History of Atomic Theory

Question 1

John Dalton

Answer 1

• proposed the first atomic theory
• he believed that:
• elements are made of extremely small particles called atoms
• atoms of a given element are identical in size, mass and other properties; atoms of different elements differ in size, mass and other properties. → isotopes are the same element, but they vary in the number of neutrons
• atoms cannot be subdivided, created or destroyed → smaller subatomic particles
• Atoms of different elements combine in simple whole-number ratios to form chemical compounds
• in reactions, atoms are combined separated or rearranged.

Question 2

J.J Thomson (Joseph John)

Answer 2

Cathode ray tube:
- he used a cathode ray tube - a sealed glass tube with metal terminals at each end to act as electrodes
- when electricity passed through the tube, a stream of particles was emitted and the tube glowed with a coloured light
- he discovered that regardless of the gas in the tuibe or the material of the metal electrodes, the particles would flow from the negative to the positive electrode, and they were deflected by a negative electrode
- he concluded that the particles must be negatively charged since it’s attracted to the postively charged metal, and deflected by the negative charge

-Plum pudding model:
- he realises that since atoms are neutrally charged, there must be a postive charge in the atom to balance out the negatively charged particles
- he thought that negative particle were spread thoughout a positively charged sphere like the raisins in a plum pudding
- this would mean that the negatively charged particles cancelled out the positively charged particles

Question 3

Ernest Rutherford

Answer 3

• Rutherford fired a beam of positively charged alpha particles at a thin sheet of gold foil
• most of the alpha particles passed through the foil and produced flashes on the fluorescent screen on the other side
• however, some of the particles were deflected and some bounced back
• based on this, rutherford some conclusions:
  
  • because most of the particles managed to get straight through, most of the atom must be empty space
  • because some of the particles were deflected, there must be something small but hard inside each atom, so that when it’s hit, the particles deflect
  • because of the different angles that the particles deflect and the fact that alpha particles are postively charged, they reasoned that the nucleus is positvely charged, because like charges repel

Question 4

Niels Bohr

Answer 4

• he thought that nucleus was in the middle (like rutherford)
• but he also thought that:
  
  • electrons revolve arond the nucleus in fixed, circular orbits
  • electrons orbits correspond to specific energy levels in the atom
  • electrons can only occupy these fixed energy levels and can’t exist between two energy levels
  • electron orbits of larger radii correspond to energy levels of higher energy
• he also suggested that it’s possible for electrons to move between the energy levels by absorbing or emitting energy in the form of light

Question 5

James Chadwick

Answer 5

• he discovered that there are neutrons, neutrally charged particles, in the nucleus of an atom
• bombarded beryllium with alpha particles