2.1—The Atomic Theory - AP Chemistry - Chemistry - 2011-09-09

2.1—The Atomic Theory

Democritus- believed that there were small, indivisible particles, which he named atomos.

In 1808 John Dalton formed a precise definition of the building blocks of matter that we call atoms.

Dalton’s Atomic Theory

  1. Elements are composed of small particles called atoms.
  2. All atoms of a given element are identical, having the same size, mass, and chemical properties. The atoms of one element are different from the atoms of all other elements.
  3. Compounds are composed of atoms of more than one element. In any compound, the ratio of the numbers of atoms of any two of the elements present is either an integer of a simple fraction.
  4. A chemical reaction involves only the separation, combination, or rearrangement of atoms; it does not result in their creation or destruction.

Joseph Prout’s law of definite proportions states that different samples of the same compound always contain its constituent elements in the same proportion by mass.

The law of multiple proportions states that if two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element are in ratios of small whole numbers. (This is supported by Dalton’s third hypothesis).

Dalton’s 4th hypothesis is the same as the law of conservation of mass, which states that matter can be neither created nor destroyed.



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