An element is the smallest part of a substance that is capable of independent existence. An element cannot be broken down into any other substance. Krypton, Bismuth etc are all elements.
A compound is a combination of two or more elements which are chemically combined together e.g NH3, CO2 etc.
A mixture is any combination of substances that are not chemically combined together. E.g Pizza, milk etc
Magnesium oxalate<span> is an inorganic compund </span><span> comprising a magnesium </span> cation<span> with a 2+ charge bonded to an oxalate </span> anion.