Download Chemistry- History of the Atom Notes Democritus

Chemistry- History of the Atom Notes
Democritus- (400 B.C.) called nature’s basic particle an atom, from the Greek meaning
indivisible.
Three Basic Laws
1. Law of Conservation of Mass (1782)- states that matter is neither created nor destroyed
during ordinary chemical reactions or physical changes.
2. Law of Definite Proportions(1797)- a chemical compound contains the same elements in
the exact same proportions by mass regardless of the size of the sample or the source of
the compound. (Example: water is ALWAYS H2O regardless of the source; water has a 2:1
ratio of hydrogen to oxygen)
3. Law of Multiple Proportions(1803)- if two or more different compounds contain the same
elements, then the ratio of masses of the second element combined with a certain mass
of the first element is always a ratio of small, whole numbers. (Example: water (H2O) and
hydrogen peroxide (H2O2) both contain hydrogen and oxygen)
Dalton’s Atomic Theory
(1808) John Dalton proposed an explanation for the law of conservation of mass, the law of
definite proportions, and the law of multiple proportions.
1. All matter is composed of atoms.
2. Atoms of a given element are identical in size, mass, and other properties; atoms of
different elements differ in size, mass, and other properties.
3. Atoms cannot be subdivided, created, or destroyed.
4. Atoms of different elements combine in simple whole number rations to form chemical
compounds.
5. In chemical reactions, atoms are combined, separated, or rearranged.
Daltons turned Democritus’s idea into scientific theory that could be tested by experiment.
Modern Atomic Theory
Not all aspects of Dalton’s atomic theory have been proven correct, but atomic theory has not
been discarded. Instead it has been modified with the most important concepts being
(1) all matter is composed of atoms and
(2) atoms of any one element differ in properties from atoms of another element.
Discovery of Subatomic Particles
Scientists first discovered electrons and concluded that the mass is very small and that
electrons have a negative charge. This led them to make the following inferences:
1. Since atoms are electrically neutral, they must contain a positive charge to balance the
negative electrons
2. Since the mass of the electron is so much less than the mass of the atom, they must
contain other particles that account for most of their mass.
JJ Thomson- discovered the electron using a cathode ray tube; thought that there must be
positives and negatives evenly dispersed in the atom and called it “plum pudding”
Rutherford- found that there was a small, dense area in the atom where most of the mass was
contained that had a positive charge and he called it the nucleus. He discovered the nucleus in
his famous gold foil experiment where he shot alpha particles at a thin piece of gold foil. Most
of the particles traveled through the foil like nothing was there but very few particles were
deflected back towards the source. Since alpha particles have a positive charge he concluded
that they must be hitting something small that had a positive charge (the nucleus).