Sulfuric Acid `H_2SO_4` is a compound, made up of elements. These elements can by separated, making it possible to decompose sulfuric acid.
Ammonia `NH_3` is a compound, it can be broken down by decomposition.
Water `H_2O` is a highly stable compound, but it is a compound and can be broken down, just add a little baking soda and a 9v battery and you'll soon have Hydrogen and Oxygen.
Argon Ar, is an element, one of the noble gases. Because it is an element it can't be broken down chemically. Breaking the atoms down into subatomic particles requires nuclear reactions.
Argon cannot be decomposed by chemical change.