Number of Subatomic Particles

Specification Point 1.10:
  • Calculate the numbers of protons, neutrons and electrons in atoms given the atomic number and mass number.
  • The atomic number is equal to the number of protons in an atom.
  • Since atoms are neutral, then it is also the same as the number of electrons.
  • The mass number is the number of protons plus neutrons.
  • The number of neutrons can thus be calculated by subtracting the atomic number from the mass number.
  • Beryllium for example has an atomic mass of 4, therefore it has 4 protons and 4 electrons.
  • The mass number of beryllium is 9, so it has 9 – 4 = 5 neutrons.

Element Key Beryllium, Edexcel GCSE Chemistry

The symbol key for beryllium as represented on the Periodic Table

Isotopes & Relative Atomic Mass

Specification Point 1.11:
  • Explain how the existence of isotopes results in relative atomic masses of some elements not being whole numbers.

Relative Atomic mass

  • The symbol for the relative atomic mass is Ar and it is sometimes written as R.A.M.
  • It is calculated from the mass number and relative abundances of all the isotopes of a particular element.
  • Since isotopes have the same atomic number but a different mass number, they are all atoms of the same element but have different atomic masses.
  • The relative atomic mass is an average of these masses, hence it is not always a whole number.

Mass number & atomic number, Edexcel GCSE Chemistry

Symbol, mass number and atomic number of chlorine

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.