Atomic Number / Atomic Mass 

In order to understand nuclear physics we need some additional vocabulary.  These words are just to classify certain sub atomic particles and how nuclear physicists count these particles.  


   A nucleon is one of the particles that makes up the nucleus. Each atomic nucleus consists of one or more nucleons, and each atom in turn consists of a cluster of nucleons surrounded by one or more electrons. There are two kinds of nucleon: the neutron and the proton. The mass number of a given atomic isotope is identical to its number of nucleons.

 Atomic Mass Number:

 Atomic mass number (A) is the total number of protons and neutrons in an atomic nucleus  The mass number is different for each different isotope of an element.

 Atomic Number: 

 The atomic number (Z) which denotes the number of protons in a nucleus, and thus uniquely identifies an element. Hence, the difference between the mass number and the atomic number gives the number of neutrons (N) in a given nucleus: N=A−Z.

using the diagrams to the left we can count our particles.  Starting with the hydrogen atom.

  • Hydrogen has 1 electron (Yellow) and 1 proton (Red).  The number of nucleons is 1 because it has one particle in the nucleus (the electron is not in the nucleus and will never be consider as an nucleons).  The atomic mass number is the number of proton and neutrons, hydrogen (at least stable hydrogen) has no neutrons and one proton so the atomic mass number is 1.  The atomic number is the number of protons hydrogen has 1 proton so the atomic number is 1 
  • Helium has 2 electron (Yellow) and 2 protons (Red) and 2 neutrons (Blue).  The number of nucleons is 4 because it has 4 particles (2 protons and 2 neutrons). The atomic mass number is 4 which is number of protons plus the number of neutrons.  The atomic mass is the number of protons and for helium is 2 
  • Lithium has 3 electrons (not important) 3 protons and 3 neutrons.  The number of nucleons and the atomic mass number is 6 and the atomic number is 3 (remember its the number of protons) 
  • Sodium has 11 electrons, 11 protons, and 11 neutrons.  The number of nucleons and atomic mass number is 22 and 11 is the atomic mass number. 

N (number of neutrons) = A (atomic mass number or the number of nucleons) - Z (atomic number)