Question

Predict the shape of a molecule with each of the following: a. three bonded atoms and no lone pairs b. two bonded atoms and one lone pair

Short answer

a. Trigonal planar; b. Bent or angular.

Step-by-step solution

Identify the basic concept

The shape of a molecule is predicted using the Valence Shell Electron Pair Repulsion (VSEPR) theory, which states that electron pairs around a central atom will arrange themselves to minimize repulsion.

Determine electron pairs for part (a)

For a molecule with three bonded atoms and no lone pairs, there are a total of three electron pairs around the central atom. According to VSEPR, these pairs will arrange themselves to minimize repulsion, resulting in a trigonal planar shape.

Determine electron pairs for part (b)

For a molecule with two bonded atoms and one lone pair, there are a total of three electron pairs around the central atom. However, one of these pairs is a lone pair, which takes up more space and pushes the bonded atoms closer together. According to VSEPR, this results in a bent or angular shape.

Key concepts

molecular geometry

The shape or structure of a molecule is called its molecular geometry. The molecular geometry is crucial because it helps in predicting the behavior and reactivity of the molecule. It is determined by the arrangement of atoms around a central atom. The VSEPR theory is often used to predict this geometry, where electron pairs, both bonding pairs and lone pairs, are considered.

If a molecule has three bonded atoms and no lone pairs, as in part (a) of the exercise, the three electron pairs around the central atom will arrange themselves in a trigonal planar shape. Here, the bond angles are 120 degrees, which minimizes repulsion between electron pairs.

For a molecule with two bonded atoms and one lone pair, like in part (b) of the exercise, there are also three regions of electron density. However, because one region is a lone pair, it exerts greater repulsion, causing the bonded atoms to come closer together. This results in a bent or angular shape, with bond angles slightly less than 120 degrees.

electron pairs

Electron pairs play a vital role in determining the shape of a molecule. They include both bonding pairs and lone pairs. Bonding pairs are shared between atoms to form bonds, while lone pairs are localized on a single atom.

According to the VSEPR theory, whether they are involved in bonding or not, electron pairs will repel each other and arrange themselves to minimize repulsion around the central atom. This arrangement determines the molecule's shape. For example:

• In the case of three bonded atoms and no lone pairs (part a), the three pairs arrange in a trigonal planar geometry.
• With two bonded atoms and one lone pair (part b), the shape becomes bent or angular as the lone pair repels the bonded pairs more strongly.

By understanding the interaction and spatial demands of these electron pairs, we can predict the molecular geometry accurately.

central atom

The central atom is the atom in a molecule to which other atoms are bonded. It is the reference point for determining the geometry of the molecule. The central atom is usually the one with the lowest electronegativity or the one that can form the most bonds.

In VSEPR theory, the central atom is surrounded by electron pairs (bonding and lone pairs), and the geometry of the molecule depends on minimizing the repulsion between these pairs. For example:

• In part (a) of the given exercise, the central atom has three bonded atoms and no lone pairs, resulting in a trigonal planar shape.
• In part (b), it has two bonded atoms and one lone pair, leading to a bent or angular shape.

Choosing the correct central atom and analyzing the electron pairs around it is crucial for predicting the molecular geometry effectively.