This is the number of protons in the nucleus of each atom. (tes.com)
Atoms of each new element are packed with protons in their nuclei, giving the four atomic numbers of 113, 115, 117 and 118. (scientificamerican.com)
In this experiment, a 5.5 - GeV beam of electrons was directed onto a target of liquid hydrogen, which has a single proton in its nucleus. (sciencedaily.com)