@trinsec @mc
The word "lead" has a lot of meaning, it can be a noun or a verb. By saying "is", that implies that the first "lead" is most likely a a noun, then the second would be a noun or a adjective. But the only adjective forms of lead are “leadless” or “leaden”, so that second “lead” in the sentence can’t be an adjective.
>"Why not 'to lead' is 'Pb' (heavy)?"
"To lead" is a different form than just "lead". If you wanted to say that, it would be "To lead is leaden", or "Leading is leaden", or "Leadership is leaden".