Atentative scientific explanation (an educated quess) which may or may not be rejected upon further experimentation is called a

The correct answer is hypothesis.
A hypothesis is an idea about something - a scientist comes up with an idea that he or she wants to prove. At this point, it is only a hypothesis, at least until the moment the scientist manages to prove it through experiments, when it can become a theory if it is accepted.

The correct answer is "atomic weight".