electrons and Fluorine has 7. Since there are 5 fluorine atoms, the total number of electrons is 5(Phosphorus) + 5x7(Fluorine) = 40. 2. Place the central atom, Phosphorus, and add fluorine atoms around it. 3. Create bonds between each fluorine and phosphorus by sharing a pair of electrons. 4. Distribute the remaining electrons to outer atoms (fluorine) to fill their octets. 5. As there are no additional electrons left, the Lewis Structure for PF5 is complete.

2. Drawing the Lewis structure for SF4 (sulfur tetrafluoride)

: 1. Count valence electrons: Sulfur has 6 valence electrons and Fluorine has 7. Since there are 4 fluorine atoms, the total number of electrons is 6(Sulfur) + 4x7(Fluorine) = 34. 2. Place the central atom, Sulfur, and add fluorine atoms around it. 3. Create bonds between each fluorine and sulfur by sharing a pair of electrons. 4. Distribute the remaining electrons to outer atoms (fluorine) and sulfur to fill their octets or expanded octets. 5. The Lewis Structure for SF4 is complete.

3. Drawing the Lewis structure for ClF3 (chlorine trifluoride)

: 1. Count valence electrons: Chlorine has 7 valence electrons and Fluorine has 7 as well. Since there are 3 fluorine atoms, the total number of electrons is 7(Chlorine) + 3x7(Fluorine) = 28. 2. Place the central atom, Chlorine, and add fluorine atoms around it. 3. Create bonds between each fluorine and chlorine by sharing a pair of electrons. 4. Distribute the remaining electrons to outer atoms (fluorine) and chlorine to fill their octets or expanded octets. 5. The Lewis Structure for ClF3 is complete.

4. Drawing the Lewis structure for Br3- (tribromide ion)

: 1. Count valence electrons: Bromine has 7 valence electrons. Since there are 3 bromine atoms and the ion has an extra electron, the total number of electrons is 3x7(Bromine) +1 = 22. 2. Place one Bromine atom as the central atom and other two Bromine atoms around it. 3. Create bonds between central bromine and surrounding bromines by sharing a pair of electrons. 4. Distribute the remaining electrons to outer atoms (bromine) and the central bromine to satisfy their octets or expanded octets. 5. The Lewis Structure for Br3- is complete.

5. Identifying elements that can have more than eight electrons around them and rationalization

: From the drawn structures, the central atoms in the molecules PF5, SF4, and ClF3, as well as the ion Br3-, have more than eight electrons. These elements are Phosphorus, Sulfur, Chlorine, and Bromine. The central atoms of these compounds belong to the third period and beyond, which have a d-orbital available for electron occupation. They can accommodate more than eight electrons in their valence shell, called expanded octets, by using their d-orbital in addition to the s and p orbitals. As a result, they can form stronger and more stable molecules by having more than 8 valence electrons in their outer shell.

Key concepts

Lewis structures

Lewis structures are visual representations of molecules that show how atoms within them are bonded. They enable us to understand how electrons are arranged in these molecules and help predict molecular geometry, reactivity, and properties. To create a Lewis structure, you must follow a few simple steps:

• Identify and count the total valence electrons in the molecule.
• Choose the central atom, typically the least electronegative, to begin drawing.
• Use lines to represent bonds using pairs of electrons between atoms.
• Distribute remaining electrons to fill the octets of surrounding atoms (typically achieved with nonmetals).
• Ensure the central atom has a stable electron configuration, which may mean using an expanded octet if necessary.

These diagrams simplify complex molecular insights and are crucial for understanding molecular behavior.

expanded octet

The concept of an expanded octet refers to the ability of certain atoms to hold more than eight electrons in their valence shell. This might seem like an odd exception to the well-known octet rule, where atoms strive to have eight electrons in their outermost shell to become stable.

However, atoms in the third period and beyond, such as phosphorus, sulfur, chlorine, and bromine, can do this because they have vacant d-orbitals. These d-orbitals allow them additional space to "expand" their capacity. For example, in phosphorus pentafluoride (\( PF_5 \)), phosphorus uses 10 electrons,where an extra pair occupies these d-orbitals.

• Expanded octets stabilize the molecule by accommodating more electron pairs.
• They enable formation of stronger bonds and complex molecular structures.
• This is common in central atoms surrounded by small, highly electronegative atoms.

Expanded octets are a fascinating aspect of chemical bonding, providing an exception that enriches our understanding of molecular chemistry.

valence electrons

Valence electrons are the outermost electrons of an atom and play a pivotal role in chemical bonding and reactions. These are the electrons available for bond formation, and their arrangement determines an atom's chemical properties.

For instance, phosphorus has five valence electrons, whereas fluorine has seven. When drawing Lewis structures like for \( PF_5 \), understanding the number of valence electrons is critical for depicting accurate electron dot structures.

• Valence electrons determine how atoms interact with one another.
• The octet rule aims for a total of eight electrons, a common goal for stability.
• Understanding valence helps identify potential reactions and compound stability.

Knowing how valence electrons function allows chemists to predict how atoms will connect, guiding the synthesis of everything from basic molecules to complex compounds.

third period elements

Third period elements on the periodic table are intriguing because they have access to d-orbitals, allowing for more versatile bonding compared to elements of the second period. Elements like phosphorus, sulfur, and chlorine can form compounds with more than eight electrons, taking advantage of these orbitals for expanded octets.

These elements are thus capable of "breaking" the octet rule under certain conditions, engaging in binding configurations not possible for lighter elements.

• Third period elements can bind in multiple ways due to d-orbitals.
• They offer a deeper understanding of molecular complexity and bonding. They are essential for forming compounds with complex geometries and properties.
• Their chemistry is significant in understanding a wide range of biological and industrial compounds.

Their ability to form expanded octets exemplifies the fascinating complexity and the exceptions within chemical rules that lead to diverse and functional molecular structures.