Which element has the same oxidation number in all of its compounds?

Sodium, potassium, calcium, magnesium etc

Explanation:

To determine the species which is being oxidized or reduced in a reaction, we make use of oxidation number. While oxidation involves electron loss, reduction involves electron gain. The specie which gain electrons is being reduced and thus experience a decrease in oxidation number and thus said to be the oxidizing agent. The species which lose electrons experience and increase in oxidation number and thus is the reducing agent.

There are some elements however which never get oxidized or reduced in the course of a chemical reaction. What we mean by this is that they neither get oxidized nor reduced but maintain the same original oxidation number.

Examples of these kind of elements include the groups 1 and 2 metals.