• 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)

• 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 indicates 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