pf6− does not obey the octet rule. draw its lewis structure and state the type of octet-rule exception. indicate the values of nonzero formal charges. be sure to include lone pair electrons on all atoms.
The Correct Answer and Explanation is :
Lewis Structure of PF₆⁻
To draw the Lewis structure for the PF₆⁻ ion, we start by determining the total number of valence electrons. Phosphorus (P) has 5 valence electrons, and each fluorine (F) atom has 7 valence electrons. Therefore, for six fluorine atoms, we have:
- Valence electrons from P: 5
- Valence electrons from 6 F: (6 \times 7 = 42)
- Total valence electrons: (5 + 42 + 1 \,(\text{for the negative charge}) = 48)
Steps to Draw the Structure:
- Central Atom: Place phosphorus in the center, as it can expand its octet.
- Bonding with Fluorine: Connect each fluorine atom to phosphorus with a single bond. This uses (6 \times 2 = 12) electrons (6 bonds).
- Assign Lone Pairs: After bonding, assign three lone pairs to each fluorine to complete their octets. This adds (6 \times 6 = 36) electrons.
- Final Structure: Phosphorus has six bonds (single) with six fluorine atoms, and each fluorine has three lone pairs.
Lewis Structure Representation:
F
|
F - P - F
|
F
|
F
|
FFormal Charges Calculation:
The formal charge (FC) can be calculated using the formula:
[
\text{FC} = \text{Valence electrons} – \text{(Nonbonding electrons + 0.5 Bonding electrons)}
]
- For phosphorus:
- Valence electrons = 5
- Nonbonding electrons = 0
- Bonding electrons = 12 (6 bonds)
- FC = (5 – (0 + 0.5 \times 12) = 5 – 6 = -1)
- For each fluorine:
- Valence electrons = 7
- Nonbonding electrons = 6
- Bonding electrons = 2 (1 bond)
- FC = (7 – (6 + 0.5 \times 2) = 7 – 7 = 0)
Type of Octet Rule Exception:
PF₆⁻ is an example of a hypervalent molecule. In this case, phosphorus expands its octet to accommodate more than eight electrons, thus breaking the octet rule. Hypervalent species are common in elements from the third period and below in the periodic table, where d-orbitals can be utilized for bonding.
Summary:
The Lewis structure of PF₆⁻ features phosphorus at the center with six fluorine atoms surrounding it, reflecting its ability to expand its octet. The formal charge on phosphorus is -1, while fluorine atoms remain neutral. This structure demonstrates how certain molecules can exceed the octet rule, especially in cases involving elements capable of utilizing d-orbitals for bonding.