Atomic Models

• Isotopes have the same number of protons but a different number of neutrons
• in the 19th century john dalton first described atoms as solid spheres
• J.J Thompson discovered the electron and proposed the plum pudding model
• in 1909 Ernest Rutherford conducted the gold foil experiment firing positively charged alpha particles at a very thin sheet of gold. from the plum pudding model they expected most of the particles to be deflected however most of them passed through and only a small number were deflected. from these observations, he came up with the nuclear model
• Neil Bohr proposed a new model where

  • electrons are in fixed orbitals
  • each orbital has a fixed energy
  • when an electron moves between a shell electromagnetic radiation is emitted or absorbed
  • because the energy of the orbitals is fixed, the frequency of the radiation is also fixed
• the most accurate model of the atom is the quantum model
• Relative atomic mass is the average mass of an atom of an element relative to the mass of 1/12 of a carbon 12 atom
• Relative isotopic mass is the average mass of an atom of an isotope relative to the mass of 1/12 of a carbon 12 atom
• relative molecular mass is the average mass of a molecule relative to the mass of 1/12 of a carbon 12 atom
• relative formula mass is the average mass of a formula unit on a scale where an atom of carbon 12 is exactly 12