Molecular polarity describes the overall distribution of electric charge across a molecule, which can be either symmetrical or asymmetrical. This fundamental concept governs how a molecule interacts with its environment and determines many physical and chemical properties, such as solubility and boiling point. When electrons are unevenly shared or distributed spatially, the molecule develops distinct positive and negative ends, classifying it as polar. Determining if a specific compound is polar or nonpolar requires examining its chemical bonds and its three-dimensional structure. To determine the polarity of selenium hexachloride (\(\text{SeCl}_6\)), we must first analyze the nature of the bonds between the selenium and chlorine atoms.
The Polarity of the Selenium-Chlorine Bond
Bonds are classified as polar or nonpolar based on the difference in electronegativity between the bonded atoms. Electronegativity is the measure of an atom’s tendency to attract shared electrons toward itself.
Selenium (Se) has an electronegativity value of 2.55, while chlorine (Cl) is significantly more electronegative at 3.16. This difference of 0.61 establishes a clear polarity in the \(\text{Se}-\text{Cl}\) bond. Because chlorine pulls the shared electrons closer, it develops a partial negative charge (\(\delta^-\)), leaving a partial positive charge (\(\delta^+\)) on the central selenium atom. Consequently, each individual \(\text{Se}-\text{Cl}\) bond possesses a bond dipole moment, confirming the presence of polar bonds.
Determining the Molecular Shape of \(\text{SeCl}_6\)
The overall polarity of a molecule depends on its three-dimensional geometry, which dictates how individual bond dipoles are arranged. The Valence Shell Electron Pair Repulsion (VSEPR) theory predicts this geometry by minimizing repulsion between electron pairs around the central atom.
In \(\text{SeCl}_6\), selenium is the central atom, contributing six valence electrons, all of which form single covalent bonds with the six surrounding chlorine atoms. This results in a central atom with six bonding pairs and zero lone pairs, categorized as an \(\text{AX}_6\) system. To minimize electron-pair repulsion, the molecule adopts an ideal octahedral geometry. This configuration places the six chlorine atoms equally spaced at the vertices of an octahedron, resulting in bond angles of \(90^{\circ}\) and \(180^{\circ}\).
Why \(\text{SeCl}_6\) is Nonpolar
Although the \(\text{Se}-\text{Cl}\) bonds are polar, the highly symmetrical octahedral shape of \(\text{SeCl}_6\) determines its overall nonpolar classification. Molecular polarity is measured by the net dipole moment, which is the vector sum of all individual bond dipole moments. A net dipole moment of zero indicates a nonpolar molecule.
In the octahedral structure, the six equivalent \(\text{Se}-\text{Cl}\) bond dipoles are oriented symmetrically in three-dimensional space. For every bond dipole pointing in one direction, there is an identical bond dipole pointing in the exact opposite direction, leading to precise cancellation. This symmetrical cancellation results in a net dipole moment of zero, meaning the molecular charge distribution is perfectly balanced. Therefore, despite having polar bonds, selenium hexachloride (\(\text{SeCl}_6\)) is classified as a nonpolar molecule.
How Molecular Polarity Affects Chemical Behavior
The classification of \(\text{SeCl}_6\) as nonpolar directly impacts its chemical behavior, particularly its solubility and intermolecular interactions. The principle of “like dissolves like” states that substances with similar polarities readily mix.
Because \(\text{SeCl}_6\) is nonpolar, its primary intermolecular forces are weak London Dispersion Forces. Consequently, \(\text{SeCl}_6\) is expected to be highly soluble in nonpolar solvents, such as hexane or carbon tetrachloride, where compatible weak forces exist. Conversely, \(\text{SeCl}_6\) will be poorly soluble in polar solvents, most notably water. Water is a highly polar solvent that relies on strong dipole-dipole forces and hydrogen bonding. The nonpolar \(\text{SeCl}_6\) molecules cannot form the necessary strong attractive forces with polar water molecules, causing the compound to remain undissolved.