Molecular polarity describes the unequal sharing of electrons within a molecule, leading to the formation of partial positive and partial negative charges. This uneven distribution of electrical charge results in a dipole within the molecule. The overall polarity of any molecule is determined by two factors: the polarity of its individual bonds and its three-dimensional shape. Antimony Trifluoride (\(\text{SbF}_3\)) is classified as a polar molecule because it satisfies both requirements. This polarity dictates many of the compound’s physical and chemical properties, such as solubility and boiling point.
The Role of Electronegativity in Bond Polarity
Molecular polarity begins with the nature of the chemical bonds between the atoms. Electronegativity measures an atom’s ability to attract a shared pair of electrons within a bond. Differences in this value determine if a bond is nonpolar or polar. For Antimony Trifluoride, we examine the difference between the central Antimony (\(\text{Sb}\)) atom and the surrounding Fluorine (\(\text{F}\)) atoms.
Antimony has an electronegativity value of 2.05, while Fluorine holds a value of 3.98. This creates a significant difference of 1.93, classifying the \(\text{Sb}-\text{F}\) bond as highly polar. This large difference means the shared electrons spend considerably more time near the Fluorine atoms.
As electrons are pulled closer to Fluorine, each Fluorine atom develops a partial negative charge (\(\delta-\)). Conversely, the central Antimony atom develops a partial positive charge (\(\delta+\)). These opposing partial charges create an individual bond dipole moment, which points toward the more negative Fluorine atom. The presence of these three polar \(\text{Sb}-\text{F}\) bonds is the first condition for the \(\text{SbF}_3\) molecule to be polar.
Molecular Geometry and VSEPR Theory
Polar bonds alone are not sufficient to ensure a polar molecule; the three-dimensional arrangement must also be asymmetrical. We apply the Valence Shell Electron Pair Repulsion (VSEPR) theory to understand the shape of Antimony Trifluoride. VSEPR predicts geometry based on the idea that electron pairs around a central atom minimize repulsion.
Antimony is in Group 15, meaning its neutral atom has five valence electrons. In \(\text{SbF}_3\), Antimony forms three single covalent bonds with the three Fluorine atoms. The remaining two valence electrons form a single non-bonding electron pair, or lone pair, on the central Antimony atom.
The central Antimony atom has four electron domains: three bonding pairs and one lone pair. VSEPR theory dictates these four domains arrange into a tetrahedral electron geometry. However, the molecular geometry, which describes the arrangement of the atoms, is influenced by the strong repulsive force of the lone pair.
Lone pairs occupy more space than bonding pairs, pushing the three bonded Fluorine atoms away. This distortion results in a trigonal pyramidal molecular geometry. The three Fluorine atoms form the base of a pyramid, with the lone pair sitting at the apex. This specific shape is inherently asymmetrical, which is the second condition for determining overall molecular polarity.
The Net Dipole Moment of the Molecule
Molecular polarity is finally determined by whether the individual bond dipoles combine or cancel out. The overall polarity is quantified by the net dipole moment, which is the vector sum of all individual bond dipole moments. In a perfectly symmetrical molecule, dipoles cancel out, resulting in a net dipole moment of zero and a nonpolar molecule.
In Antimony Trifluoride, the trigonal pyramidal geometry prevents this cancellation. All three \(\text{Sb}-\text{F}\) bond dipoles point downward toward the base of the pyramid. Because the molecule is asymmetrical, these three vectors add together rather than neutralizing one another. The lone pair also contributes significantly to this net dipole, as its electron density is directed away from the central atom.
The result is a substantial net molecular dipole moment. The presence of this non-zero net dipole moment confirms that Antimony Trifluoride is a polar molecule. This polarity enables \(\text{SbF}_3\) to interact strongly with other polar substances, influencing its physical properties.