History of the atom

Question 1

John Dalton

Answer 1

Dalton’s Atomic theory (the first atomic theory): 1803

1. All elements are composed of tiny invisible particles called atoms
2. Atoms of the same element are identical. Atoms of any other element are different
3. Atoms of different elements combine in single whole-numbered ratios to form chemical compounds
4. In chemical reactions, atoms are combined, separated or rearranged - but never changed into atoms of another element
   - Dalton was wrong about all elements of the same type being identical → Atoms CAN have different numbers of neutrons and thus have different mass numbers (isotopes)
   - Dalton also proposed the Billiard Ball model in 1804 → All matter is composed of small, pure solid particles

Question 2

JJ Thomson

Answer 2

• Thompson came up with the idea of having charges embedded within Dalton’s “Billiard Balls”
• He used the cathode ray experiment to discover the existence of the electron in 1897
• He came up with the plum pudding model → negatively charged particles embedded within a positively charged ‘soup’. This was in 1904
• This new model still kept Dalton’s key ideas intact

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Question 3

Ernest Rutherford

Answer 3

• He used the gold foil experiment
  
  • particles shot through thin sheet of gold
  • most shots went straight through
  • a small amount were deflected
  • hence, the atoms must be made of mostly empty space with a small dense nucleus
• Conclusions
  
  • the nucleus is small
  • the nucleus is dense
  • the nucleus is positively charged

Question 4

Niels Bohr

Answer 4

• Discovered that electrons exist in several distinct layers of levels
• Electrons revolve around the central positive nucleus
• Electrons can only revolve in certain orbits, or at certain energy levels (the energy levels are QUANTISED - restricted to certain values)
• This represents the Bohr Model (1913)

Question 5

James Chadwick

Answer 5

• in 1932, Chadwick bombarded beryllium atoms with Alpha particles.
• An unknown radiation was produced
• Chadwick interpreted this radiation as being composed of particles with a neutral electrical charge and the approximate mass of a proton
• This became known as the neutron

Question 6

Currently

Answer 6

Quantum Mechanical model