I can't imagine scientists think "as far as we know, we haven't managed to decompose that stuff yet and in so far as we don't even know at all what it is, let's assert it's an element until detractors crop up" that sounds wrong.
It is possible to prove that a theory is wrong by doing an experiment that directly contradicts it.
However, it is impossible to prove experimentally that a theory is right. You may do thousands of measures that prove it, yet you can't be absolutely sure the next experiment will confirm it...
To make it simple: scientist propose ideas, theoretical models... and check them in the lab. If all the experiments are conclusive, the theory is validated beyond reasonable doubts
. For example, you have dead theories that were accepted for a long time, like the phlogiston http://en.wikipedia.org/wiki/Phlogiston
but have been proved wrong.
An example of incomplete theory: Newton theory of gravitation has been working perfectly to describe motions from apples to planets... It was confirmed by countless measurements. And yet, it was incomplete, as shown by Einstein relativity. Similarly, relativity and quantum mechanics have been confirmed experimentally many many times. Yet, those two theory contradict each other when you try to use them together --> both are obviously incomplete as well. But the unifying theory has still to be worked out... So, we have to make do with those theory before getting a better one.
At best, a theory is only valid beyond reasonable doubts, not more:
- a good experiment is the one that invalidates your model, forcing you to move forward with a better one.
- a good scientist is someone who tries hard to break down his own theory and model! Not someone who tries to prove them.
The mindset of trying to break ones own work to see how robust it is, is not natural. As human, we would rather try to protect it. I see it as the main reason why religions (where you try to protect the dogma, no matter what) have been around for thousands of years while science is a new comer a few hundred years old.