The History of The Atom

• When Democritus first conceived of atomic theory, around 500 BC, how did he describe atoms?


  Separated from each other by empty space

  The smallest possible unit of matter 

  Small spheres
• In the 1800's, John Dalton described atoms as solid spheres, and suggested that different types of spheres make up the different elements
• In 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 deflectedoff 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, surround 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 prevents the atom from collapsing. 
• In the 20th century, Chadwick discovered neutral particles in the atomic nucleus. What are these particles called?


  Neutrons
• Summarise John Dalton’s theory


  John Dalton described atoms as solid spheres, and said that different-sized spheres made up the different elements.
• Summarise J J Thompson's findings and resulting theory

  J J Thomson found that atoms contain even smaller, negatively charged particles - electrons.

  He proposed a new theory in which the atom was a ball of positive charge, with discrete electrons throughout it. It was known as the 'plum pudding model’.
• What was the 'plum pudding model’, and who proposed it?

  The 'plum pudding model' was a theory proposed by J J Thompson in which the atom is a ball of positive charge, with discrete electrons throughout it. Like a 'pudding' with bits of ‘plum’ spread throughout it. 
• Summarise the experiment carried out by Ernest Rutherford, and its results.


  Experiment: Ernest Rutherford and his student conducted the alpha particle scattering experimentswhere they fired positively charged alpha particles at an extremely thin sheet of gold.

  Results: Rather than all of the alpha particles passing straight through the sheet of gold as you would expect from the plum pudding model, some were deflected to the side, and a small number were even deflected backwards.
• What is the nuclear model of the atom, and who proposed it? 

  Proposed by Ernest Rutherford.

  The nuclear model of the atom is the idea that each atom is composed of a small positively charged nucleus at the centre, surrounding by a ‘cloud’ of negative electrons
• What was the main issue with the nuclear model of the atom?

  The electrons should be attracted to the positive nucleus, so the atom should just collapse
• What did Niels Bohr propose?

  Niels Bohr suggested that electrons orbit the nucleus in shells, which are a fixed distance from the nucleus.

  This was in contrast to Ernest Rutherford's theory in which the electrons were spread in a ‘general cloud’ around the nucleus.
• What did James Chadwick discover?

  James Chadwick discovered neutral particles called neutrons.