What does "atomic number 91" mean?

a short-lived radioactive metallic element formed from uranium and disintegrating into actinium and then into lead

What is a synonym for "atomic number 91"?

Common synonyms include: pa, protactinium, protoactinium.

Rare ·Advanced level

Noun

1
a short-lived radioactive metallic element formed from uranium and disintegrating into actinium and then into lead wordnet