What is an underneath symbol in chemistry?

Answer 1

Do you mean the subscript on an element in a chemical formula, for example H₂O?

The subscript is the number of atoms of that element in the chemical formula. For the example above H₂O, the 2 means 2 atoms of hydrogen H in the formula. For the oxygen O, no subscript means 1 atom.

In the molecule C₃H₈ (propane), there are 3 atoms of C (carbon) and 8 atoms of H (hydrogen).

Let me know if you were talking about something else.