Question

Describe the geometry of the species in which there are, around the central atom, (a) four single bonds, two unshared pairs of electrons. (b) five single bonds. (c) two single bonds, one unshared pair of electrons. (d) three single bonds, two unshared pairs of electrons. (e) two single bonds, two unshared pairs of electrons. (f) five single bonds, one unshared pair of electrons.

Short answer

Question: Describe the geometry of species in each case: a) Four single bonds, two unshared pairs of electrons. b) Five single bonds. c) Two single bonds, one unshared pair of electrons. d) Three single bonds, two unshared pairs of electrons. e) Two single bonds, two unshared pairs of electrons. f) Five single bonds, one unshared pair of electrons. Answer: a) Square planar b) Trigonal bipyramidal c) Bent or V-shaped d) T-shaped e) Bent or V-shaped f) Square pyramidal

Step-by-step solution

a) Four single bonds, two unshared pairs of electrons.

In this case, the central atom is surrounded by 4 bonding electron pairs and 2 lone (unshared) pairs. According to VSEPR theory, these 6 electron pairs arrange in an octahedral electron pair geometry. However, the molecular shape will be square planar due to the presence of two unshared electron pairs.

b) Five single bonds.

Here, the central atom has 5 bonding electron pairs and 0 lone pairs. The electron pairs will arrange themselves in a trigonal bipyramidal geometry. As there are no lone pairs, the molecular shape will also be trigonal bipyramidal.

c) Two single bonds, one unshared pair of electrons.

In this case, there are 2 bonding electron pairs and 1 lone pair around the central atom. The three electron pairs will form a trigonal planar electron pair geometry. Due to the presence of one unshared electron pair, the molecular shape will be bent or V-shaped.

d) Three single bonds, two unshared pairs of electrons.

Here, there are 3 bonding electron pairs and 2 lone pairs surrounding the central atom. The five electron pairs form a trigonal bipyramidal electron pair geometry. The molecular shape will be T-shaped due to the presence of two unshared electron pairs.

e) Two single bonds, two unshared pairs of electrons.

In this case, there are 2 bonding electron pairs and 2 lone pairs around the central atom. The four electron pairs will arrange themselves in a tetrahedral electron pair geometry. However, due to the presence of two unshared electron pairs, the molecular shape will be bent or V-shaped.

f) Five single bonds, one unshared pair of electrons.

Here, the central atom has 5 bonding electron pairs and 1 lone pair. The 6 electron pairs will form an octahedral electron pair geometry. Due to the presence of one unshared electron pair, the molecular shape will be square pyramidal.

Key concepts

VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) Theory focuses on minimizing the repulsion between electron pairs in the valence shell of an atom. This theory explains why molecules assume specific shapes based on the arrangement of electron pairs around a central atom. According to VSEPR, electron pairs—whether involved in bonds or existing as lone pairs—will attempt to maximize their distance from one another. This distance-minimization helps reduce the electrostatic repulsion between these pairs.

Using this theory allows chemists to predict the three-dimensional structure of molecules just by analyzing the number of bonding and non-bonding electron pairs. For example, in the case of a central atom with four bonding pairs and two lone pairs, the VSEPR theory predicts an octahedral arrangement for these electron pairs, which results in a square planar molecular shape due to the lone pairs' presence.

Electron Pair Geometry

Electron pair geometry refers to the spatial arrangement of all electron pairs (bonding and lone pairs) around a central atom. It is purely determined by these electron pairs without considering the actual molecular shape, which also depends on the presence of atoms connected via these bonds.

For instance, consider a central atom surrounded by five bonding pairs. The electron pair geometry will be trigonal bipyramidal because of the five electron pairs. Considering lone pairs, the geometry can change without altering the central geometric idea set by electron pairs. Lone pairs can lead to a difference between the electron pair geometry and the actual shape of the molecule.

Molecular Shape

The molecular shape of a compound is defined by the positions of the nuclei (or atoms) in a molecule, excluding lone pairs. VSEPR theory helps predict these shapes by considering only the placements of atoms after accounting for the repulsions between electron pairs.

A central atom with two single bonds and one lone pair results in a bent or V-shaped molecular shape due to the repulsion between the bonding pairs and the lone pair. Similarly, if there are three single bonds and two lone pairs, a T-shaped molecular shape emerges. Here, the lone pairs still exert influence, pushing the bonding pairs closer to form specific angles and creating distinct shapes.

Lone Pairs

Lone pairs refer to the pairs of valence electrons that are not involved in bonding but still occupy space near an atom. These pairs are crucial in determining both electron pair geometry and molecular shape. Their presence increases repulsion as their negative charge repels other electron pairs, especially bonding pairs.

For example, in a molecule where the central atom is bonded to two other atoms and has two lone pairs, the lone pairs cause greater repulsion than bonding pairs would alone, leading to a bent molecular shape. Lone pairs can effectively change a molecule from a theoretical shape—like trigonal bipyramidal with no lone pairs—to a more constrained shape when present.

Bonding Electron Pairs

Bonding electron pairs form the primary links between atoms through covalent bonds. These pairs are especially important in determining the molecular structure as they link the atoms together.

The arrangement of bonding electron pairs around a central atom informs the electron pair geometry. For a scenario involving four single bonds, the electron pair geometry and the molecular shape align as tetrahedral. It's essential to differentiate between the arrangement of all electron pairs versus just the placement of the bonded atoms to fully grasp the molecular architecture. In essence, the presence and position of bonding electron pairs serve as a fundamental blueprint for the molecule's shape.