Question

Predict the molecular structure (including bond angles) for each of the following. (See Exercises 115 and 116.) a. \(\mathrm{ICl}_{5}\) b. \(\mathrm{XeCl}_{4}\) c. \(\mathrm{SeCl}_{6}\)

Short answer

The molecular structures and bond angles for the given molecules are as follows: - \(\mathrm{ICl}_{5}\): Trigonal bipyramidal structure with axial bond angles of 180° and equatorial bond angles of 120°. - \(\mathrm{XeCl}_{4}\): Square planar structure with bond angles of 90°. - \(\mathrm{SeCl}_{6}\): Octahedral structure with bond angles of 90°.

Step-by-step solution

Find the number of valence electrons

Count the valence electrons in the molecule. Iodine (I) has 7 valence electrons, and each chlorine (Cl) has 7 valence electrons. As there are 5 chlorine atoms, the total number of valence electrons is: \(7 + 5\times7 = 42\)

Determine the electron-pair geometry

Place the central atom (I) in the center and distribute the valence electrons among the surrounding atoms (Cl) to satisfy the octet rule. Since there are 5 surrounding atoms and no lone pairs on the central atom, the steric number is 5. This corresponds to a trigonal bipyramidal electron-pair geometry.

Predict the molecular structure and bond angles

The molecular structure will match the electron-pair geometry since there are no lone pairs on the central atom. Thus, the molecular structure of \(\mathrm{ICl}_{5}\) is trigonal bipyramidal. In this structure, the axial bond angle is 180°, and the equatorial bond angles are 120°. For molecule \(\mathrm{XeCl}_{4}\):

Find the number of valence electrons

Count the valence electrons in the molecule. Xenon (Xe) has 8 valence electrons, and each chlorine (Cl) has 7 valence electrons. As there are 4 chlorine atoms, the total number of valence electrons is: \(8 + 4\times7 = 36\)

Determine the electron-pair geometry

Place the central atom (Xe) in the center and distribute the valence electrons among the surrounding atoms (Cl) and the central atom. Since there are 4 surrounding atoms and 2 lone pairs on the central atom, the steric number is 6. This corresponds to an octahedral electron-pair geometry.

Predict the molecular structure and bond angles

The molecular structure will be a square planar based on the octahedral electron-pair geometry with two lone pairs on opposite positions. The bond angle between any two chlorine atoms surrounding the central Xe atom is 90°. Thus, the molecular structure of \(\mathrm{XeCl}_{4}\) is square planar with 90° bond angles. For molecule \(\mathrm{SeCl}_{6}\):

Find the number of valence electrons

Count the valence electrons in the molecule. Selenium (Se) has 6 valence electrons, and each chlorine (Cl) has 7 valence electrons. As there are 6 chlorine atoms, the total number of valence electrons is: \(6 + 6\times7 = 48\)

Determine the electron-pair geometry

Place the central atom (Se) in the center and distribute the valence electrons among the surrounding atoms (Cl). Since there are 6 surrounding atoms and no lone pairs on the central atom, the steric number is 6. This corresponds to an octahedral electron-pair geometry.

Predict the molecular structure and bond angles

The molecular structure will match the electron-pair geometry since there are no lone pairs on the central atom. Thus, the molecular structure of \(\mathrm{SeCl}_{6}\) is octahedral. In this structure, the bond angles between any two chlorine atoms surrounding the central Se atom are 90°. Summary: - \(\mathrm{ICl}_{5}\) has a trigonal bipyramidal molecular structure with axial bond angles of 180° and equatorial bond angles of 120°. - \(\mathrm{XeCl}_{4}\) has a square planar molecular structure with bond angles of 90°. - \(\mathrm{SeCl}_{6}\) has an octahedral molecular structure with bond angles of 90°.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and play a crucial role in determining how atoms bond to form molecules. These electrons are responsible for forming chemical bonds, which can be shared or transferred between atoms. Understanding the number of valence electrons in an element is essential for predicting the molecular structure. To find the number of valence electrons, you can refer to the element's group number in the periodic table:

• Elements in group 1 have 1 valence electron.
• Elements in group 17 have 7 valence electrons.
• For elements like Iodine (I), Xenon (Xe), and Selenium (Se) involved in the provided examples, they respectively have 7, 8, and 6 valence electrons.

In molecules, the total number of valence electrons is found by adding up all the valence electrons of the atoms that make up the molecule. This count helps in determining how to distribute electrons in the molecule to satisfy bonding requirements.

Electron-Pair Geometry

Electron-pair geometry involves the arrangement of electron pairs around a central atom. This includes both bonding pairs (shared between atoms) and lone pairs (not shared). The geometry is determined by considering the steric number, which is the total number of bonded atoms and lone pairs around the central atom.The shapes related to different steric numbers include:

• Steric number 2: Linear
• Steric number 3: Trigonal planar
• Steric number 4: Tetrahedral
• Steric number 5: Trigonal bipyramidal
• Steric number 6: Octahedral

In the exercise solutions:- \(\mathrm{ICl}_{5}\) is trigonal bipyramidal, correlating to a steric number of 5.- \(\mathrm{XeCl}_{4}\) is octahedral with two lone pairs, giving a square planar molecular shape.- \(\mathrm{SeCl}_{6}\) is octahedral, neatly matching the steric number 6 scenario.

Bond Angles

Bond angles are the angles formed between the bonds of three atoms within a molecule. These angles are significant as they help define the shape and symmetry of a molecule. The ideal bond angles are affected by the electron-pair geometry and the number and shape of lone pairs and bonded atoms.For example:

• In a trigonal bipyramidal structure like \(\mathrm{ICl}_{5}\), there are axial 180° and equatorial 120° bond angles due to how the atoms are spaced.
• In a square planar configuration like \(\mathrm{XeCl}_{4}\), all the bond angles between chlorine atoms are 90° because they form a square around the central xenon.
• Similarly, in an octahedral shape like \(\mathrm{SeCl}_{6}\), there are only 90° angles between closest atom pairs, creating a symmetric shape.

Bond angles provide vital information for deducing the precise spatial arrangement of atoms in a molecule.

Octet Rule

The octet rule is a guideline in chemistry, stating that atoms tend to form bonds in such a way that they each have eight electrons in their valence shell, attaining a noble gas configuration. This rule is instrumental in predicting molecular structures, although there are exceptions.Key points about the octet rule include:

• Elements like carbon, nitrogen, and oxygen strictly follow it, seeking to achieve eight valence electrons.
• Some elements, such as sulfur or phosphorus, can exceed the octet, known as "expanded octets." This often occurs with elements beyond period 2 on the periodic table.
• The central atoms in the examples provided (I, Xe, Se) tend to either follow or expand beyond the octet rule to accommodate bonding with multiple chlorine atoms. For instance, \(\mathrm{SeCl}_{6}\) uses selenium’s ability to expand its octet to accommodate six bonding chlorine atoms.

While the octet rule is widely applicable, it's essential to consider these exceptions when predicting molecular structures.