What Is A Polyatomic Ion?

Polyatomic ions are covalently bonded groups of atoms with a positive or negative charge caused by the formation of an ionic bond with another ion. Compounds formed from such ion combinations are called polyatomic ionic compounds, but the polyatomic ion behaves as a single unit.

Polyatomic ions and ionic compounds take part in chemical reactions such as acid-base, precipitation and displacement just like monatomic metallic ions. They dissolve in water, conduct electricity and dissociate in solution just like other ions. While they behave externally like monatomic ions, their internal structure is more complicated due to the presence of two or more atoms in the polyatomic ion.

Many common chemicals are polyatomic compounds and contain polyatomic ions. For example, sulfuric acid, with the chemical formula H₂SO₄, contains hydrogen ions and the polyatomic sulfate anion SO₄^-2. The sulfur atom has six electrons in its outer shell and shares them covalently with the oxygen atoms that also have six electrons in their outer shells. The four oxygen atoms would need to have eight electrons shared between them, leaving a deficit of two. In sulfuric acid, the sulfate radical forms ionic bonds with the hydrogen atoms that donate an electron each to become hydrogen ions, H⁺. The sulfate radical receives the two electrons to become SO₄^-2.

Most polyatomic ions contain oxygen and are negatively charged anions because the oxygen atoms attract electrons. Ammonium is one of the few positively charged polyatomic ions or cations and doesn't contain oxygen.

Nitrogen has five electrons in its outermost shell, and it has room for eight. When it shares electrons covalently with four hydrogen atoms, four electrons are available from the hydrogen or one more than is needed. When ammonium forms an ionic bond with a hydroxide OH radical, the extra electron is transferred to complete the outermost shell of the OH oxygen atom, which needs two electrons but has only one from the OH hydrogen atom. The electron from the NH₄ radical is transferred to the OH radical creating an OH^- ion and a NH₄⁺ ion.

Like any acid and base, polyatomic ionic acids and bases react to form water and a salt in a neutralization reaction. For example, the two polyatomic compounds above, sulfuric acid and ammonium hydroxide, will react to form water and ammonium sulfate. The polyatomic ions stay together, each maintaining their covalent bonds, while the hydrogen and hydroxide ions combine to form water.

The chemical reaction takes place as follows:

2NH₄OH + H₂SO₄ = (NH₄)₂SO₄ +2H₂O

The ammonium hydroxide in water dissociates into ammonium and hydroxide ions. The sulfuric acid dissociates into hydrogen and sulfate ions. In solution, the hydrogen and hydroxide ions combine to form water while the ammonium and sulfate ions stay in solution. If the water is removed, ammonium sulfate crystallizes out of solution as a new polyatomic ionic compound.