Problem 77 - Chemistry Principles and Practice

Question

Write the Lewis structures for the following species, and indicate whether each is an odd-electron species, an electron-deficient species, or an expanded valence shell species. There is only one central atom in each. (a) \(\mathrm{XeF}_{2}\) (b) \(\mathrm{BeCl}_{2}\) (c) \(\mathrm{XeO}_{2} \mathrm{~F}_{4}\) (both \(\mathrm{O}\) and \(\mathrm{F}\) are terminal atoms)

Step-by-Step Solution

Answer

(a) Expanded valence shell. (b) Electron-deficient. (c) Expanded valence shell.

Step 1: Identify Total Valence Electrons

For each molecule, sum the valence electrons from each atom to find the total number of valence electrons.- \( \mathrm{XeF}_{2} \): \( \mathrm{Xe} \) has 8 valence electrons, and each \( \mathrm{F} \) has 7, totalling (8 + 7*2) = 22 valence electrons.- \( \mathrm{BeCl}_{2} \): \( \mathrm{Be} \) has 2, and each \( \mathrm{Cl} \) has 7, totalling (2 + 7*2) = 16 valence electrons.- \( \mathrm{XeO}_{2} \mathrm{F}_{4} \): \( \mathrm{Xe} \) has 8, each \( \mathrm{O} \) has 6, and each \( \mathrm{F} \) has 7, totalling (8 + 6*2 + 7*4) = 48 valence electrons.

Step 2: Determine Central Atom and Terminal Atoms

Identify the central and terminal atoms for each species.- In \( \mathrm{XeF}_{2} \), \( \mathrm{Xe} \) is the central atom with \( \mathrm{F} \) atoms as terminal.- In \( \mathrm{BeCl}_{2} \), \( \mathrm{Be} \) is the central atom with \( \mathrm{Cl} \) atoms as terminal.- In \( \mathrm{XeO}_{2} \mathrm{F}_{4} \), \( \mathrm{Xe} \) is the central atom, while both \( \mathrm{O} \) and \( \mathrm{F} \) are terminal atoms.

Step 3: Sketch Initial Structures

Draw an initial Lewis structure connecting each central atom to its terminal atoms using single bonds.- \( \mathrm{XeF}_{2} \) is represented as \( \mathrm{F} - \mathrm{Xe} - \mathrm{F} \).- \( \mathrm{BeCl}_{2} \) is \( \mathrm{Cl} - \mathrm{Be} - \mathrm{Cl} \).- \( \mathrm{XeO}_{2} \mathrm{F}_{4} \) is \( \mathrm{F} - \mathrm{Xe} - \mathrm{F} \) with \( \mathrm{O} \) atoms also attached to \( \mathrm{Xe} \).

Step 4: Assign Lone Pairs and Complete Octets

Add lone pairs to complete the octet for terminal atoms, and then adjust for the central atom.- \( \mathrm{XeF}_{2} \): Add 3 lone pairs to each \( \mathrm{F} \) and 3 lone pairs to \( \mathrm{Xe} \).- \( \mathrm{BeCl}_{2} \): Add 3 lone pairs to each \( \mathrm{Cl} \); \( \mathrm{Be} \) is electron-deficient.- \( \mathrm{XeO}_{2} \mathrm{F}_{4} \): Add 3 lone pairs to each \( \mathrm{F} \) and 2 lone pairs to each \( \mathrm{O} \); \( \mathrm{Xe} \) uses an expanded valence shell.

Step 5: Determine Species Categories

Classify each species based on their electron configuration.- \( \mathrm{XeF}_{2} \): Has an expanded valence shell.- \( \mathrm{BeCl}_{2} \): Electron-deficient.- \( \mathrm{XeO}_{2} \mathrm{F}_{4} \): Has an expanded valence shell.

Key Concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and play a crucial role in the formation of chemical bonds. Understanding how to count and distribute these electrons is fundamental in drawing Lewis structures, which provide a visual representation of the atomic structure.To begin, let's identify the number of valence electrons for the atoms in our example molecules:

• For \( \mathrm{XeF}_2 \), xenon (\( \mathrm{Xe} \)) contributes 8 valence electrons, and each fluorine (\( \mathrm{F} \)) contributes 7, totaling 22 valence electrons.
• In \( \mathrm{BeCl}_2 \), beryllium (\( \mathrm{Be} \)) provides 2 valence electrons, while each chlorine (\( \mathrm{Cl} \)) provides 7, reaching a total of 16 valence electrons.
• Finally, \( \mathrm{XeO}_2 \mathrm{F}_4 \) involves xenon contributing 8, each oxygen (\( \mathrm{O} \)) contributing 6, and each fluorine 7, resulting in a total of 48 valence electrons.

The distribution of these electrons in the Lewis structure helps identify whether the structure meets the octet rule or if there are deviations like electron-deficiency or expanded valence shells.

Electron-Deficient Species

Electron-deficient species are molecules that do not have enough valence electrons to complete the octet around their central atom. These species typically involve elements that are not able to complete their valence shells during bonding.In our examples:

• \( \mathrm{BeCl}_2 \) is classified as an electron-deficient species. Beryllium (\( \mathrm{Be} \)) typically holds only four electrons after forming single bonds with two chlorine atoms, falling short of the typical eight electrons needed for a full octet. This is common with lighter elements where completing an octet might be energetically unfavorable or impossible.

An electron-deficient molecule can accept electron pairs from donors, forming coordinate covalent bonds in some scenarios. This property influences their chemical reactivity and structural stability.

Expanded Valence Shell

An expanded valence shell occurs when an atom bonds in such a way that its outer shell holds more than eight electrons. This mostly happens with elements located in the third period or beyond, as they have d orbitals available to hold additional electrons.Considering our molecules:

• Both \( \mathrm{XeF}_2 \) and \( \mathrm{XeO}_2 \mathrm{F}_4 \) showcase expanded valence shells. In \( \mathrm{XeF}_2 \), xenon (\( \mathrm{Xe} \)) accommodates more than eight electrons by forming bonds with two \( \mathrm{F} \) atoms and retaining three lone pairs.
• In \( \mathrm{XeO}_2 \mathrm{F}_4 \), the central xenon atom bonds with six terminal atoms, utilizing its d orbitals to hold more electrons than a typical valence shell would allow.

Elements capable of expanding their valence shells can accommodate more bonds, hence, forming more complex structures. This flexibility enables them to participate in unique bonding environments and physical properties distinct from other molecules constrained by the octet rule.