• All matter is composed of atoms, which are the smallest parts of an element that can take place in chemical reactions
• Atoms are mostly made up of empty space around a very small, dense nucleus that contains protons and neutrons
• The nucleus has an overall positive charge
  • The protons have a positive charge and the neutrons have a neutral charge
• Negatively charged electrons are found in orbitals in the empty space around the nucleus

The basic structure of an atom (not to scale)

• The protons, neutrons and electrons that an atom is made up of are called subatomic particles
• These subatomic particles are so small that it is not possible to measure their masses and charges using conventional units (such as grams or coulombs)
• Instead, their masses and charges are compared to each other, and so are called ‘relative atomic masses’ and ‘relative atomic charges’
• These are not actual charges and masses but charges and masses of particles relative to each other
  • Protons and neutrons have a very similar mass, so each is assigned a relative mass of 1
  • Electrons are 1836 times smaller than a proton and neutron, and so their mass is often described as being negligible
• The relative mass and charge of the subatomic particles are:

Relative mass & charge of subatomic particles table

• The atomic number (or proton number) is the number of protons in the nucleus of an atom and has the symbol Z
  • The atomic number is also equal to the number of electrons present in a neutral atom of an element
  • E.g. the atomic number of lithium is 3, meaning that a neutral lithium atom has 3 protons and therefore, also has 3 electrons
• The mass number (or nucleon number) is the total number of protons + neutrons in the nucleus of an atom, and has the  symbol A
• The number of neutrons can be calculated by:

Number of neutrons = mass number – atomic number

• • Protons and neutrons are also called nucleons, because they are found in the nucleus