In order to answer your question, we need to turn it
around and look at the chemistry of living things, and then say that things that don't
match that definition are non-living.
Living things, on
Earth at least, are based on just a few elements, carbon being the most important.
Because each carbon atom can form four covalent (sharing) bonds, carbon is able to
create complex molecules. Theses molecules always have some hydrogen atoms in them as
well, and usually there's oxygen in them too. Those are the "big three", the commonest
life molecules. Then there are nitrogen, calcium, phosphorus, chlorine, potassium,
sulfur, sodium, and magnesium; this list is in descending order of amount found in the
human body.
Non-living things can be composed of any
combination of elements, but will always lack the carbon-hydrogen-oxygen main
structure.
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