Nucleon & Proton Number (CIE A Level Physics)

• The atomic symbol of an element is used to describe the constituents of the nuclei
• An example of this notation for Lithium is:

• Where:
  • Li = lithium
  • 7 = mass number
  • 3 = atomic number

• When given an atomic symbol, you can figure out the number of protons, neutrons and electrons in the atom:
  • Protons: The atomic number
  • Electrons: Atoms are neutrals, so the number of negative electrons is equal to the number of positive protons. Therefore, this is also the atomic number
  • Neutrons: Subtract the proton number from the mass number

• For the lithium atom, these numbers would be:
  • Protons: 3
  • Electrons: 3
  • Neutrons: 7 − 3 = 4

• The term nucleon is the used to mean a particle in the nucleus – i.e. a proton or neutron
• The term nuclide is used to refer to a nucleus with a specific combination of protons and neutrons

• 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 different number of neutrons
• The isotopes of hydrogen are deuterium and tritium:

Isotopes of Hydrogen

The three atoms shown above are all forms of hydrogen, but they each have different numbers of neutrons

• Remember, the neutron number of an atom is found by subtracting the proton number from the nucleon number
• Since nucleon number includes the number of neutrons, an isotope of an element will also have a different nucleon/mass number
• Since isotopes have an imbalance of neutrons and protons, they are unstable. This means they constantly decay and emit radiation to achieve a more stable form
• This can happen from anywhere between a few nanoseconds to 100,000 years

• Atomic symbols are written in a specific notation called AZX notation

${}_{\mathrm{Z}}{}^{\mathrm{A}}\mathrm{X}$

• Where:
  • X = chemical symbol of the element
  • A = nucleon (mass) number
  • Z = proton (atomic) 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