The Mass of Atoms (Oxford AQA IGCSE Combined Science Double Award)

Relative Masses

• Protons, neutrons and electrons are so small that it is not practical to measure their mass using conventional units, such as grams

• Instead, their masses are compared to each other

  • This is why they are called relative masses

• Protons and neutrons have a very similar mass

  • So, they are both assigned a relative mass of 1

• Electrons are roughly 2000 times smaller than a proton and neutron

  • So, the mass of an electron is described as very small or negligible

• The relative masses of the subatomic particles are:

Table of relative masses

Atomic Number & Mass Number

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

• The atomic / proton number is unique to each element, so no two elements have the same number of protons

Mass Number 

• The mass number is the total number of protons and neutrons in the nucleus of an atom

  • The symbol for this number is A

Representing Atoms

• Every element is shown on the periodic table

• Each element has its own symbol, mass number and atomic number and is represented as shown

Atomic Number & Mass Number diagram

• Using the information from a chemical symbol, it is possible to calculate the number of protons, neutrons and electrons