Final answer:
The oxidation number of a monoatomic ion equals its charge, with positive or negative signs indicating the nature of the charge. For example, in MgCl2, magnesium has an oxidation number of +2 and chlorine has an oxidation number of -1. These rules help determine the oxidation states in monoatomic and polyatomic species.
Step-by-step explanation:
For monoatomic ions, the oxidation number is determined by the charge of the ion. This means that if an ion has a positive charge, like the magnesium ion in MgCl2 with a +2 charge, its oxidation number is +2. Conversely, for a negative charge such as the chlorine ion in MgCl2 which has a charge of -1, the oxidation number of chlorine is -1. Oxidation numbers should be written with the sign first, followed by the magnitude, to differentiate from charges.
Alkali metals have an oxidation number of +1, while alkaline earth metals typically have an oxidation number of +2. For nonmetals, the common oxidation numbers assigned are as follows:
- Hydrogen: +1 when combined with nonmetals, -1 when combined with metals
- Oxygen: -2 in most compounds, -1 in peroxides (O22-), and varying positive values when combined with fluorine
- Halogens (e.g., fluorine): -1 in most cases, except when combined with oxygen or halogens where positive oxidation numbers may be assigned
It is also important to note that in compounds, the sum of oxidation numbers should equal the charge on the species; for neutral compounds, that sum should be zero.