Magnesium chloride, represented by the chemical formula \(\text{MgCl}_2\), is a compound frequently encountered in industrial processes, biology, and common household applications. The question of whether this substance is polar or nonpolar often arises when trying to understand its behavior, particularly its high solubility in water. Polarity is a concept that applies most directly to molecular compounds, which are formed by the sharing of electrons between atoms. Magnesium chloride is classified as an ionic compound. This means that while its bonds are based on an extreme separation of charge, the compound does not possess a singular molecular polarity in the traditional sense, but rather a structure defined by its charged ions within an ionic lattice.
What Makes a Chemical Bond Polar
Chemical bonds are formed when atoms interact by sharing or transferring electrons, and polarity determines how equally those electrons are distributed. Polarity arises from the unequal sharing of electrons between two atoms that possess different values of electronegativity. Electronegativity is an intrinsic property of an atom that quantifies its power to attract a shared pair of electrons toward itself within a chemical bond.
When two atoms of the exact same element bond, the electron pair is shared equally, resulting in a nonpolar covalent bond. If the atoms are different, a difference in electronegativity exists, causing the electrons to spend more time closer to the atom with the higher attractive force. This creates a polar covalent bond, where one atom develops a slight negative charge (\(\delta^-\)) and the other a slight positive charge (\(\delta^+\)), establishing a dipole moment.
The magnitude of the electronegativity difference (\(\Delta\text{EN}\)) determines where the bond falls on a continuous spectrum of bond types. A small difference results in a nonpolar covalent bond, while a moderate difference yields a polar covalent bond. As the difference increases significantly, the sharing of electrons becomes so unequal that it is described as a complete transfer of electrons, defining the ionic bond.
The terms “polar” and “nonpolar” are generally used to describe the nature of covalent bonds and molecular structures. Bonds with an electronegativity difference between approximately 0.4 and 1.7 are classified as polar covalent. When the difference exceeds 1.7, the bond character is predominantly ionic, meaning the atoms have essentially become ions held together by electrostatic attraction.
The Ionic Nature of Magnesium Chloride
The compound \(\text{MgCl}_2\) is formed between Magnesium (\(\text{Mg}\)), a metal, and Chlorine (\(\text{Cl}\)), a nonmetal, which is the classic pairing for forming an ionic compound. Magnesium is electropositive, readily giving up electrons to achieve a stable electron configuration. Chlorine, a halogen, is highly electronegative and has a strong tendency to gain electrons to fill its outer shell.
The electronegativity value for Magnesium is approximately 1.31, while the value for Chlorine is significantly higher at about 3.16. This results in an electronegativity difference (\(\Delta\text{EN}\)) of approximately 1.85, a value that falls firmly into the range traditionally associated with ionic bonding.
This large difference causes the Magnesium atom to completely transfer its two valence electrons, one to each of the two Chlorine atoms. This electron transfer results in the formation of a positively charged Magnesium cation (\(\text{Mg}^{2+}\)) and two negatively charged Chloride anions (\(\text{Cl}^{-}\)). The chemical formula \(\text{MgCl}_2\) is established because two single-charged chloride ions are required to balance the \(2+\) charge of the single magnesium ion.
Because the bonding is characterized by the electrostatic attraction between these fully charged ions, the \(\text{Mg}-\text{Cl}\) bond is classified as ionic, not covalent. The compound exists as a crystal lattice structure in its solid state, where the ions are arranged in a repeating, three-dimensional array. While the individual \(\text{Mg}-\text{Cl}\) bonds are highly polar due to the extreme charge separation, the term “molecular polarity” is inapplicable to the bulk solid.
How Magnesium Chloride Interacts with Water
The ionic nature of magnesium chloride has direct consequences for its behavior, particularly its high solubility in water. Water (\(\text{H}_2\text{O}\)) is a bent molecule with a significant net dipole moment, making it a highly polar solvent. The oxygen atom carries a partial negative charge, and the two hydrogen atoms carry partial positive charges.
When solid magnesium chloride is introduced to water, the polar water molecules begin to interact with the crystal lattice. The partially negative oxygen end of the water molecule is strongly attracted to the positively charged \(\text{Mg}^{2+}\) ions, while the partially positive hydrogen ends are drawn to the negatively charged \(\text{Cl}^{-}\) ions. This attraction is known as an ion-dipole interaction.
If the energy released by these stabilizing ion-dipole attractions (the hydration energy) is greater than the energy holding the ionic lattice together (the lattice energy), the ions are pulled apart and the compound dissociates. The separated \(\text{Mg}^{2+}\) and \(\text{Cl}^{-}\) ions are then surrounded by a shell of water molecules, forming a hydration sphere. This process explains why magnesium chloride is highly soluble in water. The ability of the polar water molecules to overcome the strong electrostatic forces of the ionic lattice confirms that while \(\text{MgCl}_2\) is not a polar molecule, its extreme charge separation makes it readily dissolvable by polar solvents.