Development of the models of the atom

• When Democritus first conceived of atomic theory, around 500 BC

  , how did he describe atoms?

  Everything is made up of tiny particles, that cant be broken down, separated by empty spaces.
• How does John Dolton describe atoms as? what year?

  In the 1800's John Dalton described atoms as solid spheres, and suggested that different types of spheres make up the different elements.
• what is the plum pudding model, and when was it theorised?

  In 1897 Thomson theorised that an atom consists of a ball of positive charge, with negative electrons mixed throughout it.
• How did Rutherford developed the nuclear model?

  In Rutherford's experiments, alpha particles were fired at a thin sheet of gold foil. Most particles passed through, but some were deflected off course.

  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. As a result he developed the nuclear model of the atom, in which there was a central positive nucleus, surround by negative electrons. 
• What was one issue with Rutherford's nuclear model?

  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.
• What was Bohr response to Rutherford's Nuclear model?

  In response to this, in 1913 Bohr suggested that electrons orbit the nucleus in shells, which keep the atom from collapsing.
• What was discovered in the 20th century in the nucleus of the atom and by who?

  In the 20th century, Chadwick discovered neutral particles in the atomic nucleus, which we now call neutrons.