history of the atom ✩

• The first major theory that had tried to explain what stuff is made out of was atomic theory
• Atomic theory

  The idea that everything is made up from tiny little particles that can't be broken down any further and that they're separated from each other by empty space
• Democritus proposed atomic theory

  Around 500 BC
• When atomic theory was improved upon

  1800s
• John Dalton's atomic model

  Atoms as solid spheres, different types of spheres make up different elements
• J.J. Thompson proposed the plum pudding model

  1897
• Plum pudding model

  • Atoms contain negatively charged particles (electrons)
  • Atoms have a general ball of positive charge with discrete electrons stuck in it
• Rutherford and students made a big discovery

  1909
• Rutherford's experiment

  1. Fired positively charged alpha particles at a thin sheet of gold

  2. Some alpha particles were deflected to the side or back, proving the plum pudding model wrong
• Rutherford's nuclear model

  Atoms have a compact nucleus containing all the positive charge, with a cloud of negative charge around it
• Rutherford's nuclear model had a flaw - it didn't explain why the atom doesn't collapse
• Niels Bohr suggested a solution to the flaw in Rutherford's model

  1913
• Bohr's model

  Electrons orbit the nucleus in shells, preventing the atom from collapsing
• Further experiments by Rutherford found the positive charge in the nucleus is made up of protons
• James Chadwick provided evidence for neutral particles (neutrons) in the nucleus
• The current understanding of the atom is based on the Bohr model with a few small changes
• In the 1800's, John Dalton described atoms as solid spheres, and suggested that different types of spheres make up the different elements. 
• n 1897 J. J. Thomson theorised that an atom consisted of a ball of positive charge, with negative electrons mixed throughout it. What do we call this model? 

  plum pudding model
• how Rutherford developed the nuclear model

  1. In Rutherford's experiments, alpha particles were fired at a thin sheet of gold foil.
  2. Most particles passed through, but some were deflected off course. 
  3. This caused him to hypothesise that there was a dense region of positive charge at the centre of the atom that repelled the alpha particles.
  4. As a result he developed the nuclear model of the atom, in which there was a central positive nucleus, surrounded by negative electrons. 
• One issue with Rutherford's nuclear model was that the atom should collapse as the negative electrons would be attracted to the positive nucleus, causing them to rush inwards.

  In response to this, in 1913 Bohr suggested that electrons orbit the nucleus in shells, which prevent the atom from collapsing.
• In the 20th century, Chadwick discovered neutral particles in the atomic nucleus. What are these particles called?

  neutrons
• • In 1803, John Dalton presented his atomic theory based on three key ideas:
  • Matter is made of atoms which are tiny particles that cannot be created, destroyed, or divided
  • Atoms of the same element are identical, and atoms of different elements are different
  • Different atoms combine together to form new substances
• the evolution of models of atomic structure

  
• • In 1897, physicist J.J. Thomson discovered the electron
  • Using a cathode-ray tube, he conducted an experiment which identified the electron as a negatively charged subatomic particle, hence proving that atoms are divisible

  
• • Based on his investigations, Thomson proposed a model of the atom known as the plum pudding model which depicted negative electrons spread throughout soft globules of positively charged material