Molecular polarity, which describes the distribution of electrical charge, profoundly influences a substance’s chemical properties, including its boiling point and solubility. Determining if a molecule is polar or nonpolar requires examining its constituent atoms and its three-dimensional shape. This analysis will determine the overall polarity of Beryllium Hydride (\(\text{BeH}_2\)).
What Determines Molecular Polarity
Molecular polarity arises from two factors: the polarity of individual chemical bonds and the molecule’s geometric shape. Bond polarity is determined by electronegativity, an atom’s ability to attract bonding electrons. Atoms with similar electronegativity values share electrons nearly equally, resulting in a nonpolar bond.
When two atoms possess significantly different electronegativities, electron density is pulled closer to the more electronegative atom. This uneven sharing creates a partial negative charge (\(\delta^-\)) and a partial positive charge (\(\delta^+\)), forming a polar bond. This charge separation is referred to as a bond dipole moment. The molecule’s shape dictates how these individual bond dipoles interact and whether they cancel each other out.
Analyzing the Polarity of the Beryllium-Hydrogen Bond
To assess the Beryllium-Hydrogen (\(\text{Be}-\text{H}\)) bond, we compare the electronegativity values of the two elements. Beryllium (\(\text{Be}\)) has a value of 1.57, and Hydrogen (\(\text{H}\)) has a value of 2.20.
The difference in electronegativity (\(\Delta \text{EN}\)) is \(2.20 – 1.57 = 0.63\). This difference classifies the \(\text{Be}-\text{H}\) bond as a slightly polar covalent bond. A small bond dipole exists because electron density is pulled marginally closer to the Hydrogen atoms.
The Linear Geometry of Beryllium Hydride
The overall shape of Beryllium Hydride is governed by the Valence Shell Electron Pair Repulsion (VSEPR) theory. This model predicts geometry based on the idea that electron pairs repel and arrange themselves as far apart as possible. In \(\text{BeH}_2\), the central Beryllium atom forms two single bonds with the two Hydrogen atoms.
The Beryllium atom uses its two valence electrons for bonding, leaving zero lone pairs on the central atom. With only two electron regions, the maximum separation possible is a \(180^\circ\) angle. This arrangement results in a perfectly linear molecular geometry, placing the two Hydrogen atoms directly opposite each other.
Why Beryllium Hydride is Nonpolar
Although the individual \(\text{Be}-\text{H}\) bonds are slightly polar, the symmetrical, linear geometry of the \(\text{BeH}_2\) molecule causes it to be nonpolar overall. The bond dipole moment created by the first \(\text{Be}-\text{H}\) bond is exactly matched by the equal and opposite dipole moment from the second \(\text{Be}-\text{H}\) bond. Because the two dipoles are equal in magnitude and point in opposite directions, they effectively cancel each other out.
This cancellation results in a net dipole moment of zero for the entire molecule. The electron density is distributed symmetrically across the whole molecule, preventing the formation of a positive end and a negative end. Thus, \(\text{BeH}_2\) is classified as a nonpolar molecule.