of the Atomic Theory and the Structure of the Atom
Development of the Atomic Theory
Aristotle (384-322 BC) was a proponent of
the continuum. He believed in the four elements of air, earth, water
and fire. Aristotle felt that regardless of
the number of times
you cut a form of matter in half, you would always have a smaller piece of
that matter. This view held sway for 2000 years primarily because
Aristotle was the tutor of Alexander the Great.
Johann Becher (1635-1682) and Georg Stahl
(1660-1734) developed the Phlogiston theory which dominated chemistry
between 1670 and 1790. Basically, when something burned, it lost
phlogiston to the air (after all, you could see the phlogiston
leaving) A problem with the theory was that burning of metals resulted
in an increase in the mass. This problem was solved by assigning
negative mass to phlogiston.
Joseph Priestly (1733-1804) discovered
oxygen (which he called "dephlogisticated air") in 1774.
Priestly was an ardent phlogistonist until his dying day. Priestly was
also an early anti-war activist who favored both the American and French
Revolutions. He was shipped to the U.S. in 1791 where he lived a quiet
life in Pennsylvania. His house was used as a starting point for the
American Chemical Society in 1876. The Priestly Medal is the highest
award given by to an American chemist by the Society.
Antoine Lavoisier (1743-1794) was the
first person to make good use of the balance. He was an excellent
experimenter. After a visit with Priestly in 1774, he began careful
study of the burning process. He proposed the Combustion Theory which
was based on sound mass measurements. He named oxygen. He also
proposed the Law of Conversation of Mass which represents the beginning of
modern chemistry. To support his work, Lavoisier was associated with a
tax-collecting firm and was married to the daughter of the one of the firm's
executives. Some people believe that Madame Lavoisier was every bit as
good a scientist as her husband. Unfortunately, this
relationship with the tax firm led to Lavoisier's beheading at the guillotine
Marie and Antoine Lavoisier
Joseph Proust (1754-1826) proposed the the
Law of Constant Composition in 1799. This law was very radical at the
time and was hotly contested by Claude Berthollet (1748-1822).
John Dalton (1776-1844) proposed the Law
of Multiple Proportions. This law led directly to the proposal of the
Atomic Theory in 1803. He also developed the concept of the mole and
proposed a system of symbols to represent atoms of different elements.
(The symbols currently used were developed by J.J.
Berzelius(1779-1848)). Dalton recognized the existence of atoms of
elements and that compounds formed from the union of these atoms. He
therefore assumed that simplest ratios would be used in nature and came up
with a formula for water of HO. He then assigned a relative atomic
weight of one to hydrogen and developed a relative atomic weight scale from
percent composition data and assumed atomic ratios. Today we would
refer to these as equivalent masses. John Dalton also discovered color
blindness, an affliction from which he suffered. He determined that
five percent of the male population and less than one-tenth percent of the
female population was color blind.
Joseph Gay-Lussac ( 1778-1850) announced
the Law of Combining Volumes in 1808. He showed that at the same
temperature and pressure, two volumes of hydrogen gas reacted with one
volume of oxygen gas to produce two volumes of water (as a gas).
Amadeo Avogadro (1776-1856) proposed what
is now known as Avogadro's Hypothesis in 1811. The hypothesis states
that at the same temperature and pressure, equal volumes of gases contain
the same number of molecules or atoms. When this is combined with Gay-Lussac's
Law of Combining Volumes, the only possible formulas for hydrogen, oxygen
and water are H2, O2 and H2O,
respectively. The solution to the atomic weight problem was at hand in
1811. However, Avogadro's Hypothesis was a radical statement at the
time and was not widely accepted until fifty years later.
Stanislao Cannizzaro (1826-1910), in 1860
at the Karlsruhe Conference, proposed that Avogadro's Hypothesis be accepted
and the implications used for a period of five years. At the end of
this five year period, a new conference would be called to discuss any
problems that might develop; this second conference was never called.
(1834-1907) proposed the periodic law and developed the first periodic table
in 1869. Medeleev's table was arranged according to increasing atomic
weight and left holes for elements that were yet to be discovered.
Development of Atomic Structure
J. J. Thomson (1856-1940) identified the
negatively charged electron in the cathode ray tube in 1897. He deduced
that the electron was a component of all matter and calculated the charge to
mass ratio for the electron.
e/m = -1.76 x 108 coulombs/g
Thomson and others also studied the positive rays in the cathode ray tube and discovered that the charge to mass ratio depended on filling gas in the tube. The largest charge to mass ratio (smallest mass) occurred when hydrogen was the filling gas. This particle was later identified as the proton.
e/m = +9.58 x 104 coulombs/g
Thomson proposed the "plum pudding" model of the atom. In this model, the volume of the atom is composed primarily of the more massive (thus larger) positive portion (the plum pudding). The smaller electrons (actually, raisins in the plum pudding ) are dispersed throughout the positive mass to maintain charge neutrality.
Joseph John Thomson
Robert Millikan (1868-1953) determined
the unit charge of the electron in 1909 with his oil drop experiment
at the University of Chicago.
Thus allowing for the calculation of the mass of the electron and the
positively charged atoms.
e = 1.60 x 10-19 coulombs
Ernst Rutherford (1871-1937) proposed
the nuclear atom as the result of the gold-foil experiment in 1911.
Rutherford proposed that all of the positive charge and all of the mass of
the atom occupied a small volume at the center of the atom and that most of
the volume of the atom was empty space occupied by the electrons. This
was a very radical proposal that flew in the face of Newtonian
Physics. Although positive particles had been discussed for some time,
it was Rutherford in 1920 that first referred to the hydrogen nucleus as a
proton. Also in 1920, Rutherford proposed the existence of the third
atomic particle, the neutron.
(1887-1915) discovered that the energy of x-rays emitted by the elements
increased in a linear fashion with each successive element in the periodic
table. In 1913, he proposed that the relationship was a function of the
positive charge on the nucleus. This rearranged the periodic table by
using the atomic number instead of atomic mass to represent the progression of
the elements. This new table left additional holes for elements that
would soon be discovered. Unfortunately, Moseley was killed at Gallipoli
(1877-1945) invented the mass spectrograph in 1920. He was the first
person to observe isotopes. For example he observed that there were
three different kinds of hydrogen atoms. While most of the atoms had a
mass number of 1 he also observed hydrogen atoms with mass numbers of 2 and 3.
Modern atomic masses are based on mass spectral analysis. His work led
Rutherford to predict the existence of the neutron.
James Chadwick (1891-1974) discovered
the neutron in 1932. Chadwick was a collaborator of
Rutherford's. Interestingly, the discovery of the neutron led directly
to the discovery of fission and ultimately to the atomic bomb.