The atomic number of an element is what distinguishes it from all other elements. An atom's atomic number is the number of protons there are in the nucleus. Hydrogen's atomic number is 1. Helium's atomic number is 2. Any atom that has an atomic number of 1 is a hydrogen atom no matter how many electrons or neutrons the atom has.
The mass number is the number of neutrons added to the number of protons. The mass number of the most common isotope can be obtained from the periodic table. If you take the decimal number on the periodic table and round it to the nearest whole number, you have the mass number. For example the atomic weight of Iron(Fe) is 55.847. When rounded it gives a mass number of 56.
The atomic number of Fe is 26. so most Fe
atoms have 30 (56-26) neutrons. In addition, all neutral Fe atoms have 26
protons and 26 electrons. Atoms of the same element with a different
number of neutrons are called isotopes. The most common isotope of an
element is the one that is on the periodic table.
The above graphic shows two isotopes of Hydrogen. The picture on the left is the most common isotope of hydrogen with one electron and one proton. The picture on the right is another isotope of hydrogen with one proton, one electron, and a neutron. The most common isotope of uranium is uranium-238 which has 92 protons, 92 electrons, and 146 neutrons. Another isotope is uranium-235 with 92 protons, 92 electrons, and 143 neutrons.
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