Predict the molecular structure (including bond angles) for each of the following. (See Exercises 25 and \(26 .\) ) a. ICls b. \(\mathrm{XeCl}_{4}\) c. \(\mathrm{SeCl}_{6}\)

The steric number is a fundamental concept in predicting the molecular structure of a compound. It is the total count of bonded atoms plus the number of lone electron pairs around the central atom in a molecule. Understanding the steric number is crucial because it directly influences molecular geometry, which can affect the physical and chemical properties of substances.

Determining the steric number involves totaling the number of single, double, or triple bonds and any lone pairs around the central atom. For example, in the molecule ICl₅, iodine is the central atom with five single bonds connecting to chlorine atoms, and it has no lone pairs. Thus, the steric number is 5. Similarly, for XeCl₄, xenon has four chlorine atoms bonded and two lone pairs, contributing to a steric number of 6. Knowing the steric number, we can then proceed to predict molecular geometry.

Once the steric number is determined, the molecular geometry of a compound can be predicted. Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. It's shaped by the electronic interactions between the bonding pairs and lone pairs in the valence shell of the central atom.

For instance, ICl₅, with a steric number of 5, adopts a square pyramidal shape. This is because of the presence of five bonding pairs and no lone pairs resulting in a unique spatial arrangement. On the other hand, XeCl₄ with a steric number of 6 forms a square planar geometry due to the equidistant arrangement of the four chlorine atoms and the two lone pairs minimizing repulsion. Similarly, SeCl₆, also with a steric number of 6 but without any lone pairs, takes on an octahedral shape, which reflects an even distribution of electron pairs around the central atom. These geometries are not just theoretical; they have practical relevance in understanding the properties and reactivity of the molecules.

Bond angles are the angles between adjacent lines representing bonds that come out of an atom. They are significant in chemistry as they affect the molecular shape and, therefore, the physical and chemical properties of the molecule.

The bond angles can be roughly predicted based on the idealized angles for a given geometry, modified by the repulsion from lone pairs if present. In the square pyramidal geometry of ICl₅, the bond angles are around 90 degrees for the square base and 120 degrees for the pyramid. For XeCl₄'s square planar geometry, the bond angles are 90 degrees because of the four groups of electrons are in one plane. SeCl₆, being octahedral, has bond angles of precisely 90 degrees between any two adjacent bonds. It is worth noting that in real molecules, these bond angles can be slightly less or more than the ideal angles due to various factors including lone pair repulsion and bond length differences.