The Dimensions of the Atom

  • Atoms are extremely small with a radius of about 1 x 10-10 metres.
  • The central nucleus contains protons and neutrons only which are packed close together in a small region of space.
  • The radius of the nucleus is about 10.000 times smaller than that of the atom so it is an extremely small region of space compared to the overall size of the atom.
  • To put this into perspective, if an atom were magnified to the size of a football stadium, then:
    • The nucleus would be the size of a small pea at the centre of the pitch where kick-off takes place.
    • The electrons would be orbiting the pea at the outermost seats of the stadium.
    • In between the nucleus and the electrons is nothing but empty space.
  • This means that rather than being evenly spread out throughout the atom, virtually all of the atom’s mass is concentrated inside the nucleus.
  • Electrons have a much smaller mass than protons and neutrons (1 proton has the same mass of around 1840 electrons) and move in the space outside the nucleus in orbits.

Sub-atomic particle comparison -Mass, IGCSE & GCSE Chemistry revision notes

Atomic Number

  • The atomic number (or proton number) is the number of protons in the nucleus of an atom. The symbol for this number is Z.
  • It is also the number of electrons present in an atom and determines the position of the element on the Periodic Table.
  • The proton number is unique to each element so no two elements have the same number of protons.
  • Electrons come and go during chemical processes but the proton number doesn’t change.

Mass Number

  • The mass number (or nucleon number) is the total number of protons and neutrons in the nucleus of an atom. The symbol for this number is A.
  • The nucleon number minus the proton number gives you the number of neutrons of an atom.
  • Note that protons and neutrons can collectively be called nucleons.
  • The atomic number and mass number for every element is on the Periodic Table.

Periodic Table Symbols - AQA, IGCSE & GCSE Chemistry revision notes

Diagram showing the notation used on the Periodic Table


  • Isotopes are atoms of the same element that contain the same number of protons and electrons but a different number of neutrons.
  • The symbol for an isotope is the chemical symbol (or word) followed by a dash and then the mass number.
  • So C-14 is the isotope of carbon which contains 6 protons, 6 electrons and 14 – 6 = 8 neutrons.
  • It can also be written as 14C.
  • Isotopes display the same chemical characteristics.
  • This is because they have the same number of electrons in their outer shells and this is what determines their chemistry.
  • The difference between isotopes is the neutrons which are neutral particles within the nucleus and add mass only.

Hydrogen isotopes, IGCSE & GCSE Chemistry revision notes

The atomic structure and symbols of the three isotopes of hydrogen

Calculating PEN Numbers

  • 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.
  • The PEN numbers for beryllium are thus:
    • p = 4
    • e = 4
    • n = 9 – 4 = 5

Periodic Table Symbol for Beryllium - AQA, IGCSE & GCSE Chemistry revision notes

The symbol key for beryllium as represented on the Periodic Table

AQA GCSE Chemistry Notes

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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.