Use the calculated oxidation numbers and number of atoms to calculate the charge on the Polyatomic IonsFirst we will review electronegativity and how to calculate the oxidation number for each element in a compound. A detailed discussion of these topics can be found in our Electronegativity section.
After reviewing electronegativity and oxidation number, we then devote ourselves to applying the above 4 steps to each major type of Inorganic Polyatomic Ions.
Electronegativity and Oxidation Number
The Oxidation Number (O.N.) is the difference between the number of valence electrons and the sum of the number of shared and unshared electrons in a bond between two elements.
The valence electrons is the number of electrons in the outer shell of an element. This topic is discussed in detail in our sections on Atomic Structure and the Periodic Table.
The number of shared and unshared electrons of elements in a bond are determined based on the relative electronegativity values as detailed in the first three steps in the picture above.
The Oxidation Number (O.N.) is then calculated by the equation in step 4 in the picture above.
The First H-O Series - Water as an Acid
The first major class of Polyatomic Ions is the H-O series which involves atoms of the elements of hydrogen and oxygen.
In the first of this series water is the acid and the hydroxide ion is the anion. The Lewis Structure for water is given in the picture above. In the green box above the Oxidation Numbers for oxygen and hydrogen are calculated as -2 and +1, respectively.
Since there are one oxygen atom and two hydrogen atoms in the water molecule, the charge on the water molecule is 0 (2 + 2*(+1)) = 0.
Since there are one oxygen atom and only one hydrogen atom in the hydroxide anion, the charge for the hydroxide anion is -1 (-2 + (+1)) = -1.
The Second H-O Series - Water as a Base
In the second H-O series water is the base and the hydronium ion is the cation. The Lewis Structure for water is given in the picture above. In the green box above the Oxidation Numbers for oxygen and hydrogen are calculated as -2 and +1, respectively.
Since there are one oxygen atom and two hydrogen atoms in the water molecule, the charge on the water molecule is 0 (2 + 2*(+1)) = 0.
Since there are one oxygen atom and three hydrogen atoms in the hydronium cation, the charge for the hydronium cation is +1 (-2 + 3*(+1)) = +1.
The Third H-O Series - Hydrogen Peroxide as an Acid
In the third H-O series hydrogen peroxide is the acid and the peroxide ion is the anion. The Lewis Structure for hydrogen peroxide is given in the picture above. In the green box above the Oxidation Numbers for oxygen and hydrogen are calculated as -1 and +1, respectively.
Note: While oxygen usually has an oxidation number of -2, in this special case of peroxide oxygen has an oxidation number of -1.
Since there are two oxygen atoms and two hydrogen atoms in the hydrogen peroxide molecule, the charge on the hydrogen peroxide molecule is 0 (2*(-1) + 2*(+1)) = 0.
Since there are two oxygen atoms and zero hydrogen atoms in the peroxide anion, the charge for the peroxide anion is -2 (2*(-1) + 0) = -2.
All three of the H-O series are summarized in the table below.
The H-O Series
The next major class of Polyatomic Ions is the H-N-O series involving atoms of the elements hydrogen, nitrogen and oxygen.
The H-N-O Series - Ammonia as a Base
In the first of this series actually does not have any oxygen atoms participating. One nitrogen atom combines with three hydrogen atoms to form the base ammonia and the resulting polyatomic ion is the ammonium cation.
The Lewis Acid Structure for ammonia is shown in the picture above. In the green box above the Oxidation Numbers for nitrogen and hydrogen are calculated as -3 and +1, respectively.
Since there are one nitrogen atom and three hydrogen atoms in the ammonia molecule, the charge on the ammonia molecule is 0 (-3 + 3*(+1)) = 0.
Note: As we shall see below, when nitrogen combines with oxygen the Oxidation Number for nitrogen is +3 or +5. However, when nitrogen combines with hydrogen, as in the present case of ammonia, the Oxidation Number of nitrogen is -3. This difference in Oxidation Number graphically illustrates the importance of relative electronegativities in covalent bonding.
Since there are one nitrogen atom and four hydrogen atoms in the ammonium cation, the charge on the ammonium cation is +1 (-3 + (+4)) = +1.
The H-N-O Series - Nitrous Acid
In the second of H-N-O series, one nitrogen atom combines with two oxygen atoms and one hydrogen atom to form nitrous acid and the resulting polyatomic ion is the nitrite anion.
