Hydrogen bromide (HBr) is a simple diatomic molecule consisting of one hydrogen atom bonded to one bromine atom. This colorless gas is widely used in chemical synthesis, often dissolved in water to form hydrobromic acid. Determining the polarity of the chemical bond between hydrogen and bromine requires understanding how electrons are shared.
What Makes a Chemical Bond Polar
Covalent bonding occurs when atoms share electrons, but this sharing is not always equal. The distribution of electron density determines the bond’s polarity. When electrons are shared equally between identical atoms, the bond is nonpolar covalent because the electrical charge is distributed symmetrically.
A polar covalent bond arises when the two atoms are different, causing the electron pair to spend more time closer to one nucleus. This unequal sharing results in a separation of charge across the bond. The atom that attracts electrons more strongly develops a partial negative charge (\(\delta-\)), while the other develops a partial positive charge (\(\delta+\)). This charge separation is quantified by the dipole moment. A molecule possessing a measurable dipole moment is considered polar.
How Electronegativity Determines Polarity
The fundamental property governing unequal sharing is electronegativity, which measures an atom’s power to attract a bonding pair of electrons. The greater the difference in electronegativity (\(\Delta\)EN) between the two bonded atoms, the more polar the bond will be.
On the Pauling scale, Hydrogen (H) has a value of approximately 2.20, and Bromine (Br) is 2.96. Calculating the difference yields 0.76. Since a general guideline states that an electronegativity difference of 0.4 or greater indicates a polar covalent bond, the H-Br bond falls into the polar category.
The greater electronegativity of the bromine atom means it exerts a stronger pull on the shared electrons compared to hydrogen. This disparity confirms that the electron density is not distributed evenly, classifying the bond as distinctly polar covalent.
Why Hydrogen Bromide is a Polar Molecule
Hydrogen bromide is a polar molecule because of the difference in the electron-attracting power of its constituent atoms. The bromine atom, having a greater pull, draws the electron density closer to itself. This electron shift creates a permanent dipole moment pointing toward the bromine atom.
The bromine end of the molecule carries the partial negative charge, while the hydrogen end carries the partial positive charge. This charge separation dictates HBr’s behavior and directly influences its physical properties, such as its relatively high boiling point compared to nonpolar molecules.
The polarity also explains why hydrogen bromide is highly soluble in polar solvents like water. The partially charged ends of the HBr molecule are strongly attracted to the oppositely charged ends of the water molecules. This allows HBr to readily dissolve and ionize in solution, forming the strong electrolyte known as hydrobromic acid.