AQA GCSE Chemistry

Revision Notes

1.1.8 Size & Mass of Atoms

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
  • 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 Mass Comparison Table

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

Exam Tip

Most of the atom is actually empty space, with the mass being concentrated in the nucleus and the electrons orbiting in shells around it.

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 can be lost, gained, or shared during chemical processes but the proton number of an atom does not change in a chemical reaction

Exam Tip

Both the atomic number and the mass number are given on the periodic table, but it can be easy to confuse them. Think MASS = MASSIVE, as the mass number is always the bigger of the two numbers, the other smaller one is thus the atomic / proton number

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 and 6 electrons, but the 14 signifies that it has 8 neutrons (14 – 6 = 8)
    • 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

Exam Tip

For atoms to be isotopes of each other, they must both be from the same element, hence they must have the same atomic number. E.g., C-13 and C-14 are isotopes whereas C-13 and H-2 are not

Calculating PEN Numbers

  • The atomic number is equal to the number of protons (p) in an atom
  • Since atoms are neutral, then it is also the same as the number of electrons (e)
  • The mass number is the number of protons plus neutrons
  • The number of neutrons (n) can thus be calculated by subtracting the atomic number from the mass number
  • For example, beryllium has an atomic number 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, downloadable IGCSE & GCSE Chemistry revision notes

The symbol key for beryllium as represented on the periodic table

Exam Tip

The PEN numbers refer to the numbers of protons, electrons, and neutrons in an atom. It is a good study habit to write down the PEN numbers first before answering calculation questions on Atomic Structure

Size & Scale of Atoms

  • To put the size of an atom 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
  • Another way to think about the size of an atom is to consider that the size of a full stop on a piece of paper is usually 0.003 m = 3 x 10-3 The width of a hydrogen atom is about 1.06 x 10-10
  • This means that you could fit almost 28,301,887 hydrogen atoms across the diameter of a full stop!

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.

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