Question

In the molecule \(\mathrm{PCl}_{3}\), the four electron pairs around the phosphorus atom are arranged in a tetrahedral geometry. However, the shape of the molecule is called trigonal pyramidal. Why does the shape of the molecule have a different name from the name of the electron pair geometry?

Short answer

The presence of a lone pair on phosphorus causes the shape to be trigonal pyramidal, different from the tetrahedral electron pair geometry.

Step-by-step solution

- Identify the Electron Pair Geometry

The electron pair geometry considers all electron pairs (bonding and lone pairs) around the central atom. In \(\mathrm{PCl}_{3}\)\, phosphorus has three bonding pairs and one lone pair of electrons. These four electron pairs are arranged in a tetrahedral geometry.

- Determine the Molecular Shape

The molecular shape only considers the positions of the atoms, ignoring lone pairs. In \(\mathrm{PCl}_{3}\)\, the three chlorine atoms form a base, while phosphorus is at the vertex, resulting in a trigonal pyramidal shape.

- Explain the Difference

The shape of the molecule (trigonal pyramidal) is different from the electron pair geometry (tetrahedral) because lone pairs occupy space and influence the molecule's shape but are not visible in the determination of the molecular shape which only considers the positions of the atoms.

Key concepts

electron pair geometry

When we talk about electron pair geometry, we are considering the arrangement of all electron pairs around a central atom. This includes both bonding pairs, which form covalent bonds with other atoms, and lone pairs, which are non-bonding. In the molecule \(\text{PCl}_3\), phosphorus has four electron pairs around it: three bonding pairs with chlorine atoms, and one lone pair. These four electron pairs arrange themselves to minimize repulsion, resulting in a tetrahedral geometry. Tetrahedral geometry has bond angles of about 109.5° between each pair.

trigonal pyramidal

The term trigonal pyramidal describes the shape of a molecule when only the positions of atoms are considered, ignoring lone pairs. For \(\text{PCl}_3\), the three chlorine atoms and the phosphorus atom arrange themselves with the phosphorus at the peak and the chlorines forming a base, similar to a pyramid with a triangular base. This shape results from the lone pair on the phosphorus pushing the three bonded chlorine atoms closer together, leading to bond angles of slightly less than 109.5°.

• The lone pair takes up more space compared to bonding pairs, increasing repulsion.
• This distortion causes the trigonal pyramidal shape.

tetrahedral geometry

Tetrahedral geometry is a common electron pair geometry where four pairs of electrons are arranged symmetrically around a central atom, optimizing space to minimize electron-electron repulsion.

• Chemically, it involves four equivalent positions in 3D space.
• Every position forms a 109.5° angle with another.

In \(\text{PCl}_3\), though the molecule has a trigonal pyramidal shape, the underlying electron pair arrangement is tetrahedral. The distinction is crucial: electron pair geometry considers lone pairs, while molecular shape does not. This explains why the actual molecule's shape differs from the electron pair arrangement.