Question

Draw the Lewis structures of the acid \(\mathrm{HPO}_{2} \mathrm{F}_{2}\) and its anion \(\mathrm{PO}_{2} \mathrm{F}_{2}^{-} .\) What is the molecular geometry and hybridization for the phosphorus atom in each species? (H is bonded to an O atom in the acid.)

Short answer

HPO2F2: tetrahedral, sp3; PO2F2-: tetrahedral, sp3.

Step-by-step solution

Count total valence electrons for HPO2F2

The molecule consists of hydrogen (H), phosphorus (P), oxygen (O), and fluorine (F). Hydrogen has 1 valence electron, phosphorus has 5, oxygen has 6 (each), and fluorine has 7 (each). Calculate the total valence electrons: \[ 1 + 5 + 2(6) + 2(7) = 32 \] valence electrons.

Draw skeleton structure for HPO2F2

Place phosphorus (P) in the center because it's the least electronegative atom and connected to H, O, and two F atoms based on the molecular formula. Start by connecting each atom with single bonds to phosphorus.

Assign remaining electrons and satisfy octets for HPO2F2

After forming single bonds, distribute remaining electrons to satisfy the octet rule. The phosphorus is connected via single bonds to one oxygen bonded to hydrogen, and two fluorines each receive lone pairs to reach 8 electrons around them. The P=O bond should be formed for complete octet satisfaction.

Determine geometry of HPO2F2

The phosphorus atom in HPO2F2 is surrounded by one double bond and three single bonds (including lone pairs on molecule), corresponding to four regions of electron density. This yields a tetrahedral electron geometry which is consistent with a bond angle of approximately 109.5°.

Determine hybridization of phosphorus in HPO2F2

Phosphorus in HPO2F2 has four regions of electron density. It uses \( sp^3 \) hybridization to form these bonds, typical of a tetrahedral shape.

Count electrons for anion PO2F2-

The anion is formed by removing H and adding an extra electron for the charge, modify the total count by +1. Calculate valence electrons in PO2F2⁻: \[ 5 + 2(6) + 2(7) + 1 = 33 \] valence electrons.

Draw skeleton of anion PO2F2-

Place P in the center, similar to the acid, connecting to O and F atoms through single bonds.

Assign electrons for anion PO2F2-

Distribute remaining electrons to the outer atoms first, then to phosphorus. The resonance forms or completion of the octet may require one of the oxygen atoms forming a double bond with phosphorus.

Determine geometry of PO2F2-

In PO2F2-, phosphorus is surrounded by overall four regions: one double bond and three single bonds as before. The geometry is tetrahedral, similar to the neutral acid, with electron regions determining the geometry.

Determine hybridization of phosphorus in PO2F2-

Phosphorus in the anion uses \( sp^3 \) hybridization, accounting for the same tetrahedral geometry seen in the acid.

Key concepts

Molecular Geometry

Molecular geometry is an essential concept when determining the shape and orientation of molecules. It refers to the three-dimensional arrangement of the atoms within a molecule. To understand the molecular geometry, we need to consider electron pair repulsion that dictates the shape of the molecule based on Valence Shell Electron Pair Repulsion (VSEPR) theory.
In the case of \( ext{HPO}_2 ext{F}_2\) and its anion \( ext{PO}_2 ext{F}_2^−\), phosphorus (P) is at the center of the molecule. Here, the phosphorus atom is bonded to four atoms: one oxygen through a double bond, another oxygen connected to a hydrogen, and two fluorine atoms by single bonds. This arrangement leads to four regions of electron density which result in a tetrahedral shape. This is a common geometric arrangement for molecules with a central atom connected to four substituents.
The tetrahedral shape is recognized by its bond angles of approximately 109.5°, a direct result of symmetrical electron pair repulsion.

Valence Electrons

Valence electrons are the outermost electrons of an atom and play a pivotal role in chemical bonding and reactions. These electrons are used to form bonds with other atoms, leading to molecule formation.

• Hydrogen (H) has 1 valence electron.
• Phosphorus (P) possesses 5 valence electrons.
• Oxygen (O) carries 6 valence electrons per atom.
• Fluorine (F) has 7 valence electrons per atom.

For \( ext{HPO}_2 ext{F}_2\), the total count of valence electrons is the sum of each atom's valence electrons, amounting to 32. After accounting for the bonds, remaining electrons are assigned as lone pairs to satisfy the octet rule, except for hydrogen. This distribution involves creating a double bond between phosphorus and one oxygen to ensure full octet completion for stability. For the anion \( ext{PO}_2 ext{F}_2^−\), there are 33 valence electrons, courtesy of the negative charge contributing an extra electron. This additional electron must be considered when constructing the Lewis structure to ensure all atoms achieve stable electronic configurations.

Tetrahedral Shape

The tetrahedral shape is one of the simplest yet most common molecular geometries. It arises in molecules where a central atom is surrounded by four atoms or groups of electrons. Each group of electrons tries to maximize its distance from others due to repulsion forces, ultimately arranging themselves symmetrically around the central atom.
For both \( ext{HPO}_2 ext{F}_2 \) and its anion \( ext{PO}_2 ext{F}_2^- \), the central phosphorus atom takes on a tetrahedral geometry. This is achieved due to it being bonded to four atoms—oxygen, another oxygen bound to hydrogen, and two fluorine atoms. The tetrahedral geometry ensures that the electron regions are evenly spaced, resulting in bond angles of approximately 109.5°. This symmetry and spacing minimize electron repulsion, maintaining the molecule's stability and shape.

Hybridization

Hybridization is a critical concept used to explain observed molecular shapes and bond formations. It involves the mixing of atomic orbitals to create new hybrid orbitals, which are degenerate.
For phosphorus in both \( ext{HPO}_2 ext{F}_2 \) and \( ext{PO}_2 ext{F}_2^- \), the hybridization is \( sp^3 \). This is because phosphorus forms four sigma bonds, which align with the four hybrid orbitals created by sp3 hybridization. In this process, the one s orbital and three p orbitals mix, resulting in four equivalent sp3 hybrid orbitals.
This type of hybridization corresponds to the tetrahedral electron geometry, facilitating the arrangement of bonds and lone pairs around phosphorus. Understanding hybridization is fundamental for predicting and understanding molecular geometry and structure.