Determining whether a molecule is polar or nonpolar is a fundamental exercise in chemistry, providing insights into a substance’s physical properties, such as its solubility, melting point, and boiling point. Polarity dictates how molecules interact with one another, which governs whether a substance will dissolve in water or oil. To find the polarity of Gallane, or Gallium trihydride (\(\text{GaH}_3\)), it is necessary to examine two distinct chemical features: the nature of the bonds within the molecule and the molecule’s three-dimensional shape.
Understanding Polarity: Bonds Versus Molecules
The discussion of polarity must begin at the atomic level, specifically with the concept of bond polarity. Bond polarity arises from a difference in electronegativity—the ability of an atom to attract shared electrons toward itself. When two bonded atoms have different electronegativity values, the sharing of electrons is unequal, creating a polar bond with a slight positive end (\(\delta+\)) and a slight negative end (\(\delta-\)).
For Gallane, the Gallium (Ga) atom has an electronegativity value of approximately 1.81, while the Hydrogen (H) atom has a value of about 2.20. This difference of 0.39 units is significant enough to classify the individual \(\text{Ga-H}\) bonds as weakly polar. Since the hydrogen atom is slightly more electronegative than gallium, the electron density is pulled toward each hydrogen atom, giving each \(\text{Ga-H}\) bond its own dipole moment.
Molecular polarity, however, is not determined by bond polarity alone. A molecule is considered polar only if it possesses a net dipole moment, meaning the individual bond dipoles do not cancel each other out. If a molecule contains polar bonds but is perfectly symmetrical, the opposing forces of the bond dipoles can effectively nullify one another. Therefore, the arrangement of these individual polar bonds in three-dimensional space is the second, equally important factor in determining the molecule’s overall polarity.
The Molecular Geometry of Gallane
The specific three-dimensional arrangement of atoms, known as molecular geometry, is the factor that governs whether bond dipoles cancel out. This shape is predicted using the Valence Shell Electron Pair Repulsion (VSEPR) theory. For Gallane (\(\text{GaH}_3\)), the central atom is Gallium, which is bonded to three Hydrogen atoms.
The Gallium atom in \(\text{GaH}_3\) forms three single bonds with the Hydrogen atoms and does not possess any lone pairs of valence electrons. According to VSEPR theory, three electron groups surrounding a central atom with no lone pairs will maximize their separation by adopting a trigonal planar geometry. In this shape, all four atoms lie in the same flat plane, with the three Hydrogen atoms arranged symmetrically around the central Gallium atom at equal \(120^\circ\) angles.
This flat, symmetrical structure is crucial for the final determination of polarity. The three hydrogen atoms are perfectly distributed around the central gallium, creating a uniform charge distribution. Their symmetrical placement is the defining feature. The overall molecular shape is highly regular, which sets the stage for the cancellation of any internal electrical forces.
The Final Verdict: Is \(\text{GaH}_3\) Polar or Nonpolar?
Gallane (\(\text{GaH}_3\)) is determined to be a nonpolar molecule. This conclusion results from the interplay between the slightly polar nature of the individual \(\text{Ga-H}\) bonds and the highly symmetrical molecular geometry. Although each \(\text{Ga-H}\) bond has a small dipole moment pointing toward the more electronegative hydrogen atom, the symmetrical trigonal planar arrangement ensures that the net molecular dipole moment is zero.
The three bond dipoles are vectors of equal magnitude pointing outward from the central Gallium atom toward the three Hydrogen atoms. Because they are all separated by \(120^\circ\) angles in a single plane, the vector sum of these three equal and opposing forces is zero. This is similar to three people of equal strength pulling on ropes attached to a central ring, each pulling in a direction \(120^\circ\) from the others; the ring would not move. The perfect symmetry of the trigonal planar structure means the electron cloud is uniformly distributed around the molecule, despite the minor charge separation within the bonds.