A specific number that differs for each element, equal to the number of protons in the nucleus of each of its atoms. So in essence, the atomic number equals the number of protons that an atom has which defines what type of an atom it is. The amount of protons within an atom differs amongst each element. Hydrogen atoms only contain one proton, oxygen atoms carry eight, and carbon atoms possess six protons. The amount of protons in an element also influences its chemical behavior. You can see in the periodic table that the elements are placed in a sequence where atomic numbers are rising.