To their credit, the alchemists discovered several new elements and learned to prepare acids like sulfuric acid and nitric acid. The age of the Greek definition of elements was over.
Combustion (1600)
Due to the rise of the importance of coal, understanding combustion (burning) took center stage in the 1600s. In 1700 a German doctor named Georg Ernest Stahl proposed the best theory yet on what was going on when something burned. He proposed that all inflammable objects contained a substance he called "phlogiston" (Greek for "to set on fire") and it was their phlogiston content that allowed them to burn. During combustion, the burning substance lost phlogiston and what was left was a substance completely lacking in phlogiston and thus could burn no more. Wood and coal were his examples, for they could burn (they contained phlogiston) and the ash that was left could not (all the phlogiston was gone).
Stahl saw a similarity with rusting iron which he said was simply a slow burn. The iron lost its phlogiston as it rusted and could rust no more when the metal was gone since it had lost all its phlogiston.
Things looked promising since wood lost weight upon burning. Of course it should, since it was losing phlogiston. But, iron GAINED weight upon combustion (rusting). If lost phlogiston, how could it weigh more? It quickly became obvious that exact measurement in chemistry was essential to understanding what was going on during chemical reactions.
Quantitative analysis (Late 1700s)
In the 1790's, accurate measurement of masses became widespread for the first time due to improvement of balances. This led scientists to measure the masses of the elements and compounds they were working with. What followed was the establishment of several new laws of chemistry:
1) Law of conservation of mass : "In a chemical reaction, mass is neither created nor destroyed."
Antoine Lavoisier of France carefully weighed the reactants and products of various chemical reactions and formulated this law. (He was beheaded during the French Revolution due to his investment in a tax- collecting firm and his marriage to the daughter of one of the company's executives)
2) Law of definite proportion : "A given compound always contains exactly the same proportion of elements by mass."
Joseph Proust of France proposed this which can be interpreted simply by saying that a certain compound always has the same chemical formula. For example, water is ALWAYS H2O, it is never HO2, or H3O, etc.
3) Law of multiple proportions : "If two or more 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."
This can be interpreted by showing that if carbon and oxygen form more that one compound, they are CO (carbon monoxide) and CO2 (carbon dioxide). There could never be CO1.2. Atoms always combine in whole numbers. There is no such thing as part of an atom. There are many examples of this idea in compounds of nitrogen and oxygen. NO, NO2, N2O3, N2O, N2O4, N2O5.
John Dalton (1808)
All of this information stimulated John Dalton, an English school teacher to propose his atomic theory.
See the 5 points on pg 66 of your text.
Discovery of the electron - J.J. Thomson ( 1897)
Cathode ray tube, negative charge found
See pg 70-71 of your text Neon signs, TVs, computer monitors are CRTs.
Mass of the electron - Robert A. Millikan (1909)
1/1837 the mass of the simplest atom (hydrogen) or 9.109 x 10-31 kg
Plum Pudding Model
Based on Thomson and Millikan's work, it was assumed that
1) atoms must contain positive charges since they are electrically neutral
2) atoms must contain other particles since electrons weigh so little
Discovery of the atomic nucleus - Ernest Rutherford (1911)
1) Thin gold sheets were bombarded by alpha particles ( radioactive decay particles - 2 protons, 2 neutrons and 4 times the mass of a hydrogen atom and positively charged)
2) Mass and charge were assumed to be uniformly distributed in the atom, so alpha particles were expected to pass through with slight deflections. Unexpectedly, there were some large deflections and 1 in 8000 actually bounced straight back! See pg 72 of your text
3) Result - a) the atom had a center of concentrated positive charge
b) the atom was mostly empty space - if nucleus was the size of a BB, size of atom was the size of a football stadium
c) this would imply that electrons orbit the nucleus like planets around the sun in this "empty space" (found later to be wrong)
Atomic Structure - a summary
1) The nucleus contains protons with a +1 charge and neutrons with no charge. Both have about the same mass (neutrons are 0.14% heavier).
2) Electrons are somewhere outside the nucleus and have a -1 charge. It takes about 1836 electrons to equal the mass of 1 proton.
Bibliography
Site: http://www2.asd.k12.ak.us/hauser/Links/html/Chemistry/Unit%202%20Building%20Blocks%20of%20Matter/handouts%20and%20Notes/Handout_on_Atomic_Structure%20_History.htm