The Lewis Acid Structure for nitrous acid is shown in the picture above. In the green box above the Oxidation Numbers for oxygen, nitrogen and hydrogen are calculated as -2, +3 and +1, respectively.
Since there are two oxygen atoms, one nitrogen atom and one hydrogen atom in the nitrous acid molecule, the charge for the nitrous acid molecule is 0 (2*(-2) + (+3) + (+1)) = 0.
Since there are two oxygen atoms, one nitrogen atom and zero hydrogen atoms in the nitrite anion, the charge for the nitrate anion is -1 (2*(-2) + (+3) + 0) = -1.
The H-N-O Series - Nitric Acid
In the third of H-N-O series one nitrogen atom combines with three oxygen atoms and one hydrogen atom to form nitric acid and the resulting polyatomic ion is the nitrate anion.
The Lewis Acid Structure for nitric acid is shown in the picture above. In the green box above the Oxidation Numbers for oxygen, nitrogen and hydrogen are calculated as -2, +5 and +1, respectively.
Since there are three oxygen atoms, one nitrogen atom and one hydrogen atom in the nitric acid molecule, the charge for the nitric acid molecule is 0 (3*(-2) + (+5) + (+1)) = 0
Since there are three oxygen atoms, one nitrogen atom and zero hydrogen atoms in the nitrate anion, the charge for the nitrate anion is -1 (3*(-2) + (+5) + 0 = -1).
All three of the H-N-O series of Polyatomic Ions are summarized in the table below.
The H-N-O Series
The next major class of Polyatomic Ions is the H-P-O series involving atoms of the elements hydrogen, phosphorous and oxygen.
The H-P-O Series - Phosphoric Acid
In this series one phosphorous atom combines with four oxygen atoms and three hydrogen atoms to form the phosphoric acid molecule and the resulting polyatomic ions are referred to as the phosphate anion series.
The Lewis Acid Structure for phosphoric acid is shown in the picture above. In the green box above the Oxidation Numbers for oxygen, phosphorous and hydrogen are calculated as -2, +5 and +1, respectively.
As can be seen in the above picture, phosphoric acid is what is called a 'tri-protic' acid, which means that it has three ionizable hydrogen ions and thus can form three different polyatomic anions - dihydrogen phosphate, hydrogen phosphate and phosphate.
Since there are four oxygen atoms, one phosphorous atom and three hydrogen atoms in the phosphoric acid molecule, the charge on the phosphoric acid molecule is 0 (4*(-2) + (+5) + 3*(+1)) = 0.
Since there are four oxygen atoms, one phosphorous atom and two hydrogen atoms in the dihydrogen phosphate anion, the charge for the dihydrogen phosphate anion is -1 (4*(-2) + (+5) + 2*(+1)) = -1.
Since there are four oxygen atoms, one phosphorous atom and one hydrogen atom in the hydrogen phosphate anion, the charge for the hydrogen phosphate anion is -2 (4*(-2) + (+5) + 1*(+1)) = -2.
Since there are four oxygen atoms, one phosphorous atom and zero hydrogen atoms in the phosphate anion, the charge for the phosphate anion is -3 (4*(-2) + (+5) + 0) = -3.
The H-P-O series of Polyatomic Ions is summarized in the table below.
The H-P-O Series
The next major class of Polyatomic Ions is the H-S-O series involving atoms of the elements hydrogen, sulfur and oxygen.
The First H-S-O Series - Sulfurous Acid
In the first H-S-O series, one sulfur atom combines with three oxygen atoms and two hydrogen atoms to form the sulfurous acid molecule and the resulting polyatomic ions are referred to as the sulfite anion series.
The Lewis Acid Structure for sulfurous acid is shown in the picture above. In the green box above the Oxidation Numbers for oxygen, sulfur and hydrogen are calculated as -2, +4 and +1, respectively.
As can be seen in the above picture, sulfurous acid is what is called a 'di-protic' acid, which means that it has two ionizable hydrogen ions and thus can form two different polyatomic anions - hydrogen sulfite and sulfite.
Since there are three oxygen atoms, one sulfur atom and two hydrogen atoms in the sulfurous acid molecule, the charge on the sulfurous acid molecule is 0 (3*(-2) + (+4) + 2*(+1)) = 0.
