Hydrogen selenide (\(\text{H}_2\text{Se}\)) is an inorganic compound existing as a colorless, flammable gas at room temperature. Understanding this molecule requires an examination of its polarity, a fundamental chemical property describing the distribution of electrical charge within a molecule. Molecular polarity dictates how a substance interacts with others, influencing its boiling point and solubility. This uneven distribution of charge arises from differences in the electron-pulling power of its constituent atoms.
The Foundation of Polarity
Molecular polarity begins with the nature of the chemical bonds holding its atoms together. Bond polarity is determined by electronegativity, which measures an atom’s tendency to attract shared electrons within a bond. When two atoms with differing electronegativity values bond, the electron cloud is pulled closer to the more electronegative atom, creating a separation of charge called a bond dipole. This unequal sharing gives one end of the bond a partial negative charge (\(\delta^-\)) and the other a partial positive charge (\(\delta^+\)). For \(\text{H}_2\text{Se}\), the central selenium (\(\text{Se}\)) atom (2.55) is more electronegative than the hydrogen (\(\text{H}\)) atoms (2.20). The resulting difference of 0.35 indicates that the electrons in the \(\text{Se-H}\) bonds are slightly drawn toward selenium, creating two distinct polar covalent bonds.
Molecular Geometry of Hydrogen Selenide
Bond polarity alone is not sufficient to determine molecular polarity; the three-dimensional arrangement of the atoms must also be considered. The structure of \(\text{H}_2\text{Se}\) is determined by the Valence Shell Electron Pair Repulsion (VSEPR) model. Selenium, being in Group 16, possesses six valence electrons. It uses two electrons to form single bonds with hydrogen, leaving two non-bonding lone pairs. These four total electron groups arrange themselves in a tetrahedral electron geometry. However, the molecular geometry is not tetrahedral because the lone pairs exert a stronger repulsive force. This repulsion pushes the two hydrogen atoms closer together, resulting in a non-linear, “bent” or “V-shaped” molecular geometry. The \(\text{H-Se-H}\) bond angle is approximately \(91^\circ\).
Determining the Overall Molecular Polarity
Determining molecular polarity requires combining the polar nature of the \(\text{Se-H}\) bonds with the molecule’s bent geometry. Polarity is visualized using a net dipole moment, which is the vector sum of all individual bond dipoles. In a perfectly symmetrical molecule, even if the bonds are polar, the individual dipoles cancel out, resulting in a nonpolar molecule. However, the bent shape of hydrogen selenide ensures that the individual bond dipoles do not cancel. Since the selenium atom is at the “point” of the V-shape, the two bond dipoles (directed toward the electronegative selenium) are added together vectorially. This addition results in a significant net dipole moment for the entire molecule. The net dipole moment points through the selenium atom, meaning the center of negative charge is located on the selenium side. The asymmetry created by the two lone pairs and the resulting bent structure prevents charge cancellation. Therefore, hydrogen selenide possesses an uneven distribution of charge and is definitively classified as a polar molecule.
Impact of Polarity on Physical Properties
The established polarity of \(\text{H}_2\text{Se}\) has direct consequences for its physical properties, primarily by dictating its intermolecular forces (IMFs). Polar molecules engage in dipole-dipole interactions, which are stronger than the London dispersion forces found in nonpolar substances. These forces require more energy to overcome during phase changes, thus affecting the boiling and melting points. The boiling point of \(\text{H}_2\text{Se}\) is approximately \(-41.3^\circ\text{C}\), and its melting point is around \(-65.73^\circ\text{C}\). These values are higher than those of nonpolar molecules of similar size, reflecting the added energy required to break the dipole-dipole attractions. The principle of “like dissolves like” governs the solubility of hydrogen selenide. Since \(\text{H}_2\text{Se}\) is polar, it is highly soluble in polar solvents, such as water. When dissolved, it engages in favorable dipole-dipole interactions with water molecules, with solubility reaching approximately 289 milliliters per 100 grams of water at \(20^\circ\text{C}\).