Oxidation number
Oxidation number, also known as oxidation state, is a concept in chemistry that describes the hypothetical charge that an atom would have if all bonds were purely ionic. It is a way to keep track of electron transfer in chemical reactions.
The oxidation number of an atom can be positive, negative, or zero. It can also be a fraction or a decimal. The oxidation number of an element in its elemental state is always zero. For example, the oxidation number of oxygen in O2 is zero.
The rules for assigning oxidation numbers are as follows:
1. The oxidation number of an atom in a pure element is zero.
2. The oxidation number of a monatomic ion is equal to its charge. For example, the oxidation number of Na+ is +1.
3. In most compounds, the oxidation number of hydrogen is +1, and the oxidation number of oxygen is -2.
4. Group 1 elements (i.e., alkali metals) have an oxidation number of +1 in all compounds.
5. Group 2 elements (i.e., alkaline earth metals) have an oxidation number of +2 in all compounds.
6. The sum of oxidation numbers in a neutral compound is zero. For example, in H2O, the oxidation number of hydrogen is +1 and the oxidation number of oxygen is -2, so the sum is zero.
7. Polyatomic ions have an oxidation number that equals the charge of the ion. For example, the oxidation number of sulfate (SO42-) is -2.
It is important to note that oxidation numbers are not necessarily real charges on atoms, but a way to keep track of electron transfer in chemical reactions.