Chemical polarity describes the separation of electric charge within a substance, resulting from the uneven sharing of electrons between atoms. This creates distinct regions with partially positive and partially negative character. For Strontium Dibromide (\(\text{SrBr}_2\)), the question of polarity is nuanced because it is an ionic compound. \(\text{SrBr}_2\) is formed by the complete transfer of electrons, meaning its bonds are extremely polar, but it is not classified using the same molecular polarity rules applied to covalent molecules.
The Role of Electronegativity in Chemical Bonding
The fundamental concept governing all chemical bonding is electronegativity, which represents an atom’s power to attract a shared pair of electrons toward itself within a bond. The difference in electronegativity (\(\Delta\text{EN}\)) between two bonded atoms determines the nature of the chemical bond formed, establishing a spectrum from nonpolar to fully ionic.
If atoms have identical electronegativities, electrons are shared equally, resulting in a nonpolar covalent bond. A moderate difference in electronegativity leads to uneven sharing, pulling electrons closer to the more attractive atom. This uneven sharing creates partial charges (\(\delta+\) and \(\delta-\)), forming a polar covalent bond.
When the electronegativity difference is very large, one atom removes the electron entirely from the other. This complete electron transfer forms an ionic bond, characterized by the creation of full positive and negative ions.
Determining the Bond Type in Strontium Dibromide
Applying the concept of electronegativity difference confirms Strontium Dibromide (\(\text{SrBr}_2\)) is an ionic compound. Strontium (\(\text{Sr}\)), an alkaline earth metal, has a low electronegativity (0.95), while Bromine (\(\text{Br}\)), a halogen, has a high electronegativity (2.96). The resulting difference (\(\Delta\text{EN}\)) is approximately 2.01, which is significantly above the threshold for ionic character, dictating electron transfer rather than sharing.
Each Strontium atom loses two valence electrons, becoming a cation (\(\text{Sr}^{2+}\)). Two Bromine atoms each gain one electron, forming two anions (\(\text{Br}^{-}\)). The strong electrostatic attraction between these ions constitutes the ionic bond.
Crystal Structure
Solid \(\text{SrBr}_2\) is not a discrete molecule typically assessed for molecular polarity. Instead, the ions arrange themselves into a highly ordered, repeating three-dimensional crystal lattice structure. In this lattice, the ions are held together by powerful ionic bonds, and the overall compound is a neutral solid.
Polarity Versus Ionic Character: The Solubility of Strontium Dibromide
The practical question of whether \(\text{SrBr}_2\) behaves as a polar substance is best answered by examining its interaction with solvents. The rule “like dissolves like” states that polar substances dissolve in polar solvents, and nonpolar substances dissolve in nonpolar solvents. Water is a highly polar solvent.
Strontium Dibromide is highly soluble in water, confirming that \(\text{SrBr}_2\) exhibits “polar behavior” due to its ionic character. When placed in water, the positive end of the water molecule is attracted to the negative Bromide ions, and the negative end is attracted to the positive Strontium ions.
This attraction overcomes the strong electrostatic forces holding the crystal lattice together, causing the ions to dissociate. The separated \(\text{Sr}^{2+}\) and \(\text{Br}^{-}\) ions are then surrounded and stabilized by the polar water molecules in a process called hydration. Its full charge separation and resulting solubility place \(\text{SrBr}_2\) at the extreme end of the polar spectrum.