Question

What is it called when an element has 2 little numbers on top of each other on the left of the element?

Answer 1

$_{n}^{m} A \\ \\ If\ we\ have\ an\ element\ A,\ then\ n\ is\ an\ atomic\ number, \\ \\ and\ m \ is\ a\ mass\ number.$

Atomic number shows the number of protons, and charge of the nucleus.
Mass number shows sum of the number proton and neurons in the nucleus.