The Atom: From Philosophical Idea to Scientific Theory Democritus was one of first to propose the idea of a basic particle of matter (400 B.C.) Called it the atom (means indivisible in Greek). Aristotle lived after Democritus (300 B.C.), but believed in all matter being continuous. Aristotle’s view was accepted until the late 1700s. No solid evidence existed to support Aristotle’s views. Foundations of Atomic Theory As technology improved, so did scientific theories. In the 1790s, improvements in balances enabled scientists to accurately measure masses of substances they were studying. This led to three new laws: ○ The law of conservation of mass. ○ The law of definite proportions. ○ The law of multiple proportions. The Law of Conservation of Mass Mass is neither created nor destroyed during ordinary chemical reactions or physical changes. The Law of Definite Proportions A chemical compound contains the same elements in exactly the same proportions by mass regardless of the size of the sample or source of the compound. 11.9% hydrogen 88.1% oxygen 11.9% hydrogen 88.1% oxygen The Law of Multiple Proportions If two or more different compounds are composed of the same two elements, then the ratio of the masses of the second element combined with a certain mass of the first element is always a ratio of small whole numbers. Dalton’s Atomic Theory In 1808, John Dalton proposed an explanation for the laws. All matter is made up of small particles (atoms). Every element has atoms that are unique for that element and identical. Atoms cannot be subdivided, created, or destroyed. Atoms of different elements combine in simple whole-number ratios to form chemical compounds. In chemical reactions, atoms are combined, separated, or rearranged. Modern Atomic Theory Changes to Dalton’s Theory Atoms ARE divisible (nuclear reactions). Atoms of a given element CAN have different masses (isotopes). Putting it all together… Why were Dalton’s ideas accepted and not Aristotle or Democritus’? What laws did Dalton explain using his theory?