Question

Some molecules have central atoms with steric numbers greater than 4 . For example, the central sulfur atom in \(\mathrm{SF}_{6}\) has a steric number of 6 . Its actual geometry (called octahedral), is shown below (all the bond angles are \(90^{\circ}\) or \(180^{\circ}\) ). (a) Are the bonds in \(\mathrm{SF}_{6}\) polar, polar covalent, or ionic? Explain. (b) Would you predict the \(\mathrm{SF}_{6}\) molecule to be polar or nonpolar? Justify your answer.

Short answer

(a) The bonds in SF6 are polar covalent, as the electronegativity difference between sulfur and fluorine is 1.4, which falls within the range for polar covalent bonds (0.5 ≤ electronegativity difference < 2.0). (b) The SF6 molecule is predicted to be nonpolar, as its octahedral geometry causes the polarities of the polar covalent bonds to cancel each other out.

Step-by-step solution

(a) Identify the type of bonds in SF6

To determine the type of bond between the sulfur (S) and fluorine (F) atoms in SF6, we need to consider the electronegativity difference between the two elements. Electronegativity values can be found on many periodic tables or using reference materials. Sulfur has an electronegativity of 2.58, and fluorine has an electronegativity of 3.98. Calculate the electronegativity difference between sulfur and fluorine: Electronegativity difference = |3.98 - 2.58| = 1.4 Using the electronegativity difference, we can determine the type of bond: - Nonpolar covalent: Electronegativity difference < 0.5 - Polar covalent: 0.5 ≤ Electronegativity difference < 2.0 - Ionic: Electronegativity difference ≥ 2.0 Since the electronegativity difference between sulfur and fluorine is 1.4, the bond is polar covalent.

(b) Determine the overall polarity of the SF6 molecule

To determine the overall polarity of the SF6 molecule, we need to consider both its geometry and bond polarities. As mentioned in the problem, SF6 has an octahedral geometry with bond angles of 90° and 180°. In octahedral geometries, the polarities of the polar covalent bonds tend to cancel each other out, resulting in a nonpolar molecule. Since all of the bond dipoles in SF6 cancel each other out due to its octahedral geometry, we can predict that the SF6 molecule is nonpolar.

Key concepts

Steric Number

The steric number is a fundamental concept in chemistry, referring to the total number of atoms bonded to a central atom plus the number of lone pair electrons on the central atom. For instance, in sulfur hexafluoride (\texttt{SF}\(_6\)), the central sulfur atom is bound to six fluorine atoms with no lone pairs, resulting in a steric number of six.

This number is important because it dictates the molecular geometry, which influences various physical and chemical properties of the molecule. Knowing the steric number, you can predict the molecular shape using VSEPR (Valence Shell Electron Pair Repulsion) theory, a crucial step in understanding the behavior of molecules in chemical reactions and physical processes.

Polar Covalent Bond

A polar covalent bond is formed when two atoms with different electronegativity values share electrons unequally in a chemical bond. This disparity in electronegativity leads to a partial charge distribution, with the more electronegative atom attracting the shared electrons more strongly, thus acquiring a slight negative charge, while the less electronegative atom becomes slightly positive.

For example, in \texttt{SF}\(_6\), the bond between sulfur and fluorine is polar covalent, as indicated by the electronegativity difference of 1.4. As a result, each sulfur-fluorine bond has a dipole moment—a measure of the separation of charge in the bond. Understanding the nature of chemical bonds is key to predicting the physical properties and reactivity of molecules.

Octahedral Geometry

Octahedral geometry is one of the shapes that molecular compounds can adopt. A molecule with an octahedral shape has a central atom surrounded by six atoms at equal distances, forming 90° and 180° angles between them. This geometry is a direct consequence of a steric number of six, where the spatial distribution of the atoms around the central atom minimizes repulsion between the electron pairs and creates a symmetric shape.

In the context of \texttt{SF}\(_6\), which is mentioned as having octahedral geometry, the molecule will have all sulfur-fluorine bonds spaced evenly apart. The symmetry of this geometry plays a crucial role in determining whether the molecule will be polar or nonpolar as a whole, despite having polar covalent bonds.

Electronegativity Difference

Electronegativity difference is a key factor in determining the type of chemical bond between two atoms. It is based on the concept of electronegativity, which is a measure of an atom's ability to attract shared electrons in a bond. The greater the difference in electronegativity between the two bonding atoms, the more polar the bond becomes.

A small difference results in a nonpolar covalent bond, while a significant difference—though not extreme enough to result in electron transfer—leads to a polar covalent bond. An even larger difference typically indicates an ionic bond, where electrons are transferred from one atom to another. In our example with \texttt{SF}\(_6\), the difference in electronegativity is substantial enough (1.4) to form polar covalent bonds but not to the extent of forming ionic bonds.