Since there are three oxygen atoms, one sulfur atom and one hydrogen atom in the hydrogen sulfite anion, which is sometimes called bisulfite, the charge for the hydrogen sulfite anion is -1 (3*(-2) + (+4) + 1*(+1)) = -1.
Since there are three oxygen atoms, one sulfur atom and zero hydrogen atoms in the sulfite anion, the charge for the sulfite anion is -2 (3*(-2) + (+4) + 0) = -2.
The Second H-S-O Series - Sulfuric Acid
In the second H-S-O series, one sulfur atom combines with four oxygen atoms and two hydrogen atoms to form the sulfuric acid molecule and the resulting polyatomic ions are referred to as the sulfate anion series.
The Lewis Acid Structure for sulfuric acid is shown in the picture above. In the green box above the Oxidation Numbers for oxygen, sulfur and hydrogen are calculated as -2, +6 and +1, respectively.
As can be seen in the above picture, sulfuric acid is what is called a 'di-protic' acid, which means that it has two ionizable hydrogen ions and thus can form two different polyatomic anions - hydrogen sulfate and sulfate.
Since there are four oxygen atoms, one sulfur atom and two hydrogen atoms in the sulfuric acid molecule, the charge for the sulfuric acid molecule is 0 (4*(-2) + (+6) + 2*(+1)) = 0.
Since there are four oxygen atoms, one sulfur atom and one hydrogen atom in the hydrogen sulfate anion, the charge for the hydrogen sulfate anion is -1 (4*(-2) + (+6) + 1*(+1)) = -1.
Since there are four oxygen atoms, one sulfur atom and zero hydrogen atoms in the sulfate anion, the charge for the sulfate anion is -2 (4*(-2) + (+6) + 0) = -2.
The H-S-O series of Polyatomic Ions is summarized in the table below.
The H-S-O Series
The next major class of Polyatomic Ions is the H-Cl-O series involving atoms of the elements hydrogen, chlorine and oxygen.
The First H-Cl-O Series - Hypochlorous Acid
In the first H-Cl-O series, one chlorine atom combines with one oxygen atom and one hydrogen atom to form hypochlorous acid and the resulting polyatomic ion is referred to as the hypochlorite anion.
The Lewis Acid Structure for hypochlorous acid is shown in the picture above. In the green box above the Oxidation Numbers for oxygen, chlorine and hydrogen are calculated as -2, +1 and +1, respectively.
Since there are one oxygen atom, one chlorine atom and one hydrogen atom in the hypochlorous acid molecule, the charge on the hypochlorous acid molecule is 0 (-2 + (+1) + (+1)) = 0.
Since there is one oxygen atom, one chlorine atom and zero hydrogen atoms in the hypochlorite anion, the charge for the hypochlorite anion is -1 (-2 + (+1) + 0) = -1.
The Second H-Cl-O Series - Chlorous Acid
In the second H-Cl-O series, one chlorine atom combines with two oxygen atoms and one hydrogen atom to form chlorous acid and the resulting polyatomic ion is referred to as the chlorite anion.
The Lewis Acid Structure for chlorous acid is shown in the picture above. In the green box above the Oxidation Number for oxygen, chlorine and hydrogen are calculated as -2, +3 and +1, respectively.
Since there are two oxygen atoms, one chlorine atom and one hydrogen atom in the chlorous acid molecule, the charge on the chlorous acid molecule is 0 (2*(-2) + (+3) + (+1)) = 0.
Since there are two oxygen atoms, one chlorine atom and zero hydrogen atoms in the chlorite anion, the charge for the chlorite anion is -1 (2*(-2) + (+3) + 0) = -1.
The Third H-Cl-O Series - Chloric Acid
In the third H-Cl-O series, one chlorine atom combines with three oxygen atoms and one hydrogen atom to form chloric acid and the resulting polyatomic ion is referred to as the chlorate anion.
The Lewis Acid Structure for chloric acid is shown in the picture above. In the green box above the Oxidation Numbers for oxygen, chlorine and hydrogen are calculated as -2, +5 and +1, respectively.
Since there are three oxygen atoms, one chlorine atom and one hydrogen atom in the chloric acid molecule, the charge on the chloric acid molecule is 0 (3*(-2) + (+5) + (+1)) = 0.
