Atomic number
For example, oxygen has 8 protons. The atomic number is the number of protons an element has.  So, if an unidentified element has 8 protons, then it is oxygen. This is because each element has a certain number of protons.  If an element had 9 protons, it would not be oxygen, but an element can have 8 protons, and 9 neutrons and still be considered oxygen; we call this an isotope.