• Atomic symbols are written in a specific notation as shown below:

Atomic symbols show the proton number and nucleon number

• The top number A represents the?nucleon?number or the?mass?number
  • Nucleon number (A)?= total number of?protons and neutrons?in the nucleus

   

• The lower number Z represents the?proton?or?atomic?number
  • Proton number (Z)?= total number of?protons?in the nucleus

   

• Note: In Chemistry, the nucleon number is referred to as the mass number and the proton number as the atomic number. The periodic table is ordered by atomic number

Isotopes

• Although all atoms of the same element always have the same number of protons (and hence electrons), the number of neutrons can vary
• An isotope is an atom (of the same element) that has an equal number of protons but a?different number of neutrons
  • For example, hydrogen has two isotopes:?deuterium?and?tritium. Both isotopes have a proton number of 1
  • Deuterium has one neutron, so its nucleon number is 2
  • Tritium has two neutrons, so its nucleon number is 3