Atomic Structure History

Question 1

State the scientists that contributed to the atomic structure in chronological order.

Answer 1

John Dalton, JJ Thomson, Ernest Rutherford, Niels Bohr, James Chadwick

Question 2

John Dalton

Answer 2

• Proposed that all matter is made up of tiny particles called atoms
• All atoms of a given element are identical having the same size, mass, and chemical properties.
• Atoms are not created nor destroyed or changed into different types during a chemical reaction.
• A chemical reaction only involves separation, combination, or rearrangement of atoms.
• Compounds are formed when atoms of more than one element combine in a specific ratio.

Question 3

JJ Thomson

Answer 3

• Realised that atoms are divisible and contain very tiny, negatively charged particles called electrons.
• Negatively charged particles which make up cathode rays (electrons) had an estimated mass of around 1/1000 of the smallest known atom.
• Plum pudding model.
  
  • Numerous very small negatively charged electrons imbedded in a much larger uniformly positively charged sphere.
    The large spherical part accounted for almost all of its mass.

Question 4

Ernest Rutherford

Answer 4

• Targeted a beam of alpha particles at a thin sheet of gold foil.
  
  • Almost all of the alpha particles passed straight through the foil.
  • A small number of alpha particles were significantly deflected.
• Atoms consist mostly of empty space occupied only by the very low mass negatively charged particles called electrons.
• Electrons orbited a tiny central region called the nucleus.
• Almost all of the atom’s mass and all of its positive charge are located in the nucleus. Positive charge comes from protons.
  
  • The alpha particles that passed close to or toward the positively charged nuclei would be deflected.

Question 5

Niels Bohr

Answer 5

• Proposed that electrons moved about the central nucleus in circular orbits, but only certain orbit radii are allowed.
• Proposed that the electron in each orbit had a specific amount of energy, with the lowest orbital radii having the lowest possible energy.
• Proposed that the electron could orbit without losing energy.
• Hypothesised that a single light photon would be emitted from an excited atom when one of its electrons fell from a higher energy orbit to a lower energy one.
  
  • The energy of the photon would equal the difference in energy of the two orbits.
  • The frequency of the photon depends on its energy.
  • His model was able to explain line spectra and absorption spectra and predict the wavelengths of light in the line spectrum of hydrogen

Question 6

James Chadwick

Answer 6

• Identified neutrons as a product of the alpha particle bombardment of Beryllium.