Since there are three oxygen atoms, one chlorine atom and zero hydrogen atoms in the chlorate anion, the charge for the chlorate anion is -1 (3*(-2) + (+5) + 0) = -1.
The Fourth H-Cl-O Series - Perchloric Acid
In the fourth H-Cl-O series, one chlorine atom combines with four oxygen atoms and one hydrogen atom to form perchloric acid and the resulting polyatomic ion is referred to as the perchlorate anion.
The Lewis Acid Structure for perchloric acid is shown in the picture above. In the green box above the Oxidation Numbers for oxygen, chlorine and hydrogen are calculated as -2, +7 and +1, respectively.
Since there are four oxygen atoms, one chlorine atom and one hydrogen atom in the perchloric acid molecule, the charge on the perchloric acid molecule is 0 (4*(-2) + (+7) + (+1)) = 0.
Since there are four oxygen atoms, one chlorine atom and zero hydrogen atoms in the perchlorate anion, the charge for the perchlorate anion is -1 (4*(-2) + (+7) + 0) = -1.
The H-Cl-O series of Polyatomic Ions is summarized in the table below.
The H-Cl-O Series
The next major class of Polyatomic Ions is the H-I-O series involving atoms of the elements hydrogen, iodine and oxygen.
The First H-I-O Series - Hypoiodous Acid
In the first H-I-O series, one iodine atom combines with one oxygen atom and one hydrogen atom to form hypoiodous acid and the resulting polyatomic ion is referred to as the hypoiodite anion.
The Lewis Acid Structure for hypoiodous acid is shown in the picture above. In the green box above the Oxidation Numbers for oxygen, iodine and hydrogen are calculated as -2, +1 and +1, respectively.
Since there are one oxygen atom, one iodine atom and one hydrogen atom in the hypoiodous acid molecule, the charge on the hypoiodous acid molecule is 0 (-2 + (+1) + (+1)) = 0.
Since there is one oxygen atom, one iodine atom and zero hydrogen atoms in the hypoiodite anion, the charge for the hypoiodite anion is -1 (-2 + (+1) + 0) = -1.
The Second H-I-O Series - Iodous Acid
In the second H-I-O series, one iodine atom combines with two oxygen atoms and one hydrogen atom to form iodous acid and the resulting polyatomic ion is referred to as the iodite anion.
The Lewis Acid Structure for iodous acid is shown in the picture above. In the green box above the Oxidation Numbers for oxygen, iodine and hydrogen are calculated as -2, +3 and +1, respectively.
Since there are two oxygen atoms, one iodine atom and one hydrogen atom in the iodous acid molecule, the charge on the iodous acid molecule is 0 (2*(-2) + (+3) + (+1)) = 0.
Since there are two oxygen atoms, one iodine atom and zero hydrogen atoms in the iodite anion, the charge for the iodite anion is -1 (2*(-2) + (+3) + 0) = -1.
The Third H-I-O Series - Iodic Acid
In the third H-I-O series, one iodine atom combines with three oxygen atoms and one hydrogen atom to form iodic acid and the resulting polyatomic ion is referred to as the iodate anion.
The Lewis Acid Structure for iodic acid is shown in the picture above. In the green box above the Oxidation Numbers for oxygen, iodine and hydrogen are calculated as -2, +5 and +1, respectively.
Since there are three oxygen atoms, one iodine atom and one hydrogen atom in the iodic acid molecule, the charge on the iodic acid molecule is 0 (3*(-2) + (+5) + (+1)) = 0.
Since there are three oxygen atoms, one iodine atom and zero hydrogen atoms in the iodate anion, the charge for the iodate anion is -1 (3*(-2) + (+5) + 0) = -1.
The Fourth H-I-O Series - Periodic Acid
In the fourth H-I-O series, one iodine atom combines with four oxygen atoms and one hydrogen atom to form periodic acid and the resulting polyatomic ion is referred to as the periodate anion.
The Lewis Acid Structure for periodic acid is shown in the picture above. In the green box above the Oxidation Numbers for oxygen, iodine and hydrogen are calculated as -2, +7 and +1, respectively.
Since there are four oxygen atoms, one iodine atom and one hydrogen atom in the periodic acid molecule, the charge on the periodic acid molecule is 0 (4*(-2) + (+7) + (+1)) = 0.
Since there are four oxygen atoms, one iodine atom and zero hydrogen atoms in the periodate anion, the charge for the periodate anion is -1 (4*(-2) + (+7) + 0) = -1.
