Question

Determine the geometry of the following molecules: a. \(\mathrm{CCl}_{4}\) b. \(\mathrm{BeCl}_{2}\) c. \(\mathrm{PH}_{3}\)

Short answer

a. Tetrahedral, b. Linear, c. Trigonal pyramidal.

Step-by-step solution

Determine the Lewis Structure

Draw the Lewis structures for each molecule to figure out the arrangement of electrons around the central atom. This helps in understanding the basic layout of the atoms.

Count Electron Pairs Around Central Atom

Count the bonding pairs and lone pairs of electrons on the central atom for each molecule. Use the Lewis structure to identify the electron pairs.

Apply VSEPR Theory

Using the Valence Shell Electron Pair Repulsion (VSEPR) theory, predict the geometry of each molecule based on the number of bonding and lone pairs around the central atom.

Determine Geometry for \(\text{CCl}_{4}\)

\[\mathrm{CCl}_{4}\]: The central carbon atom has 4 bonding pairs and no lone pairs. According to VSEPR theory, this results in a tetrahedral geometry.

Determine Geometry for \(\text{BeCl}_{2}\)

\[\mathrm{BeCl}_{2}\]: The central beryllium atom has 2 bonding pairs and no lone pairs. According to VSEPR theory, this results in a linear geometry.

Determine Geometry for \(\text{PH}_{3}\)

\[\mathrm{PH}_{3}\]: The central phosphorus atom has 3 bonding pairs and 1 lone pair. According to VSEPR theory, this results in a trigonal pyramidal geometry.

Key concepts

Lewis Structure

To understand the molecular geometry of a molecule, the first step is to determine its Lewis structure. This diagram represents the arrangement of electrons around the central atom and the other atoms in the molecule. By knowing the Lewis structure, we can figure out how atoms are bonded and identify any lone pairs of electrons. For instance, let's look at \(\text{CCl}_{4}\).
Carbon sits at the center of the molecule with four chlorine atoms surrounding it. Each carbon-chlorine bond involves a pair of shared electrons, forming a single bond. So, in the Lewis structure of \(\text{CCl}_{4}\), the carbon atom is surrounded by four bonding pairs and no lone pairs. The step-by-step drawing of these structures helps to visualize and understand how atoms are put together in 3D space.

VSEPR Theory

Once the Lewis structure is drawn, we use the Valence Shell Electron Pair Repulsion (VSEPR) theory to predict the molecular geometry. VSEPR theory states that pairs of electrons around a central atom will orient themselves as far apart as possible to minimize repulsion. This theory helps us predict the shape of the molecule:
1. For \(\text{CCl}_{4}\), there are four bonding pairs around the carbon atom. According to VSEPR theory, they arrange themselves in a tetrahedral geometry to maximize their distances.
2. In \(\text{BeCl}_{2}\), the beryllium atom in the center has two bonding pairs and no lone pairs. These pairs align linearly, a 180-degree angle separating them.
3. With \(\text{PH}_{3}\), there are three bonding pairs and one lone pair around the phosphorus atom. The lone pair causes the bonding pairs to adopt a trigonal pyramidal shape, pushing bonded atoms slightly closer together than in a perfect tetrahedral arrangement.

Electron Pairs

In VSEPR theory, both bonding pairs and lone pairs of electrons around the central atom influence molecular shape. Electron pairs repel each other, determining the molecule's geometry:
1. Bonding pairs are shared between atoms, forming chemical bonds.
2. Lone pairs are not shared but belong entirely to one atom.
3. Lone pairs take up more space than bonding pairs because they are only subject to the nucleus's attraction and not another atom's.
In \(\text{PH}_{3}\), the lone pair on phosphorus pushes the three hydrogen atoms closer together, resulting in a trigonal pyramidal shape, instead of a flat triangular shape. Ultimately, recognizing and counting these pairs is crucial to predicting molecular geometry accurately.