AZX Notation & Isotopes (OCR A Level Physics)

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Katie M


Katie M



AZX Notation & Isotopes

AZX Notation

  • A nuclide is a group of atoms containing the same number of protons and neutrons
    • For example, 5 atoms of oxygen are all the same nuclide but are 5 separate atoms

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

AZX Notation, downloadable AS & A Level Physics revision notes

Atomic symbols in AZX Notation describe the constituents of nuclei

  • 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


  • 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 defined as:

An atom (of the same element) that has an equal number of protons but a different number of neutrons

  • Hydrogen has two isotopes: deuterium and tritium

Isotopes of Hydrogen, downloadable AS & A Level Physics revision notes

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

  • 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 decay and emit radiation to achieve a more stable form
    • This can happen from anywhere between a few nanoseconds to 100,000 years

Differences Between Isotopes

  • The number of neutrons in an atom does not affect the chemical properties of an atom, such as its charge, but only its mass
    • This is because neutrons have no charge but do have mass

  • The charge of the nucleus of a particular element is always the same
  • In the periodic table, the mass number of Chlorine is often given as 35.5

Chlorine isotope, downloadable IGCSE & GCSE Physics revision notes

This section of a periodic table shows Chlorine as having a mass number of 35.5, but other elements have an integer mass number

  • The mass number of chlorine is given as 35.5 because it has 2 isotopes, one with a mass number of 35 and the other with a mass number of 37
  • Chlorine-35 is about three times more abundant than chlorine-37, so the given mass number of chlorine is closer to 35 than 37 because the mass number is a weighted average, therefore it takes into account the proportion of each isotope present
  • The number of electrons and protons in different isotopes remains the same
  • Some isotopes are unstable as they have an imbalance of protons and neutrons

Worked example

One of the rows in the table shows a pair of nuclei that are isotopes of one another.WE - Isotopes question image, downloadable AS & A Level Physics revision notesWhich row is correct?


Step 1: State the properties of isotopes

    Isotopes are nuclei with the same number of protons but different number of neutrons

    The nucleon number is the sum of the protons and neutron

    Therefore, an isotope has a different nucleon number too

Step 2: Calculate number of protons in the first nucleus

Nucleon number: 37

Neutrons: 20

Protons = 37 − 20 = 17

Step 3: Calculate number of protons in the second nucleus

Nucleon number: 35

Neutrons: 18

Protons = 35 − 18 = 17

Step 4: Conclusion

Therefore, they have the same number of protons but different numbers of neutrons and are isotopes of each other

The correct answer is therefore option B

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Katie M

Author: Katie M

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.