History of the atom

• In 1897 scientists discovered that atoms have electrons, this showed scientists that atoms must have an internal structure
• The plum pudding model is an atom that is a ball of positive charge with negative electrons embedded into it
• In the alpha scattering experiment scientists took a piece of gold foil that was only a few atoms thick. They then fired alpha particles with a positive charge at the gold foil. Mostly the particles went straight through but sometimes they were deflected off course or totally bounced back
• Due to the alpha scattering experiment scientists learnt the atom had to have a positive nucleus
• The alpha particle scattering experiment also told scientists there must be a centre of mass in the atom
• The alpha particle scattering experiment caused the plum pudding model to be replaced by the nuclear model
• In the nuclear model most of the atom is empty space with a positive nucleus
• Niels Bohr proposed that electrons orbit the nucleus in shells.
• Bohrs proposal was accepted because it was agreed with by other scientists experiments
• James Chadwick discovered that nucleus contains neutrons after Bohr discovered shells
• Atomic theory

  • Everything is made of tiny little particles that can't be broken down any further
  • Separated from each other by empty space
  • Proped by Democritus in 500 BC
• John Dalton (1800s)

  • Described atoms as solid spheres
  • Said different spheres might make up different elements
• Plum Pudding Model (1897)

  • J.J Thompson
  • Did a series of experiments showing atoms couldn't be solid spheres
  • Must contain negatively charged particles
  • He proposed the atom was a general ball of positive charge with negative particles embedded throughout
• Gold sheet experiment 1909

  • Ernst Rutherford and his students took alpha particles (positive) and fired them at a thin sheet of gold
  • If the positive charge was generally spread out the alpha particles should pass right through as the charge wouldn't be strong enough to deflect them
  • What actually happened was some were deflected to the side and even some back the way they'd come
  • Rutherford suggested his nuclear model which suggested there was a compact nucleus and the negative charge existed in a cloud around the positive centre
• The main flaw of rutherford's model was that there was nothing holding the atom together (meaning it should automatically collapse)
• Nuclear model 1913

  • Niels Bohr suggested the electrons orbited the nucleus like planets around the sun
  • He suggested they were held in shells
  • This is important as the orbiting of electrons is what prevents the atom from collapsing
  • Many experiments support this model with a few minor changes
• • Rutherford discovered the nucleus is made of small particles not just one (protons)
• • James Chadwick discovered neutrons
• • 1913- electrons
  • 1918- protons
  • 1932- nuetrons