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
'The strength and weakness of physicists is that we believe in what we can measure. And if we can't measure it, then we say it probably doesn't exist. And that closes us off to an enormous amount of phenomena that we may not be able to measure because they only happened once. For example, the Big Bang. ... That's one reason why they scoffed at higher dimensions for so many years. Now we realize that there's no alternative... '