Chemistry Information Center

MAJOR DISCOVERIES IN CHEMISTRY
B.C.
c. 450 Leucippus of Miletus ( Greek: 5th cent.) introduces concept of atom, later expanded upon by his pupil Democritus of abdera (c. 460-c. 370).

A.D.
1662 Robert Boule (Anglo-Irish: 1627-91) announces what becomes known as Boyle's law: For gas kept a constant temperature, pressure and volume vary inversely.

1775 Joseph Black (Scottish: 1728-99) discovers carbon dioxide.

1766 Henry Cavendish (English: 1731-1810) discovers hydrogen.

1772 Joseph Priestly (English-American: 1733-1804) notes that burning hydrogen produces water.

Daniel Rutherford (Scottish: 1749-1819) and several other chemists discover nitrogen. Karl Wilhelm Scheele (Swedish: 174286) discovers oxygen but does not announce discovery until after independent discovery by Joseph Priestly in 1774.

1778 Antoine-Laurent Lavoisier (French: 1743-94) discovers that air is mostly mixture of nitrogen and oxygen.

1784 Cavendish announces water is compound of hydrogen and oxygen.

1789 Lavoisier explicitly states law of conservation of matter: In chemical change, matter is neither created nor destroyed.

1791 Jeremias Benjamin Richter (German: 1762-1807) shows that acids and bases always neutralize each other in same proportion.

1803 John Dalton (English: 1766-1844) establishes atomic theory of matter.

1811 Amedeo Avogadro (Italian: 1776-1856) proposes that equal volumes of gas at same temperature and pressure contain same number of molecules (Avogadro's law).

1824 Joseph-Louis Gay-Lussac (French: 1778-1850) discovers chemical isomers, chemicals with same formula but different structures.

1828 Friedrich Wohler (German: 1800-1882) prepares organic compound from inorganic chemicals, showing that life is basically same as other matter.

1859 Gustaf Robert Kirchhoff (German: 1824-87) and Robert Wilhelm Bunsen (German: 1811-99) introduce use of spectroscope to identify elements from light they give off when heated or burned.

1868 Pierre-Jules-Cesar Janssen (French: 1824-1907) and Sir Joseph Norman Lockyer (English: 1836-1920) discover helium by observing sun's spectrum.

1869 Dmitry Ivanovic Mendeleyev (Russian: 1834-1907) publishes his first version of periodic table of elements.

1875 Paul-Emile Lecoq de Boisbaudran (French: 1838-1912) discovers gallium, the first discovery of an element predicted by Mendeleyev.

1906 Mikhail Tsvett (Russian: 1872-1919) develops paper chromatography, the beginning of modern methods of chemical analysis.

1908 Fritz Haber (German: 1868-1934) develops cheap process for making ammonia from nitrogen in the air.

1962 Neil Bartlet (English: 1932-) creates a compound of xenon, platinum, and fluorine, showing that the noble gases can form compounds.

1984 Dany Shechtman American and coworkers discover first quasicrystal, a "crystal" that violates the symmetry rules of all other crystals.

1985 Richard E. Smalley (American: 1943-) and Harry Kroto (English: 1939-) discover buckminster fullerene, a carbon molecule containing 60 carbon atoms arranged in a geodesic sphere (nicknamed "the bucky ball").

1991 Sumio Lijima of NEC Corp. discovers that carbon forms tiny hollow cylinders called nanotubes.

1996 Scientists at the society for Heavy Ion Research in Darmstadt, Germany, create a few atoms of element 112, continuing a process the society started with the manufacturer of elements 107-111 in the 1980's and 1990s.

Scientists at Lawrence Livermore National Laboratory in California use a gas gun to compress hydrogen into a metal, although it quickly returns to a nonmetal state.

1997 Various groups of chemists develop atom-by-atom methods to construct chemical molecules.

THE PERIODIC TABLE

Chemistry is concerned with the way substances interact with one another. These interactions are chiefly the result of outer electrons of an atom interacting with the outer electrons of another atom. It has increasingly become clear that the shapes of the various combinations of atoms (called molecules) also affect chemical reactions, and physical chemistry is one of the most vital parts of chemistry today. Another vital branch is biochemistry, the study of the chemistry of molecules in living organisms. Organic chemistry generally deals with chemicals formed by living organisms and other chemicals containing carbon, but it treats them as chemicals outside the organism, Inorganic chemistry is concerned with chemicals that do not contain carbon.

In the 19th century, chemists began to determine how much one atom of an element weighed with respect to another-the atomic weight (also known as the atomic mass and measured in atomic mass units, or am; one am is a mass equal to one-twelfth the mass of the most common form of carbon atom). Jones Jacob Berkelium prepared the first comprehensive list of atomic weights in 1828. When chemists made lists of elements in he order of atomic weights, they noticed that every seven or eight elements in the list had similar properties. In 1869 Dirt Minelayer went further and boldly interchanged some elements in the list and left blanks for others to make sure the properties marched for every "period" of eight elements. This was the first periodic table. Minelayer had only 63 elements to work with, but he correctly predicted three more that would make his list more complete. Today there are 112 elements in the periodic table.

Early in the 20th century, atoms were discovered to consist of protons and electrons (in 1932 it was discovered that neurons also are found in atoms). The number of protons is the (atomic number 1) to the unnamed element numbered 112. The periodic table was improved by arranging the element numbered 112. The periodic table was improved by arranging the elements in order of atomic number instead of atomic weight. This clearly showed where the blanks were-all of which have been filled in since 1945. Any other newly discovered or created elements must go at the end of the table.

Each column of the periodic table includes elements with similar properties, although hydrogen in the first column is less typical in this respect. But the other elements in the first column are all soft metals that react strongly. Similarly, the last column of the table contains only the gases that react only minimally. In general, elements are metals on the left side of the table (hydrogen is a metal only under great pressure), becoming mostly nonmetals in the last six columns. These last columns include some elements that are metals, such as aluminum. (A broken, heavy line separates the metals from the nonmetals).

The row of rare-earth elements beginning with lanthanum and the row of actinide elements beginning with actinium do not fit nearly into the rest of the table. Elements from atomic number 57 to 71 are all similar to lanthanum, while elements from atomic number 89 to 103 are similar to actinium. The rare earths are not generally rare, nor do they resemble soil. They are moderately common metals that, because of atomic structure, are very similar chemically. The actinide elements are radioactive metals.

The periodic table as shown here also includes the atomic mass as well as the atomic number. The atomic mass is essentially the sum of the protons and neutrons in an atom of an element. As protons and neutrons join to form an atomic nucleus, a little of there energy becomes mass, the amount of which depends on how many protons and neutrons there are. Consequently, a particular atom is chosen upon which to base the am. Today the atomic mass is adjusted to make the most common form of carbon have atomic mass of exactly 12 (6 protons and 6 neutrons). Most elements occur with several different atomic masses (in addition to carbon-12, for example, there are both carbon-13 and carbon-14; carbon 14 has 6 protons and 8 neutrons and is radioactive). These different forms are called isotopes. Therefore, in the periodic table, the atomic mass given for most elements is the one that would be found by averaging the different isotopes in the amounts they naturally occur. Carbon is given an atomic mass of 12.01 because there is so much more carbon-12 than there is carbon-13 or carbon-14 in an ordinary sample of carbon. For some radio-active elements, natural abundance is meaningless, since there is no stable form. For these, the atomic mass of the most stable form is given, indicated by putting the atomic mass in parentheses.