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Common Polyatomic Ions

A polyatomic ion is formed when a neutral molecule gains or loses electrons, just as ions are formed when neutral atoms gain or lose electrons. As a result, a polyatomic ion is a group of covalently bonded atoms with a net charge because the total number of electrons in the molecule does not equal the total number of protons in the molecule. The sum of the formal charges on all the atoms in the Lewis dot structure of a polyatomic ion must equal the net charge on the ion.

Knowing the most common polyatomic ions will help in recognising ionic compounds and predict their reactivity.

Table of Contents

Structure of Polyatomic Ions

A polyatomic ion can be thought of as a molecule that has been ionised by gaining or losing electrons. The group of covalently bonded atoms in a polyatomic ion carries a net charge because the total number of electrons in the molecule does not equal the total number of protons in the molecule. The overall charge on a polyatomic ion is equal to the sum of the formal charges on each atom in the ion when drawing Lewis dot structures.

List of Common Polyatomic Ions

Common Polyatomic Ion with Charge +1

Formula

Name

[NH4]+

Ammonium

[H3O]+

Hydronium

Common Polyatomic Ion with Charge -1

Formula

Name

[CH3COO]

Acetate

[HCO3]

Bicarbonate

[HSO4]

Bisulfate (or hydrogen sulfate)

[ClO]

Hypochlorite

[ClO3]

Chlorate

[ClO2]

Chlorite

[OCN]

Cyanate

CN

Cyanide

[H2PO4]

Dihydrogen phosphate-

OH

Hydroxide

[HSO4]

Hydrogen sulphate

[NO3]

Nitrate

[NO2]

Nitrite

[ClO4]

Perchlorate

[MnO4]

Permanganate

SCN

Thiocyanate

Common Polyatomic Ions with Charge +2

Formula

Name

[Hg2]2+

Mercury (I)

Common Polyatomic Ions with Charge -2

Formula

Name

[CO3]2-

Carbonate

[CrO4]2-

Chromate

[Cr2O7]2-

Dichromate

[HPO4]2-

Hydrogen phosphate

[C2O4]2-

Oxalate

[O2]2-

Peroxide

[SiO3]2-

Silicate

[SO4]2-

Sulfate

[SO3]2-

Sulfite

[S2O3]2-

Thiosulfate

Common Polyatomic Ion with charge -3

Formula

Name

[AsO3]3-

Arsenate

[BO3]3-

Borate

[PO3]3-

Phosphite

[PO4]3-

Phosphate

Frequently Asked Questions on Common Polyatomic Ions

What is the common polyatomic ion found in bases?

The common polyatomic ion found in bases is hydroxide OH.

What do the majority of the polyatomic ion names have in common?

Polyatomic ions are ions that contain more than one type of atom. The vast majority of polyatomic ions are anions with names that end in “ate” or “ite.”

How many protons and electrons are there in a hydroxide ion?

The total number of protons in a hydroxide ion can be calculated by adding the number of protons in one hydrogen atom and one oxygen atom:

Total protons are equal to the sum of protons in H and protons in O.

1 proton + 8 protons = 9 protons

Is it possible for polyatomic ions to remain together?

A polyatomic ion’s atoms are usually covalently bonded to one another and thus remain together as a single, charged unit.

How to determine if an ion is polyatomic?

All of the elements on the periodic table begin with a capital letter, and only a few of them have a lower case second letter. So, there are two capital letters together in an ion, it is a polyatomic ion.

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