Determine the molecular structure for the PH4+ molecule (FULL credit) (you must show your work to get full credit) Select one: – Name the Bond Angles – Name the Geometry – State the amount of Valence electrons – Draw the Structure – State the formal charge
The Correct Answer and Explanation is:
To determine the molecular structure of PH₄⁺ (phosphonium ion), we need to analyze its valence electrons, geometry, bond angles, and formal charge. Here is the full answer with calculations, drawing, and explanations.
✔ Final Answer:
- Valence Electrons: 8 total valence electrons
- Geometry: Tetrahedral
- Bond Angles: Approximately 109.5°
- Formal Charge on Phosphorus (P): +1
🧮 Step-by-Step Work:
1. Determine Total Valence Electrons:
- Phosphorus (P) has 5 valence electrons.
- Each Hydrogen (H) has 1 valence electron.
- There are 4 hydrogen atoms: 4 × 1 = 4 electrons.
- The positive charge (+1) means one electron is removed.
Total:
5 (P) + 4 (H) = 9
9 – 1 (due to +1 charge) = 8 valence electrons
2. Draw the Lewis Structure:
- Place P in the center.
- Connect 4 H atoms to P using single bonds (each bond uses 2 electrons).
- 4 bonds × 2 electrons = 8 electrons → all valence electrons used.
H
|
H - P - H
|
H
No lone pairs are left on P. Each H has 2 electrons, satisfying its duet rule.
3. Molecular Geometry:
- P is bonded to 4 atoms, with 0 lone pairs.
- This is the AX₄ type in VSEPR theory → Tetrahedral geometry.
- Bond Angles = 109.5°
4. Formal Charge Calculation on P:
Formal Charge (FC) = [Valence e⁻] – [Non-bonding e⁻] – ½(Bonding e⁻)
For P:
- Valence electrons = 5
- Non-bonding electrons = 0
- Bonding electrons = 8 (4 bonds × 2 e⁻)
FC = 5 – 0 – (8/2) = 5 – 4 = +1
So, the phosphorus has a formal charge of +1, matching the overall PH₄⁺ charge.
🧠 Summary Explanation
The phosphonium ion, PH₄⁺, is a positively charged molecule formed when phosphorus bonds with four hydrogen atoms. The first step in understanding its molecular structure is calculating the total number of valence electrons. Phosphorus contributes 5 electrons, and each hydrogen contributes 1, making 9 total. However, because the molecule has a positive charge (+1), one electron is removed, leaving 8 valence electrons.
These 8 electrons are used to form four single covalent bonds between the central phosphorus atom and four hydrogen atoms. Each bond uses two electrons, and no lone pairs remain on the phosphorus atom. Because the phosphorus atom is bonded to four atoms with no lone pairs, the molecule follows the AX₄ arrangement in VSEPR (Valence Shell Electron Pair Repulsion) theory. This gives the molecule a tetrahedral shape with bond angles of 109.5°, which is characteristic of tetrahedral molecules.
Next, we calculate the formal charge to ensure the charge on the central atom matches the ion’s overall charge. The formal charge on phosphorus is +1, which matches the molecular ion’s overall charge of +1. This confirms the accuracy of our Lewis structure and electron configuration.
In summary, PH₄⁺ has a tetrahedral geometry with 109.5° bond angles, uses 8 valence electrons for bonding, and has a formal charge of +1 on phosphorus. Understanding this molecular structure is essential for predicting the molecule’s shape, reactivity, and interaction with other chemical species.
