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1. How atoms have changed?

   • All matter consists of atoms that can't be divided
   • They all have same element but different atoms
   • Rutherford discovered the nucleas in 1911.

2. Properties of isotopes

   • Not all atoms of an element have same number of neutrons. Atoms of same element but different masses are isotopes.
   • Each isotope element is fixed, same in both pure and compound elements
   • Isotopes having the same number means they all react the same.

3. Dating with isotopes

   • Some elements are radioactive which decay over time

4. Relative formula mass

   • to work out RFM of a compound you find atomic mass of each element multiplied by number of atoms then add up.

5. Electronic structure

   • Electrons occupy lowest energy levels. For elements beyond calcium the number of outer electrons is the same as group number
     • Noble gas structure are in group 0, they are unreactive because they have full outer shell of electrons. When atoms combine to make compounds their outer electrons are shared, gained or lost