2.1: introduction to atoms, elements, and ions

basics of atomic history

• Democritus (460-370 BCE)—coined term atomus (“indivisible”)

  • not backed by Aristotle, who pushed elemental (water, air, earth, fire) theory which was developed by Empedocles; politics has always been involved with science

• Antoine Lavoisier—developed law of conservation of mass (1789): the total mass of substance present at the end of a chemical process is the same as the mass of substances present before the process took place

• John Dalton—developed atomic theory of matter (1803): atoms are the fundamental building block of matter

  • Dalton was the first modern scientist (used scientific method)

  • developed four postulates:

  • all matter is composed of indivisible particles, atoms (Democritus)

  • chemical reactions combine or recombine atoms, they do not destroy them (Lavoisier)

  • all atoms of an element are identical atoms of different elements are different

  • when elements react to form compounds, they react in defined, whole-number ratios

    of protons dictate which element is which

  • modeled using the “billiard ball” model

  • law of multiple proportions: if two or more compounds can be made from elements A and B, the masses of B that combine with a given mass of A are in the ratio of small whole numbers

  • when two or more compounds exist from the same elements, they cannot have the same relative number of atoms

• radioactivity: the spontaneous emission of high levels of energy

  • discovered by Marie Curie (1867-1934)
  • eventually used to discover that atoms are not the smallest particles (protons, neutrons, etc. exist—subatomic particles)

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