AZX Notation & Isotopes (OCR A Level Physics): Revision Note

AZX Notation & Isotopes

AZX Notation

• A nuclide is a type of nucleus with a particular number of protons and a particular number of neutrons

  • For example, 5 atoms of oxygen-18 are all the same nuclide but are 5 separate atoms

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

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

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 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

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

• Some isotopes are unstable because they have too many or too few neutrons for their number of protons

  • These isotopes decay and emit radiation to achieve a more stable form

  • This can happen over anything from a tiny fraction of a second to billions of years

Differences Between Isotopes

• The number of neutrons in an atom does not affect its charge or its chemical properties, 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 relative atomic mass of chlorine is given as 35.5

This section of a periodic table shows chlorine with a relative atomic mass of 35.5

• The relative atomic mass of chlorine is 35.5 because it has two 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 relative atomic mass of chlorine is closer to 35 than 37 because it 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