AQA A Level Chemistry Revision Notes

  Atomic Structure

  • 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

Subatomic Particles

  • Subatomic particles are the particles an element is made up of and include protonsneutrons 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

Mass Number & Isotopes

  • The mass of an atom is concentrated in the nucleus, because the nucleus contains the heaviest subatomic particles (the neutrons and protons)
    • The mass of the electron is negligible

  • The nucleus is also positively charged due to the protons
  • Electrons orbit the nucleus of the atom, contributing very little to its overall mass, but creating a ‘cloud’ of negative charge
  • The electrostatic attraction between the positive nucleus and negatively charged electrons orbiting around it is what holds an atom together
  • The number of protons equals the atomic (proton) number
  • The number of protons of an unknown element can be calculated by using its mass number and number of neutrons:
  • Mass number = number of protons + number of neutrons

    Number of protons = mass number - number of neutrons


  • An atom is neutral and therefore has the same number of protons and electrons
  • Ions have a different number of electrons to their atomic number depending on their charge
    • A positively charged ion has lost electrons and therefore has fewer electrons than protons
    • A negatively charged ion has gained electrons and therefore has more electrons than protons


  • 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
    • E.g. carbon-12 and carbon-14 are isotopes of carbon containing 6 and 8 neutrons respectively   
    • Isotopes have the same chemical properties but different physical properties

    Chemical properties

    • Isotopes of the same element display the same chemical characteristics
    • This is because they have the same number of electrons in their outer shells
    • Electrons take part in chemical reactions and therefore determine the chemistry of an atom

    Physical properties

    • The only difference between isotopes is the number of neutrons
    • Since these are neutral subatomic particles, they only add mass to the atom
    • As a result of this, isotopes have different physical properties such as small differences in their mass and density

    • Isotopes are different atoms of the same element that contain the same number of protons and electrons but a different number of neutrons.
      • These are atoms of the same elements but with different mass numbers

    • Because of this, the mass of an element is given as relative atomic mass (Arby using the average mass of the isotopes
    • The relative atomic mass of an element can be calculated by using the relative abundance values
      • The relative abundance of an isotope is either given or can be read off the mass spectrum
      • Average atomic mass=

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