The H-I-O series of Polyatomic Ions is summarized in the table below.
The H-I-O Series
The next major class of Polyatomic Ions is the H-Cr-O series involving atoms of the elements hydrogen, chromium and oxygen.
The First H-Cr-O Series - Chromic Acid
In the first H-Cr-O series, one chromium atom combines with four oxygen atoms and two hydrogen atoms to form the chromic acid molecule and the resulting polyatomic ions are referred to as the chromate anion series.
The Lewis Acid Structure for chromic acid is shown in the picture above. In the green box above the Oxidation Numbers for oxygen, chromium and hydrogen are calculated as -2, +6 and +1, respectively.
As can be seen in the above picture, chromic acid is what is called a 'di-protic' acid, which means that it has two ionizable hydrogen ions and thus can form two different polyatomic anions - hydrogen chromate and chromate.
Since there are four oxygen atoms, one chromium atom and two hydrogen atoms in the chromic acid molecule, the charge on the chromic acid molecule is 0 (4*(-2) + (+6) + 2*(+1)) = 0.
Since there are four oxygen atoms, one chromium atom and one hydrogen atom in the hydrogen chromate anion, the charge for the hydrogen chromate anion is -1 (4*(-2) + (+6) + 1*(+1)) = -1.
Since there are four oxygen atoms, one chromium atom and zero hydrogen atoms in the chromate anion, the charge for the chromate anion is -2 (4*(-2) + (+6) + 0) = -2.
The Second H-Cr-O Series - Dichromic Acid
In the second H-Cr-O series, two chromium atoms combine with seven oxygen atoms and two hydrogen atoms to form the dichromic acid molecule and the resulting polyatomic ions are referred to as the dichromate anion series.
The Lewis Acid Structure for dichromic acid is shown in the picture above. In the green box above the Oxidation Numbers for oxygen, chromium and hydrogen are calculated as -2, +6 and +1, respectively.
As can be seen in the above picture, dichromic acid is what is called a 'di-protic' acid, which means that it has two ionizable hydrogen ions and thus can form two different polyatomic anions - hydrogen dichromate and dichromate.
Since there are seven oxygen atoms, two chromium atoms and two hydrogen atoms in the dichromic acid molecule, the charge for the dichromic acid molecule is 0 (7*(-2) + 2*(+6) + 2*(+1)) = 0.
Since there are seven oxygen atoms, two chromium atoms and one hydrogen atom in the hydrogen dichromate anion, the charge for the hydrogen dichromate anion is -1 (7*(-2) + 2*(+6) + 1*(+1)) = -1.
Since there are seven oxygen atoms, two chromium atoms and zero hydrogen atom in the dichromate anion, the charge for the dichromate anion is -2 (7*(-2) + 2*(+6) + 0) = -2.
The H-Cr-O series of Polyatomic Ions is summarized in the table below.
The H-Cr-O Series
The next major class of Polyatomic Ions is the H-Mn-O series involving atoms of the elements hydrogen, manganese and oxygen.
The H-Mn-O Series - Permanganic Acid
In the H-Mn-O series, one manganese atom combines with four oxygen atoms and one hydrogen atom to form the permanganic acid molecule (unstable) and the resulting polyatomic ion is referred to as the permanganate anion.
The Lewis Acid Structure for permanganic acid is shown in the picture above. In the green box above the Oxidation Numbers for oxygen, manganese and hydrogen are calculated as -2, +7 and +1, respectively.
Since there are four oxygen atoms, one manganese atom and one hydrogen atom in the permanganic acid molecule, the charge on the permanganic acid molecule is 0 (4*(-2) + (+7) + (+1)) = 0.
Since there are four oxygen atoms, one manganese atom and zero hydrogen atoms in the permanganate anion, the charge for the permanganate anion is -1 (4*(-2) + (+7) + 0) = -1.
The H-Mn-O series of Polyatomic Ions is summarized in the table below.
The H-Mn-O Series
While each and every Inorganic Polyatomic Ion can be constructed from the basic atomic structure following the above outlined procedure, it is helpful to have a set of rules and memory aids to help us remember the structure and charge of the inorganic polyatomic ions.
These rules or memory aids are given in the picture below.
Inorganic Polyatomic Rules / Memory Aids
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