Extended Response

• Overarching History


  500 BCE
• Leucippus (Greek Philosopher)

  Proposed the idea that if a piece of matter is subdivided over and over again, eventually it would no longer be able to divide
• Democritus (follower of Leucippus)


  Expanded on the idea, naming this final piece of matter 'atomos' (indivisible)
• Democritus' ideas


  • Atoms have different sizes and shapes, which gave different properties
  • These ideas were not widely accepted as Aristotle believed all matter was composed of air, water, fire, earth
• John Dalton published 'A New System of Chemical Philosophy'


  1805
• John Dalton's Atomic Theory

  • All matter is made of tiny particles called atoms
  • Atoms of different elements have different weights
  • Compounds are made from a combination of the atoms of two or more elements in simple numerical ratio
  • Atoms of the same element are identical in size and mass (only from compounds in simple ratio)
• Dalton's Atomic Theory created the foundation of modern atomic theory
• Dalton was wrong about atoms being indivisible, as they are composed of subatomic particles
• Dalton recognised that atoms of elements differ
• Dalton's Atomic Theory incorporated ideas from the law of conservation, the law of definite proportions, and the law of multiple proportions
• Experimental Evidence


  • Theory originated from early studies of atmospheric gases
  • Ratio of mass of element in a compound is always the same
• JJ Thomson


  1840-1900, English physicist who discovered atoms were not as indivisible as once thought
• Cathode ray tube experiment

  1. Sealed glass tube with metal terminals as anode and cathode

  2. Electricity (cathode ray) passes through, emitting particles causing tube to glow. Always moving from negatively charged terminal to positively charged terminal

  3. He deduced particles are negatively charged and present in all atoms (called corpuscles). As he experimented with multiple metals in the cathode.

  
  

  By measuring the charge on the particles in the ray, he concluded that they were 2000 times lighter than hydrogen atoms

  
  

  4. Proposed a positive charge to cancel out the negative charge
• Plum pudding model


  Proposed by Thomson, with a general ball of positive charge and discrete electrons
• Ernest Rutherford


  • Physicist from New Zealand, went to Cambridge under Thomson
  • Performed experiment firing positively charged alpha particles at a thin sheet of gold foil
  • Most particles passed through, some deflected, some bounced back
  • Concluded most of an atom was empty space, with a tiny, dense centre (nucleus) with positive charge and surrounding negative charge orbiting it
• Rutherford's nuclear model


  Positive charge localised in the nucleus, negative charge orbits like planets
• Niels Bohr


  • Danish physicist who developed a new model of hydrogen to explain the emission of light characteristics
  • Electrons revolve around the nucleus in fixed, quantised orbits
  • Electrons can only occupy specific energy levels and cannot exist between two levels
  • Electrons in orbits of larger radius correspond to higher energy levels
• Bohr's planetary model


  • Electrons orbit at fixed distances, corresponding to specific energy levels
  • Electrons can move between levels by absorbing or emitting energy in the form of light
• Bohr's model had good agreement between calculated theoretical frequencies and observed frequencies of light emission
• Valence electrons


  Electrons in the outermost shell, require the least energy to remove and are involved in chemical reactions
• James Chadwick


  • British physicist who discovered the neutron
  • Measured the range of protons given off from radioactive beryllium, found an uncharged particle of similar mass to the proton (the neutron)
  • Neutrons help to reduce the repulsion of protons in the atomic nucleus, stabilising it