Compounds With Both Ionic and Covalent Bonds

An ionic bond is a chemical bond between two atoms in which one atom seems to donate its electron to another atom. Covalent bonds, on the other hand, appear to involve two atoms sharing electrons reach a more stable electron configuration. Some compounds contain both ​ionic and covalent bonds. These compounds contain polyatomic ions. Many of these compounds contain a metal, a nonmetal, and also hydrogen. However, other examples contain a metal joined via an ionic bond to covalently bonded nonmetals. Here are examples of compounds that exhibit both types of chemical bonding:

• NaNO₃ - sodium nitrate
• (NH₄)S - ammonium sulfide
• Ba(CN)₂ - barium cyanide
• CaCO₃ - calcium carbonate
• KNO₂ - potassium nitrite
• K₂SO₄ - potassium sulfate

In ammonium sulfide, the ammonium cation and the sulfide anion are ionically bonded together, even though all of the atoms are nonmetals. The electronegativity difference between ammonium and the sulfur ion allows for an ionic bond. At the same time, the hydrogen atoms are covalently bonded to the nitrogen atom.

Calcium carbonate is another example of a compound with both ionic and covalent bonds. Here calcium acts as the cation, with the carbonate species as the anion. These species share an ionic bond, while the carbon and oxygen atoms in carbonate are covalently bonded.

How It Works

The type of chemical bond formed between two atoms or between a metal and set of nonmetals depends on the electronegativity difference between them. It's important to remember the way bonds are classified is somewhat arbitrary. Unless two atoms entering a chemical bond have identical electronegativity values, the bond will always be somewhat polar. The only real difference between a polar covalent bond and an ionic bond is the degree of charge separation.

Remember the electronegativity ranges, so you'll be able to predict the types of bonds in a compound:

• nonpolar covalent bond - The electronegativity difference is less than 0.4.
• polar covalent bond - The electronegativity difference is between 0.4 and 1.7.
• ionic bond - The electronegativity difference between species forming a bond is greater than 1.7.

The difference between ionic and covalent bonds is a bit ambiguous since the only truly nonpolar covalent bond occurs when two elements of the same atom bond with each other (e.g., H₂, O₃). It's probably better to think of chemical bonds as being more-covalent or more-polar, along a continuum. When both ionic and covalent bonding occurs in a compound, the ionic portion is almost always between the cation and anion of the compound. The covalent bonds could occur in a polyatomic ion in either the cation or the anion.