This is the number of protons — positively charged particles — in a single atom. The atomic number identifies an atom as a specific element. Any atom of gold will have 79 protons, for instance. An atom with one more proton is no longer gold. It’s mercury. One less, and it’s platinum. Atomic number also determines the element’s location on the periodic table. Number one is hydrogen.
In a sentence
Scientists made four new elements by slamming smaller atoms together until their protons stuck, creating elements with the atomic numbers 113, 115, 117 and 118.