Nucleon & Proton Number (Cambridge (CIE) A Level Physics)

Nucleon & proton number

• The nuclide notation 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 or nucleon number

  • 3 = atomic number or proton number

• The mass number is the number of particles in the nucleus

  • The nucleus contains protons plus neutrons

• The atomic number is the number of protons in the atom

  • Since atoms are neutral, this is also equal to the number of electrons in the atom

• When given an atomic symbol, you can therefore 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 nucleon 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

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 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 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

AZX notation

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

• Where:

  • X = chemical symbol of the element

  • A = nucleon (mass) number

  • Z = proton (atomic) number

• The 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