The sum of the number of protons and neutrons is the mass number. Different isotopes have different mass numbers. The Periodic Table of the Elements There are 93 different stable atoms that occur in nature. We have managed to make new elements with atomic numbers greater than 93 in nuclear reactions in the laboratory. Of these 93 elements, only a few are very abundant in the universe.
Most elements are quite rare.
At first, there were only hydrogen atoms. Nearly all of the hydrogen that exists today formed at the Big Bang and is as old as the universe itself. As the universe cooled, stars began to form owing to the gravitational collapse of hydrogen clouds. This is an image of the Orion Nebula as taken by the Hubble space telescope. Here, young stars are being formed during the gravitational collapse of intersteller dust clouds.