Give Lewis dot structures and sketch the shapes of the PH4+:
The Correct Answer and Explanation is:
Lewis Dot Structure and Shape of PH₄⁺ (Phosphonium Ion)
Lewis Structure of PH₄⁺:
To draw the Lewis structure:
- Determine total valence electrons:
- Phosphorus (P) has 5 valence electrons.
- Hydrogen (H) has 1 valence electron, and there are 4 H atoms.
- Total from atoms: 5+4(1)=95 + 4(1) = 9
- PH₄⁺ has a +1 charge, meaning one electron is lost, so:
9−1=89 – 1 = 8 valence electrons.
- Structure:
- Place P in the center.
- Surround it with 4 H atoms, each forming a single bond with P.
- Electron usage:
- 4 bonds × 2 electrons = 8 electrons used (all electrons are bonding electrons).
- No lone pairs left on phosphorus.
Thus, the Lewis structure is:
H
|
H - P⁺ - H
|
H
The structure has 4 single bonds around phosphorus, no lone pairs, and a formal positive charge on P.
Molecular Geometry (Shape):
- PH₄⁺ has 4 bonding pairs and 0 lone pairs on the central atom.
- According to VSEPR theory, this corresponds to a tetrahedral geometry.
- Bond angles are approximately 109.5°.
Explanation
The phosphonium ion, PH₄⁺, is the conjugate acid of phosphine (PH₃) and resembles the structure of ammonium (NH₄⁺). To understand its structure, we start with the valence electron count. Phosphorus has 5 valence electrons, and each hydrogen contributes 1, totaling 9. Because the species is positively charged (+1), one electron is removed, giving a total of 8 electrons to use in the Lewis structure.
These 8 electrons are used to form four P–H single bonds, using all the available electrons, and leaving no lone pairs on phosphorus. This makes the Lewis structure symmetrical, with each hydrogen bonded directly to the phosphorus atom. A positive formal charge resides on phosphorus because it has fewer electrons than in its neutral state.
For molecular geometry, VSEPR theory (Valence Shell Electron Pair Repulsion) tells us that 4 regions of electron density (4 bonding pairs, 0 lone pairs) arrange themselves in a way to minimize repulsion. The optimal shape for this is a tetrahedral geometry, where the bond angles are about 109.5°. This structure allows for equal spacing between the bonding pairs, minimizing repulsion and resulting in a stable shape.
The tetrahedral shape of PH₄⁺ gives it a symmetric, non-polar molecular geometry, despite the formal charge. Understanding this geometry is important in inorganic and coordination chemistry, especially when predicting reaction mechanisms and bonding behavior of phosphine derivatives.
