Elements are recognized by the inability to break them down any further by chemical or physical means into simpler components. Water, for example, is a compound made up of molecules. Each of those molecules contains 2 hydrogen and 1 oxygen atom. An electric current can break down the H2O water molecule and produced hydrogen and oxygen gas. There is no further chemical or physical way to break down hydrogen or oxygen, since they are composed of atoms.
Atoms can only be broken into simpler atoms by nuclear techniques that involve splitting atomic nuclei. Hydrogen, with the simplest possible nucleus, cannot be split into any
simple atoms but oxygen nuclei could be.
Answered by: Paul Walorski, B.A., Part-time Physics Instructor
'One thing I have learned in a long life: that all our science, measured against reality, is primitive and childlike - and yet it is the most precious thing we have.'