How do you determine oxidation numbers in a compound?

Oxidation numbers in a compound are determined by applying a set of rules based on the periodic table and chemical bonding.

To determine the oxidation numbers in a compound, you need to understand the concept of oxidation states. An oxidation state, or oxidation number, is a theoretical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. It's a way of keeping track of electrons in a compound, particularly when it comes to redox reactions.

The first rule is that the oxidation number of an atom in its elemental state is always zero. This applies to elements in their pure form, such as hydrogen gas (H2), oxygen gas (O2), or a single atom of carbon (C).

The second rule is that the oxidation number of a monoatomic ion is equal to its charge. For example, the oxidation number of Na+ is +1, and the oxidation number of Cl- is -1.

The third rule is that the sum of all oxidation numbers in a neutral compound is zero. In an ion, the sum of the oxidation numbers is equal to the charge of the ion. For example, in water (H2O), the oxidation number of oxygen is -2, and since the molecule is neutral, the sum of the oxidation numbers of the two hydrogen atoms must be +2, so each hydrogen atom has an oxidation number of +1.

The fourth rule is that some elements have specific oxidation numbers based on their group in the periodic table. For example, alkali metals in Group 1 always have an oxidation number of +1, and alkaline earth metals in Group 2 always have an oxidation number of +2.

Lastly, when a compound contains more than one atom of a certain element, the oxidation number found is multiplied by the number of atoms of that element present in the compound. For example, in carbon dioxide (CO2), the oxidation number of oxygen is -2, and since there are two oxygen atoms, the total oxidation number for oxygen is -4. Therefore, the oxidation number of carbon must be +4 to balance it out.

Remember, these rules are applied in order until a solution is found. It's a systematic way to determine the oxidation numbers in a compound.