When two atoms join to form a chemical bond, the shared electrons are not always distributed equally. This unequal sharing creates molecular polarity, characterized by partial electrical charges across the molecule. Determining a molecule’s polarity is necessary for predicting its behavior, including solubility and interaction with electric fields. Bromine monofluoride (BrF) is a simple, two-atom molecule that illustrates these principles.
The Foundation of Molecular Polarity
The propensity of an atom to attract a shared pair of electrons toward itself within a chemical bond is called electronegativity. This property forms the basis for classifying the nature of any bond. If the two atoms have identical pulling strengths, the electrons are shared equally, resulting in a nonpolar covalent bond.
If the atoms possess different electronegativities, the electron density shifts toward the more attractive atom, forming a polar covalent bond. Scientists use scales, such as the Pauling scale, to assign numerical values to this power. A significant difference indicates a substantial shift in electron distribution, creating the partial charges that define a polar molecule.
Electronegativity Difference in Bromine Monofluoride
To determine the polarity of BrF, one must compare the electronegativity values of its constituent atoms. Fluorine (F) has a Pauling value of approximately 4.0, while Bromine (Br) has a lower value of around 2.8. Calculating the difference yields a result of about 1.2, which falls within the established range for a polar covalent bond.
Because Fluorine has a significantly greater pull on the shared electrons, the electron density clusters around the Fluorine nucleus. This unequal distribution means the Br-F bond is highly polarized, establishing that BrF is a polar molecule.
The Meaning of a Molecular Dipole
The unequal sharing of electrons in BrF results in a permanent molecular dipole moment, which measures the separation of charge. The high electronegativity of Fluorine causes it to acquire a partial negative charge (\(\delta^-\)), while Bromine acquires a partial positive charge (\(\delta^+\)). This creates an internal electric field within the molecule.
Since Bromine monofluoride is a linear, diatomic molecule, there are no other bonds to cancel the effect of the single polar bond. Therefore, the bond dipole is identical to the molecular dipole. This permanent dipole moment dictates how BrF behaves, causing it to align in an external electric field. This polarity also means BrF is more likely to dissolve in polar solvents, such as water.