Determining if Magnesium Oxide (MgO) is polar or nonpolar requires understanding how atoms interact to form chemical bonds. Chemical classification relies on a spectrum ranging from equal sharing of electrons to a complete transfer. While “polar” and “nonpolar” are common terms, they primarily apply to covalent compounds, making the classification of MgO complex. To accurately describe Magnesium Oxide, we must first examine the foundational principles governing the bond between magnesium and oxygen.
The Chemical Spectrum of Bonding
The nature of a chemical bond is determined by the difference in the atoms’ desire to attract electrons, a property known as electronegativity (EN). Atoms with high EN pull shared electrons toward themselves, while those with low EN allow electrons to drift away. This difference creates a continuous spectrum of bonding types.
A nonpolar covalent bond occurs when atoms share electrons almost equally, typically when the electronegativity difference (\(\Delta \text{EN}\)) is less than 0.5. If the difference is moderate (0.5 to 1.7), the result is a polar covalent bond where electrons are shared unequally. This unequal sharing creates partial positive and partial negative charges across the bond.
When the \(\Delta \text{EN}\) exceeds 1.7, the bond is classified as ionic. This large disparity results in one atom stripping electrons from the other, forming full positive and negative ions. It is often more accurate to think of bonds in terms of percent ionic character rather than rigid categories.
Analyzing the Magnesium Oxide Bond
To determine the nature of the bond between magnesium and oxygen, we must consider their electronegativity values. Oxygen (O) is a nonmetal with a high electronegativity of approximately 3.44. Magnesium (Mg), an alkaline earth metal, has a much lower electronegativity, around 1.31.
Calculating the difference in electronegativity (\(\Delta \text{EN}\)) yields 2.13, which is significantly greater than the 1.7 threshold used to define an ionic bond. This large difference means the oxygen atom exerts a much stronger pull on the electrons. Consequently, the two valence electrons from the magnesium atom are transferred to the oxygen atom.
This electron transfer forms a positively charged magnesium ion (\(\text{Mg}^{2+}\)) and a negatively charged oxide ion (\(\text{O}^{2-}\)), held together by a strong electrostatic force. The bond in Magnesium Oxide is classified as highly ionic due to this complete charge separation. While the bond possesses extreme polarization, its ionic classification places it outside the typical framework of covalent bond polarity.
Why MgO is Not a Polar or Nonpolar Molecule
The terms “polar” and “nonpolar” are most precisely applied to discrete molecules held together by covalent bonds. Magnesium Oxide is an ionic compound, meaning it does not exist as individual, isolated MgO molecules. Instead, it forms an extended structure known as a crystal lattice.
In the MgO lattice, every \(\text{Mg}^{2+}\) ion is surrounded by six \(\text{O}^{2-}\) ions, and every \(\text{O}^{2-}\) ion is surrounded by six \(\text{Mg}^{2+}\) ions. This repeating, three-dimensional arrangement adopts the cubic “rock salt” structure. This structure ensures that positive and negative charges are distributed symmetrically throughout the entire solid.
Due to this perfect symmetry and the counterbalancing of charges, the compound as a whole has no net electrical polarity or molecular dipole moment. While the bond between a single magnesium ion and an oxide ion is extremely polarized, the solid MgO compound cannot be correctly labeled as a polar or nonpolar molecule. It is accurately described as an ionic solid, a structural classification distinct from covalent molecules.
How Ionic Character Influences Physical Properties
The immense strength of the electrostatic forces within the ionic lattice dictates the macroscopic behavior of Magnesium Oxide. The strong attraction between the \(\text{Mg}^{2+}\) and \(\text{O}^{2-}\) ions requires significant energy to break the bonds and melt the solid. This results in an exceptionally high melting point of approximately \(2,852 \text{ }^{\circ}\text{C}\) and a boiling point near \(3,600 \text{ }^{\circ}\text{C}\).
This strong ionic character also makes MgO a poor electrical conductor in its solid state. The ions are locked rigidly in place within the crystal lattice and cannot move freely to carry an electric current. While MgO is only slightly soluble in water, it readily dissolves in highly polar solvents such as acids or ammonium salt solutions.