Question

Predict the geometry of the following species: (a) \(\mathrm{SO}_{2}\) (b) \(\mathrm{BeCl}_{2}\) (c) \(\mathrm{SeCl}_{4}\) (d) \(\mathrm{PCl}_{5}\)

Short answer

Question: Predict the molecular geometry of \(\mathrm{SO}_{2}\), \(\mathrm{BeCl}_{2}\), \(\mathrm{SeCl}_{4}\), and \(\mathrm{PCl}_{5}\) using the VSEPR theory. Answer: The molecular geometry of \(\mathrm{SO}_{2}\) and \(\mathrm{BeCl}_{2}\) is linear, \(\mathrm{SeCl}_{4}\) has a see-saw geometry (or distorted tetrahedron), and \(\mathrm{PCl}_{5}\) has a trigonal bipyramidal geometry.

Step-by-step solution

Predict the geometry of \(\mathrm{SO}_{2}\)

1. Identify the central atom: In this case, the central atom is S (sulfur). 2. Count the electron pairs around the central atom: Sulfur has a total of 6 valence electrons, and it is bonded to two oxygen atoms (each by a double bond). This means that there are 2 electron pairs around the sulfur atom. 3. Use the VSEPR theory to predict geometry: With 2 electron pairs around the central atom, the molecular geometry of \(\mathrm{SO}_{2}\) will be linear.

Predict the geometry of \(\mathrm{BeCl}_{2}\)

1. Identify the central atom: In this case, the central atom is Be (beryllium). 2. Count the electron pairs around the central atom: Beryllium has a total of 2 valence electrons, and it is bonded to two chlorines (each by a single bond). This means that there are 2 electron pairs around the beryllium atom. 3. Use the VSEPR theory to predict geometry: With 2 electron pairs around the central atom, the molecular geometry of \(\mathrm{BeCl}_{2}\) will also be linear.

Predict the geometry of \(\mathrm{SeCl}_{4}\)

1. Identify the central atom: In this case, the central atom is Se (selenium). 2. Count the electron pairs around the central atom: Selenium has a total of 6 valence electrons, and it is bonded to four chlorine atoms (each by a single bond). This means that there are 4 bonding electron pairs and 1 lone pair of electrons around the selenium atom. 3. Use the VSEPR theory to predict geometry: With 4 bonding electron pairs and 1 lone pair around the central atom, the molecular geometry of \(\mathrm{SeCl}_{4}\) will be a see-saw, also called a distorted tetrahedron.

Predict the geometry of \(\mathrm{PCl}_{5}\)

1. Identify the central atom: In this case, the central atom is P (phosphorus). 2. Count the electron pairs around the central atom: Phosphorus has a total of 5 valence electrons, and it is bonded to five chlorine atoms (each by a single bond). This means that there are 5 bonding electron pairs around the phosphorus atom. 3. Use the VSEPR theory to predict geometry: With 5 bonding electron pairs around the central atom, the molecular geometry of \(\mathrm{PCl}_{5}\) will be trigonal bipyramidal.

Key concepts

VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory is a powerful tool used to predict the shapes of molecules. At its core, the VSEPR theory posits that electron pairs located in the valence shell of an atom will repel each other. This repulsion is due to the negative charge of electrons, and as they try to move as far apart as possible, they shape the molecule.

The key to understanding VSEPR theory is to consider both bonding electron pairs, which are shared between two atoms, and lone electron pairs, which are not shared and only belong to a single atom. The repulsion between these electron pairs dictates the spatial arrangement of the atoms in a molecule to minimize repulsion and hence stabilize the molecule.

When using the VSEPR theory for geometry prediction, a step-by-step process is helpful:

• Identify the central atom.
• Count all electron pairs (bonding and lone pairs) around the central atom.
• Apply the VSEPR rules to determine the arrangement that minimizes repulsion, which will be the molecular geometry.

An understanding of electron geometries, like linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral, is essential, as the molecular shape will often follow these basic patterns depending on the number of electron pairs.

Valence Electrons

Valence electrons play a pivotal role in chemical bonding and molecular structure. They are the outermost electrons in an atom and are involved in forming bonds with other atoms. The number of valence electrons an atom has determines its bonding behavior, and this is what the VSEPR theory uses to predict molecular geometry.

For instance, in the molecule \texttt{SO}\(_2\), sulfur has 6 valence electrons, leading to its bonding with two oxygen atoms through two double bonds. In \texttt{BeCl}\(_2\), beryllium's 2 valence electrons form two single bonds with chlorine atoms. Recognizing the valence electrons and how they combine with the electrons of other atoms is the foundation for predicting the shape of molecules.

The study of valence electrons extends to understanding the periodic table, electron configurations, and how atoms reach a stable electronic state through bonding. Understanding the principles of valence electrons can help students grasp why certain elements form specific types of bonds and predict molecule behavior.

Electron Pair Repulsion

Electron pair repulsion is a crucial concept in determining the shape of molecules. Since electrons carry a negative charge, they repel one another. The stronger these repulsive forces, the more the molecules will adjust their shape to reduce the repulsion. The VSEPR theory is based on this phenomenon.

In molecules like \texttt{SeCl}\(_4\), selenium has a lone pair of electrons in addition to the four bonding pairs attached to chlorine atoms. This lone pair exerts additional repulsion on the bonding electron pairs, distorting the geometry from a perfect tetrahedron to a see-saw shape. Similarly, due to electron pair repulsion, \texttt{PCl}\(_5\) adopts a trigonal bipyramidal shape, spreading out five bonding pairs to minimize repulsion.

When visualizing electron pair repulsion, it's helpful to remember that lone pairs have a stronger repulsive effect than bonding pairs. Factors like the size of the electron clouds and the relative electronegativities of the atoms can also influence the degree of repulsion and the resulting molecular shape.