Question

What distinguishes one element from another

Answer 1

The number of protons,neutrons,electrons, the way the are classified (earth metal, noble gas,meteoroids) and also the way they react.

Answer 2

The number of protons in the atom of an element (is that confusing) corresponds with the atomic number of the element, say hydrogen (#1) would only have one proton.

(Well that's the simplest way)