Atomic Number & Mass Number

Atomic Number

• The number of protons in an atom is called its atomic number (it can also be called the proton number)
• Elements in the periodic table are ordered by their atomic number
• Therefore, the number of protons determines which element an atom is
• The atomic number of a particular element is always the same
• For example:
  • Hydrogen has an atomic number of 1. It always has just one proton
  • Sodium has an atomic number of 11. It has 11 protons
  • Uranium has an atomic number of 92. It has 92 protons
• The atomic number is also equal to the number of electrons in an atom
  • This is because atoms have the same number of electrons and protons in order to have no overall charge

Mass Number

• The total number of particles in the nucleus of an atom is called its mass number
• The mass number is the number of protons and neutrons in the atom
• The number of neutrons can be found by subtracting the atomic number from the mass number
  Number of Neutron = Mass Number - Atomic Number
• For example, if a sodium atom has a mass number of 23 and an atomic number of 11, then the number of neutrons would be 23 - 11 = 12

Tip

You may have noticed that the number of electrons is not part of the mass number. This is because electrons have a tiny mass compared to neutrons and protons. We say their mass is negligible when compared to the particles in the nucleus.

Nuclear Notation

• The mass number and atomic number of an atom are shown by writing them with the atomic symbol
  • This is called nuclear notation
• Here are three examples:
  
  Examples of nuclear notation for atoms of Hydrogen, Sodium and Uranium
• The top number is the mass number
  • This is equal to the total number of particles (protons and neutrons) in the nucleus
• The lower number is the atomic number
  • This is equal to the total number of protons in the nucleus
• The atomic and mass number of each type of atom in the examples above is shown in this table: