Chapter 3 Discovering the atom and subatomic particles (History of modern chemistry)

3.1 Chemistry developed out of interest in materials The ability to control fire allowed ancient people to prepare copper (7000 years ago) and iron (3200 years ago) for tools and weapons. 商代青铜器(铜锡合金)

火 热 干 土 空气 湿 冷 水 Aristotle 384 - 322 B.C. 四原说示意图 Aristotle described the composition and behavior of matter in terms of four qualities: hot, cold, moist, and dry. A material is possible to be transformed to another material by changing the relative proportion of these four qualities. Gold can be made from other metals (Alchemy).

Democritus started thinking about “atom” (not cut or that which is indivisible)

Father of modern chemistry unfortunate association with a company organized to collect taxes for King Louis died at guillotine of no use to the New Republic of France headless body buried in unmarked grave Showed conservation of mass in chemical reactions invented precise scale to weigh small amounts of matter accurate to 0.0005 grams, about 1/100 of a drop of water mass of all reactants = mass of all products cannot create matter from nothing, can only change form matter is not created from nothing, only changes form during chemical reactions

Mass is conserved in a chemical reaction 16 X + 8 Y 8 X2Y Law of mass conservation (质量守恒定律): There is no detectable change in the total mass of materials when they react chemically to form new materials

Joseph Priestley found oxygen and Lavoisier named it, which means “acid former” Priestley, Joseph (1733-1804) Philosopher, scientist and unorthodox theologian , Joseph Priestley is perhaps best known for his discovery of oxygen while performing experiments with gases in 1774. Ultimately, Priestley identified nine distinct gases, including carbon dioxide and nitrous oxide. He is credited with developing a sound method for infusing carbon dioxide into water, thus establishing himself as the father of carbonation.

The Phlogiston Theory Chemistry was so underdeveloped at the time Lavoisier gained interest in it that it could hardly be called a science. The prevailing view of combustion was the Phlogiston Theory which involved a weightless or nearly weightless substance known as phlogiston. Metals and fire were considered to be rich in phlogiston and earth was considered oxygen poor.

氧气的发现者 普利斯特里(Joseph Priestly) ，英国 舍勒(Carl Wilhelm Scheele)，瑞典 拉瓦锡(Antoine Lavoisier)，法国

Proust proposed the Law of Definite Proportions Henry Cavendish isolated hydrogen and Lavoisier named it meaning “water former”. Lavoisier also realized that water is not an element, but a compound. Joseph Proust found that hydrogen and oxygen always react in a particular mass ratio and proposed the Law of Definite Proportions: Elements combine in definite mass proportions to form compounds.

3.3 Dalton deduced that matter is made of atoms John Dalton reintroduced the atomic ideas of Democritus: Each element consists of indivisible, minute particles called atoms. (not completely correct) Atoms can be neither created nor destroyed in chemical reactions. All atoms of a given element are identical. (not completely correct) Atoms chemically combine in definite whole-number ratios to form compounds Atoms of different elements have different masses.

Arguments on the Chemical Structure of Water Dalton’s suggestion for water: HO, oxygen is eight times as heavy as hydrogen. Joseph Gay-Lussac’s finding of water: 2 liters hydrogen gas + 1 liter oxygen gas → 2 liters water vapor molecular formular for water should be H2O

Joseph Louis Gay-Lussac and Jean-Baptistse Biot in their balloon on 24 August 1804 (7000 meters)

Amadeo Avogadro gave the right explanation to Gay-Lussac’s experiment by introducing the idea of diatomic particles. But his explanation was not accepted by Dalton and the chemical community for 50 years.

Problem: How to measure the masses of atoms of different elements? At a conference in 1860, Stanislao Cannizzaro re-introduced Avogadro’s idea and also introduced a simple method to measure the relative atomic masses by weighing the gases of the same volume. This idea gradually grew to be the periodic table. 1 liter Oxygen 1 liter Hydrogen 16x 1x

Mendeleev used known relative atomic masses to create the Periodic Table In 1860s, many scientists found that the properties of elements tended to recur in cycles, exhibiting the periodicity. In 1869, Dmitri Mendeleev produced a chart summarizing the properties of known elements. Main disadvantage of Mendeleev’s chart: ordered according to atomic masses. Today’s periodic table was ordered according to atomic numbers (原子序数).

3.4 The electron was the first subatomic particle discovered Electron was found when Joseph John Thomson and Robert Millikan studied the cathode ray tube. They found that the cathode ray particles were much lighter than hydrogen atoms, indicating that the atom was not the smallest particle of matter. Now we know that electrons determine many of a material’s properties, including chemical reactivity, color, etc. Robert Andrews Millikan (1868-1953) :excelled in his experimental designs and conclusions, in addition to research, he also spent much time preparing textbooks so that his student didn’t have to rely so much on lectures Joseph John Thomson, wan one of the first director of the famous Cavendish Laboratory of Cambridge University in England, Where almost all the discoveries concerning subatomic particles and their behavior were made

3.5 The mass of an atom is concentrated in its nucleus JJ proposed the “plum-pudding model” for atoms. Thomson proposed that the atoms might be made of thousands of tiny ,negatively charged particles swarming within a cloud of positively charge, much like plums and raisins in an old-fashioned christmas plum-pudding

Ernest Rutherford found the nucleus through the “gold-foil” experiment Rutherford’s Model of the Atom

3.6 The atomic nucleus is made of protons and neutrons Proton (质子) is a positively charged particle in a nucleus. A proton is clearly 2000 times as heavy as an electron but has the same (reverse) electric charge as an electron. The number of protons each atom of a given element contains is called atomic number. Neutron (中子) is another subatomic particle in nucleus, having the similar mass as the proton but electrically neutral. It has very important role in holding the atomic nucleus together. The atom for a given element should have a set number of proton, but the number of neutron can be changed. Atoms of the same element that contain different numbers of neutrons are isotopes (同位素).

Chemistry is the study of how atoms combine to form materials Chemistry is the study of how atoms combine to form materials. So chemistry puts more attention to atoms (and electrons in atoms) but not the nucleus. The change of atoms through the change of nucleus is not the center of chemistry. It belongs to Nuclear Science and Technology. Chapter 4 (self study)