~ 450 B.C.E., Leucippus and Democritus Atomos: Point at which matter can no longer be subdivided. Atoms: The Greek Idea
~384 B.C.E., Aristotle: All matter is composed of 4 elements, and all matter is continuous, not atomistic. Atoms: The Greek Idea
THE ATOM • Around 450 B.C. a Greek philosopher, Democritus, proposed that all forms of matter were divisible into extremely small fundamental particles called atomos. (Greek for indivisible) • Due to world events, this concept was forgotten, replaced by the belief that matter was composed of four “elements” ; earth,wind, fire, & water. • From 1803 to 1810, John Dalton proposed the concept of the atom once again. Dalton backed his claim with experimentation.
John Dalton and the Atomic Theory of Matter • 1803, John Dalton: Law of Multiple Proportions • Elements may combine in more than one set of proportions, each set corresponds to a different compound.
John Dalton and the Atomic Theory of Matter • 1. All matter is composed of extremely small particles called atoms. • 2. All atoms of a given element are alike and differ from the atoms of any other element. • 3. Compounds are formed when atoms of different elements combine in fixed proportions. • 4. A chemical reaction involves the rearrangement of atoms.
Subatomic Particles • ELECTRON: Properties discovered in 1897 by JJ Thompson • PROTON: Isolated and identified in 1919 by Ernest Rutherford • NEUTRON: Discovered by James Chadwick in 1932
Nuclear Model of the Atom • Every atom contains an extremely small, extremely dense nucleus. • All of the positive charge and nearly all of the mass of an atom are concentrated in the nucleus. • The nucleus is surrounded by a much larger volume of nearly empty space that makes up the rest of the atom. • The space outside the nucleus is very thinly populated by electrons, the total charge of which exactly balances the positive charge of the nucleus.
Lavoisier: The Law of Conservation of Mass • Early 1700’s Lavoisier: Law of Conservation of Mass • During a chemical change, matter is neither created nor destroyed.
Proust: The Law of Definite Proportions • 1799, Proust: Law of Definite Proportions • A compound always contains the same elements in certain definite proportions.