Ammonium bromide (\(\text{NH}_4\text{Br}\)) is classified as an ionic compound, meaning it is held together by the strongest form of charge attraction. When considering its fundamental chemical nature, \(\text{NH}_4\text{Br}\) is not a nonpolar or polar molecule in the traditional sense. Because of the inherent full charge separation between its components, ammonium bromide is considered highly polar.
The Ionic Composition of Ammonium Bromide
Ammonium bromide exists as a crystal lattice formed by two distinct ions, not a single covalently bonded molecule. These ions are the positively charged polyatomic ammonium cation (\(\text{NH}_4^+\)) and the negatively charged bromide anion (\(\text{Br}^-\)). The ammonium ion is a group of atoms held together by covalent bonds that collectively carries a \(+1\) electrical charge.
The fundamental force binding the \(\text{NH}_4^+\) and \(\text{Br}^-\) ions is an ionic bond. This bond is purely electrostatic, arising from the strong attraction between opposite charges. This contrasts sharply with covalent compounds, where atoms share electrons. The chemical formula, \(\text{NH}_4\text{Br}\), reflects the one-to-one ratio of these two ions required to maintain electrical neutrality.
Determining Polarity Based on Electronegativity
Polarity in chemical compounds is determined by the electronegativity of the constituent atoms. Electronegativity is an atom’s ability to attract a shared pair of electrons toward itself within a chemical bond. The difference in electronegativity values (\(\Delta\text{EN}\)) dictates the type of bond formed.
A very small or zero \(\Delta\text{EN}\) results in a nonpolar covalent bond where electrons are shared equally. If the \(\Delta\text{EN}\) is moderate, electrons are shared unequally, creating a polar covalent bond with a partial positive (\(\delta+\)) and partial negative (\(\delta-\)) end. For a compound to be considered polar, the net effect of all individual bond dipoles must not cancel out due to the molecule’s geometry.
When the \(\Delta\text{EN}\) between two atoms is very large, typically exceeding \(1.7\) or \(2.0\), the electrons are not shared at all; instead, one atom completely transfers its electron to the other. This results in the formation of a cation and an anion held together by an ionic bond. The formation of discrete, fully charged ions is the maximum expression of polarity, far exceeding the partial charges found in polar covalent molecules.
Classification and Behavior of Ammonium Bromide
The definitive classification of ammonium bromide as an ionic compound dictates its highly polar nature. The bond between the \(\text{NH}_4^+\) and \(\text{Br}^-\) ions represents a complete separation of charge, which is the maximum possible degree of polarity. All ionic compounds, by definition, possess this high degree of polarity.
The physical behavior of ammonium bromide strongly supports this classification, particularly its interaction with solvents. \(\text{NH}_4\text{Br}\) is highly soluble in water, a polar solvent. This high solubility occurs because the polar water molecules are strongly attracted to and able to surround the fully charged ions, pulling them out of the crystal lattice.
Once dissolved, the compound dissociates completely into its constituent ions (\(\text{NH}_4^+(\text{aq})\) and \(\text{Br}^-(\text{aq})\)). The presence of these mobile, fully charged ions in the solution allows it to conduct electricity, making ammonium bromide a strong electrolyte. \(\text{NH}_4\text{Br}\) is also soluble in other polar solvents but is practically insoluble in nonpolar solvents, confirming its polar, ionic identity.