Problem 85 - Chemistry An Atoms First Approach

Question

The most common exceptions 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?

Step-by-Step Solution

Answer

The Lewis structures of the compounds and ions given are as follows: \(\mathrm{PF}_5\): P at the center, surrounded by 5 F atoms, forming single bonds. P has 10 electrons around it. \(\mathrm{SF}_4\): S at the center, surrounded by 4 F atoms, forming single bonds. S has 10 electrons around it. \(\mathrm{ClF}_3\): Cl at the center, surrounded by 3 F atoms, forming single bonds. Cl has 10 electrons around it. \(\mathrm{Br}_3^{-}\): 3 Br atoms in a line, forming 2 single bonds between neighboring atoms. The central Br atom has 10 electrons around it. Elements from the third period or beyond in the Periodic Table, such as P, S, Cl, and Br, can have more than eight electrons due to the availability of d-orbitals in their valence shell. This expanded octet allows for the formation of stable compounds with high coordination numbers and various molecular geometries.

Step 1: Determine the total number of valence electrons

For each compound or ion, we need to determine the total number of valence electrons for all the atoms involved. We can do this by looking at the group number in the periodic table. To account for negative ions, add the appropriate number of electrons. For \(\mathrm{PF}_5\): P has 5 valence electrons and F has 7. Therefore, the total number of valence electrons is \(5 + 5\times7 = 40\). For \(\mathrm{SF}_4\): S has 6 valence electrons and F has 7. Therefore, the total number of valence electrons is \(6 + 4\times7 = 34\). For \(\mathrm{ClF}_3\): Cl has 7 valence electrons and F has 7. Therefore, the total number of valence electrons is \(7 + 3\times7 = 28\). For \(\mathrm{Br}_3^{-}\): Br has 7 valence electrons. With the negative charge, we add another electron. Therefore, the total number of valence electrons is \(3\times7 + 1 = 22\).

Step 2: Draw the Lewis structure for each compound or ion

Using the valence electrons, we will draw the Lewis structures for each compound. For central atoms that break the octet rule having more than 8 electrons, distribute the remaining electrons to satisfy the octet rule for the outer atoms. For \(\mathrm{PF}_5\): Place P in the center, and surround it with 5 F atoms. Place single bonds between each P-F pair, and complete the octet for all the F atoms. P will have 10 electrons around it. For \(\mathrm{SF}_4\): Place S in the center, and surround it with 4 F atoms. Place single bonds between each S-F pair, and complete the octet for all the F atoms. S will have 10 electrons around it. For \(\mathrm{ClF}_3\): Place Cl in the center, and surround it with 3 F atoms. Place single bonds between each Cl-F pair, and complete the octet for all F atoms. Cl will have 10 electrons around it. For \(\mathrm{Br}_3^{-}\): Place 3 Br atoms in the line and form 2 single bonds between the neighboring Br atoms. Distribute the remaining electrons to complete the octet rule for each Br atom, adding the extra electron to the central Br atom. The central Br atom will have 10 electrons around it. The Lewis structure of these ions and compounds all have a central atom with an expanded octet, that is, having more than eight electrons around them.

Step 3: Identify the elements that can have more than eight electrons and rationalize the phenomenon

The elements that can have more than eight electrons are those from the third period and beyond in the Periodic Table. This includes P, S, Cl, and Br, as seen in the examples above. The phenomenon occurs due to d-orbitals available in the valence shell of these elements, allowing them to accommodate additional electrons beyond the usual octet. This expanded octet helps these elements form stable compounds with high coordination numbers and gives them various molecular geometries.

Key Concepts

Lewis Structures

Understanding Lewis structures is fundamental to grasping how molecules are formed and predicted. A Lewis structure is a visual representation of the bonds between atoms in a molecule and shows how valence electrons are distributed. It helps us predict molecule shape, reactivity, and how atoms are bonded. Here's a quick rundown of how to draw a Lewis structure:

• Total the valence electrons from all atoms.
• Sketch a skeletal structure, connecting atoms with single bonds, which represent two electrons shared.
• Distribute remaining electrons to complete the octet for outer atoms first.
• Consider central atom's octet. If any electrons remain, place them on the central atom.

Lewis structures often follow the "octet rule," however many compounds, especially those involving elements like P, S, and Cl, will demonstrate exceptions where the central atom has more than eight electrons.

Valence Electrons

Valence electrons are the outer electrons of an atom and are crucial in chemical bonding and reactions. They determine an atom's ability to bond with others, and in Lewis structures represent the electrons available for forming bonds. You can easily find the number of valence electrons by looking at the group number in the periodic table:

• Group 1 elements have 1 valence electron.
• Group 2 elements have 2 valence electrons.
• Groups 13-18 reflect 3-8 valence electrons respectively.

Understanding valence electrons is key for drawing Lewis structures, predicting molecule reactivity, and determining the overall stability of compounds. In exceptions to the octet rule, such as in some elements capable of forming an expanded octet, valence electrons play a role in stabilizing unusual structures.

Expanded Octet

In chemistry, some molecules deviate from what we commonly know as the 'octet rule.' This rule states that atoms tend to form bonds until they are surrounded by eight valence electrons. However, an expanded octet occurs when atoms have more than eight electrons.Understanding an expanded octet involves acknowledging that:

• It happens in elements from Period 3 and beyond.
• These atoms utilize available d-orbitals to accommodate extra electrons.
• The increased number of surrounding electrons accounts for more complex and stable structures.

Examples include \(\mathrm{PF}_{5}\), \(\mathrm{SF}_{4}\), \(\mathrm{ClF}_{3}\), and \(\mathrm{Br}_{3}^{-}\). These compounds can afford more than eight electrons because their central atoms are capable of expanding their electron cloud, which allows astonishing flexibility in molecular design.

Periodic Table Insight

The periodic table is an essential tool for understanding chemical behavior and predicting how atoms will interact. It organizes elements by increasing atomic number and similar chemical properties, which suggests trends in atomic size, electronegativity, and valence electrons. When it comes to octet exceptions:

• Elements from Period 3 and beyond—like phosphorus (P), sulfur (S), chlorine (Cl), and bromine (Br)—display these exceptions due to available d-orbitals in their valence shell.
• These elements can expand their octet, enabling them to form more bonds than the typical limit of eight electrons.
• The d-orbitals facilitate electron accommodation, which is employed in forming stable, expanded structures found in many advanced chemicals.

Using the periodic table offers insights into potential chemical reactivity and bond formation, particularly in understanding which elements can form expanded octets and why these structures are stable.