• 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 surrounding a very small, dense nucleus that contains protons and neutrons
• The nucleus has an overall positive charge due to the presence of these protons
• Negatively charged electrons are found in shells in the empty space around the nucleus

The basic structure of an atom (not to scale)

• Subatomic particles are the particles an element is made up of and include protons, neutrons and electrons
• These subatomic particles are so small that it is not possible to measure their masses and charges using conventional units (such as grams and coulombs)
• Instead, their masses and charges are compared to each other using ‘relative atomic masses’ and ‘relative atomic charges’
• These are not actual charges and masses but they are 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 whereas electrons are 1836 times smaller than a proton and neutron
  • Protons are positively charged, electrons negatively charged and neutrons are neutral
• 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 symbol Z
  • The atomic number is equal to the number of electrons present in a neutral atom of an element
  • Eg. the atomic number of lithium is 3 which suggests that the neutral lithium atom has 3 protons and 3 electrons
• The mass number (or nucleon number) is the total number of protons and neutrons in the nucleus of an atom and has symbol A
• The number of neutrons can be calculated by:

Number of neutrons = mass number – atomic number

• • Protons and neutrons are also called nucleons