A molecule with the formula AB₃ has a trigonal pyramidal geometry. How many electron groups are on the central atom (A)?

Verified step by step guidance

Step 1: Understand the concept of electron groups. Electron groups around a central atom include bonding pairs (shared electrons in covalent bonds) and lone pairs (non-bonding electrons). These groups determine the molecular geometry.

Step 2: Recall the geometry of a trigonal pyramidal molecule. A trigonal pyramidal shape arises when there are three bonding pairs and one lone pair of electrons around the central atom.

Step 3: Add the number of bonding pairs and lone pairs to determine the total number of electron groups. In this case, there are 3 bonding pairs (from the AB3 formula) and 1 lone pair, making a total of 4 electron groups.

Step 4: Relate the electron groups to the molecular geometry. The trigonal pyramidal geometry is consistent with 4 electron groups around the central atom, where one of the groups is a lone pair.

Step 5: Conclude that the central atom (A) in the AB3 molecule has 4 electron groups, which include 3 bonding pairs and 1 lone pair.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. It is determined by the number of bonding pairs and lone pairs of electrons around the central atom. In the case of AB3 with trigonal pyramidal geometry, the arrangement is influenced by the presence of lone pairs that repel bonding pairs, leading to a specific shape.

Electron Groups

Electron groups include both bonding pairs of electrons (shared between atoms) and lone pairs (non-bonding electrons) around a central atom. The total number of electron groups determines the molecular geometry according to the VSEPR (Valence Shell Electron Pair Repulsion) theory. For a trigonal pyramidal shape, the central atom must have four electron groups.

VSEPR Theory

VSEPR (Valence Shell Electron Pair Repulsion) theory is a model used to predict the geometry of molecules based on the repulsion between electron pairs. According to this theory, electron groups will arrange themselves as far apart as possible to minimize repulsion. In the case of AB3, the presence of one lone pair leads to a trigonal pyramidal shape, indicating that there are four electron groups around the central atom.