Atomic number

Every atom of an element has the same number of protons, and no two elements share that number: it is the element's atomic number, written low on the left of its symbol, and the periodic table lists the elements in its order. In a neutral atom the electrons equal the protons, so the atomic number gives both. Changing the number of protons changes the element, which no chemical reaction does: reactions only move electrons, which is why alchemists never turned lead into gold.