You need to know the rules for assigning oxidation numbers

Oxidation numbers, also known as oxidation states, are assigned to each atom or ion in a compound to indicate the degree of oxidation. These numbers represent the total number of electrons transferred to or from an atom within the compound.

Here are some rules for assigning oxidation numbers:

1. Uncombined elements (e.g. Ne) have an oxidation number of 0.
2. Atoms in molecules bonded to identical atoms (e.g. O₂) also have an oxidation number of 0.
3. A monatomic ion's oxidation number is equal to its charge. E.g., Fe^3+^ has an oxidation number of +3.
4. In a molecular ion, the sum of the oxidation numbers equals the overall charge. E.g., in SO_4_^2-^, S is +6 and O is -2, summing to -2.
5. In a neutral compound, the sum of the oxidation numbers is 0. E.g., in Al_2_O_3_, Al is +3 and O is -2, summing to 0.
6. Combined oxygen usually has an oxidation number of -2, except in peroxides, where it is -1, and in OF_2_ where it is +2.
7. Combined hydrogen usually has an oxidation number of +1, except in metal hydrides, where it is -1.

In a compound or ion containing 2 non-metals, the negative oxidation number is given to the more electronegative element. Fluorine always has an oxidation number of -1 because it is the most electronegative element.

Worked example 1 - Determine the oxidation number of carbon in H_2_CO_3_.

Determine the oxidation number of carbon in carbonic acid, H_2_CO_3_.

Step 1: Assign known oxidation numbers

• Hydrogen (H) has an oxidation number of +1.
• Oxygen (O) usually has an oxidation number of -2.

Step 2: Calculate total oxidation number of O

There are 3 oxygen atoms, each with an oxidation number of -2.

Total oxidation number from oxygen = 3 x (-2) = -6

Step 3: Calculate total oxidation number of H

There are 2 hydrogen atoms, each with an oxidation number of +1.

Total oxidation number from hydrogen = 2 x (+1) = +2

Step 4: Determine oxidation number of carbon (C)

The total oxidation number of a neutral compound is 0. Let the oxidation number of carbon be x.

Total oxidation number of H + Oxidation number of C + Total oxidation number of O = 0

2 + x + (-6) = 0

x - 4 = 0

x = 4

Therefore, the oxidation number of carbon in H_2_CO_3_ is +4.

Worked example 2 - Determine the oxidation number of chromium in K_2_Cr_2_O_7_.

Determine the oxidation number of chromium in potassium dichromate, K_2_Cr_2_O_7_.

Step 1: Assign known oxidation numbers

• Potassium (K) is in group 1, so it has an oxidation number of +1.
• Oxygen (O) usually has an oxidation number of -2.

Step 2: Calculate total oxidation number of K

There are 2 potassium atoms, each with an oxidation number of +1.

Total oxidation number from potassium = 2 x (+1) = +2

Step 3: Calculate total oxidation number of O

There are 7 oxygen atoms, each with an oxidation number of -2.

Total oxidation number from oxygen = 7 x (-2) = -14

Step 4: Determine oxidation number of chromium (Cr)

The total oxidation number of a neutral compound is 0. Let the oxidation number of chromium be x.

Total oxidation number of K + Oxidation number of Cr + Total oxidation number of O = 0

2 + 2x + (-14) = 0.

Note: There are 2 chromium atoms, so the oxidation number is multiplied by 2.

2x - 12 = 0

x = $\frac{12}{2}$ = 6

Therefore, the oxidation number of chromium in K_2_Cr_2_O_7_ is +6.

Roman numerals show variable oxidation numbers

When elements can exhibit multiple oxidation numbers, Roman numerals are used after the element's name to specify its oxidation number.

For example, iron(II) signifies iron with an oxidation number of +2, while iron(III) indicates an oxidation number of +3.

-'ate' compounds contain oxygen plus another element

1. Ions ending in '-ate' are made up of oxygen and another element. For example, sulfates include sulfur, while nitrates include nitrogen.
2. If the element bonded to oxygen has variable oxidation numbers, Roman numerals are used after the compound's name to differentiate between these numbers.

Worked example 3 - Determining the formula of copper(II) nitrate

Determine the chemical formula of copper(II) nitrate.

Step 1: Identify oxidation numbers from the name

• Copper(II) indicates copper has an oxidation number of +2.
• The nitrate ion (NO_3_^-^) has a charge of -1.

Step 2: Set up ratio to balance charges

The overall charge of the compound must equal 0. Since copper has an oxidation number of +2 (Cu^2+^) and each nitrate ion has a charge of -1 (NO_3_^-^), we need two nitrate ions to balance one copper ion.

Step 3: Determine the formula

For the charges to balance to zero, one copper ion needs to pair with two nitrate ions.

Thus, the chemical formula for copper(II) nitrate is Cu(NO_3_)2.

Worked example 4 - Determining the name for the ClO_2_^-^ ion

Determine the systematic name of the ClO_2_^−^ ion.

Step 1: Assign known oxidation numbers

Oxygen (O) has an oxidation number of -2.

Step 2: Calculate total oxidation number of O

There are 2 oxygen atoms, each with an oxidation number of -2.

Total oxidation number from oxygen = 2 x (-2) = -4.

Step 3: Calculate the oxidation number of Cl

Let the oxidation number of chlorine be x. The sum of the oxidation numbers in the ion equals the overall charge of the ion = -1. Therefore,

x + (-4) = -1

x = -1 + 4

x = 3

Step 4: Determine the systematic name

Since the oxidation number of chlorine in ClO₂^- is +3, its systematic name is chlorate(III).