Molecular polarity describes how electric charge is distributed across a molecule. An uneven distribution results in a net dipole moment, where one end carries a slight positive charge and the other a slight negative charge. The polarity of Bromine trichloride (\(\text{B}\text{r}\text{C}\text{l}_3\)) is determined by the polarity of its chemical bonds and the molecule’s three-dimensional shape. If polar bonds exist, their effects must be analyzed within the molecular structure to determine if they cancel out, revealing the overall polarity.
Polarity of the Individual Bonds
Bond polarity is established by comparing the electronegativity values of the two atoms, which is their ability to attract shared electrons. Bromine (Br) and Chlorine (Cl) are halogens, but they possess different electronegativities due to their positions on the periodic table. Electronegativity generally increases as elements move up a group.
Chlorine is situated above Bromine and is the more electronegative atom (Cl: 3.16; Br: 2.96). This difference of 0.2 means the shared electrons in the \(\text{B}\text{r}-\text{C}\text{l}\) bond are not shared equally.
Electron density is pulled closer to the more electronegative Chlorine atom. This unequal sharing creates a partial negative charge (\(\delta^-\)) on the Chlorine end and a partial positive charge (\(\delta^+\)) on the Bromine end. Consequently, all three \(\text{B}\text{r}-\text{C}\text{l}\) bonds are polar. This bond polarity is necessary for the molecule to be polar, but the final determination depends on the molecular geometry.
The Molecular Geometry of \(\text{B}\text{r}\text{C}\text{l}_3\)
The arrangement of atoms in Bromine trichloride is determined using the Valence Shell Electron Pair Repulsion (VSEPR) theory. VSEPR states that electron domains (bonding and lone pairs) around the central atom arrange themselves as far apart as possible to minimize repulsion. In \(\text{B}\text{r}\text{C}\text{l}_3\), Bromine is the central atom, possessing seven valence electrons.
Three valence electrons form single bonds with the three Chlorine atoms, leaving four non-bonding electrons that form two lone pairs. The central Bromine atom thus has five electron domains: three bonding pairs and two lone pairs. According to VSEPR theory, five electron domains arrange themselves in a trigonal bipyramidal electron geometry.
The two lone pairs occupy the equatorial positions, lying in the plane of the central atom. This placement minimizes repulsive forces between the lone pairs and the bonding pairs. The molecular geometry, which describes the arrangement of only the atoms, is revealed by ignoring the position of the lone pairs.
With the two lone pairs in the equatorial plane and the three Chlorine atoms occupying the remaining positions, the resulting shape is a T-shape. This T-shaped structure is inherently asymmetrical, which is the deciding factor in determining the overall molecular polarity.
The Final Polarity Determination
Molecular polarity depends on whether the individual bond dipoles, created by the polar \(\text{B}\text{r}-\text{C}\text{l}\) bonds, cancel out due to symmetry. In a highly symmetrical molecule, such as carbon dioxide (\(\text{C}\text{O}_2\)), the dipoles pull in equal and opposite directions, resulting in a nonpolar molecule. The T-shaped geometry of \(\text{B}\text{r}\text{C}\text{l}_3\) prevents this cancellation.
The asymmetrical arrangement of the three Chlorine atoms and the two lone pairs on the central Bromine atom ensures the molecule is unbalanced. The vector sum of the three \(\text{B}\text{r}-\text{C}\text{l}\) bond dipoles does not equal zero, meaning the molecule possesses a net dipole moment. The two lone pairs on the Bromine atom contribute to this uneven distribution of charge, reinforcing the overall asymmetry.
The result is that \(\text{B}\text{r}\text{C}\text{l}_3\) is a polar molecule. This polarity has implications for the compound’s physical properties, such as its solubility, since polar molecules dissolve in polar solvents (“like dissolves like”). The presence of a permanent dipole moment means \(\text{B}\text{r}\text{C}\text{l}_3\) molecules interact through dipole-dipole intermolecular forces, influencing its melting and boiling points.