Polyatomic Ions

The term poly means many, so a polyatomic ion is an ion that contains more than one atom. Thus it is different from monatomic ions, which contain only one atom. Examples of monatomic ions include Na+ and Cl- etc. This article will give details of polyatomic ions and their examples.

polyatomic ions

Definition of Polyatomic Ions

Polyatomic ions are covalently bonded groups of atoms and having a positive or negative charge caused by the formation of an ionic bond with another ion. Compounds formed from such a combination of ions are polyatomic ionic compounds.

But the polyatomic ion will behave as a single unit. In addition, polyatomic ions and those ionic compounds take part in chemical reactions such as acid-base, precipitation, and displacement similar to monatomic metallic ions.

They easily dissolve in water and conduct electricity and dissociate in a solution similar to other ions. Whereas they behave externally like monatomic ions, but their internal structure is more complicated. This is because there are two or more atoms present in the polyatomic ion.

For example:

NH₄⁺ :: Ammonium , CO₃²⁻ :: Carbonate

NO₂⁻ :: Nitrite HCO₃⁻ :: Hydrogen carbonate

NO₃⁻ :: Nitrate ClO⁻ :: Hypochlorite

Structure of Polyatomic Ions

Polyatomic ions can be compared with monatomic ions. Monatomic ions are the atoms which are ionized by gaining or losing electrons. The ion has a net charge over it because the total number of electrons is not balanced by the total number of protons in the nucleus.

Therefore compared to the neutral atom, they have extra electrons for negatively charged anion or not enough electrons for the positively charged cation.

Polyatomic Ionic Compound Sulfuric Acid

Many common chemicals are polyatomic compounds with polyatomic ions. For example, sulfuric acid H₂SO₄ contains H⁺ and the polyatomic SO₄^-2.

The sulfur atom has six electrons in its outer shell. It shares them covalently with the oxygen atoms having six electrons in their outer shells. The 4 oxygen atoms would need to have eight electrons shared between them, leaving a deficit of two.

In sulfuric acid, the sulfate forms ionic bonds with the hydrogen atoms that donate an electron each to become hydrogen ions, H+. The sulfate receives the two electrons to become SO₄^-2.

Polyatomic Ion NH₄⁺ or Ammonium:

In most polyatomic ions, it contains oxygen and a negatively charged anions. Ammonium is one of the few positively charged polyatomic ions and doesn’t contain oxygen.

Nitrogen has five electrons in its outermost shell, and it can have eight. When it shares electrons covalently with four hydrogen atoms. Then four electrons are available from the hydrogen or one more than needed.

When ammonium forms an ionic bond with an OH the extra electron transfers to complete the outermost shell of the OH. It needs two electrons but has only one from the OH hydrogen atom. The electron from the NH₄ transfers to the OH creating an OH^– ion and an NH₄⁺ ion.

Examples of Common Polyatomic Ions

Many of the polyatomic ions have an electrical charge of -1.

Acetate – C₂H₃O₂^–
Bicarbonate (or hydrogen carbonate) – HCO₃^–
Bisulfate (or hydrogen sulfate) – HSO₄^–
Hypochlorite – ClO^–
Chlorate – ClO₃^–
Chlorite – ClO₂^–
Cyanide – CN-
Hydroxide – OH-
Nitrate – NO3-
Nitrite – NO2-

Polyatomic ions with a -2 charge are also common.

Carbonate – CO₃^2-
Chromate – CrO₄^2-
Peroxide – O₂^2-
Sulfate – SO₄^2-
Sulfite – SO₃^2-

However, other polyatomic ions form with the -3 charge, but the borate and phosphate ions are the ones to memorize.

Borate – BO₃^3-
Phosphate – PO₄^3-

Solved Question for You

Q. How many protons and electrons are in a hydroxide ion?

Ans: We can calculate the total number of protons in a hydroxide ion by adding up the number of protons in one hydrogen atom and one oxygen atom:

Total protons=protons in H + protons in O

=1 proton + 8 protons=9 protons

In a neutral molecule, the number of protons and electrons are equal. Also, hydroxide has a net -1 charge and we know there must be an extra electron compared to the number of protons. Hence, the hydroxide ion has nine protons and ten electrons.

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