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Chemistry:
A Brief History of Atomic Theory February 28, 1999

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In the beginning of the 1800s John Dalton, an English scientist did work some work on
gases, which lead him to the creation of a complex system of symbols for all known
elements at the time. He took all the information he had collected, along with the Laws of
Conservation of Mass, Definite Composition and Multiple Proportions and updated
Aristotle’s theory of matter with the Atomic Theory of Matter, which stated: - All
matter is composed of tiny, indivisible particles called atoms. - Atoms of an element have
identical properties. - Atoms of different elements have different properties. - Atoms of
two or more elements can combine in constant ratios to form new substances. In the late
1800s a man named J. J. Thomson did some experiments, who’s results did not agree
with Dalton’s Atomic Theory. Thomson passed electricity though gases, my his
experiments, he theorized the existence negatively charged subatomic particles he called
electrons. From this theory Thomson created a model of a atom which had the electrons
placed evenly inside the atoms. In the early 1900s a Japanese scientist named H. Nagaoka
designed an atom model as a large sphere surrounded by a ring of negatively charged
electrons. Also, during the early 1900s (1898-1907) a physicist named Ernest Rutherford
worked on experiments to test current atom models. His experiments involved shooting rays
of alpha particles (small positively charged particles) though very thin pieces of gold
foil. Based on Thomson’s model, Rutherford hypothesized that the alpha particles
would travel through the gold foil mostly unaffected by the gold. He was right. Most of
the particles did pass through, but a small amount of particles were deflected. From this
Rutherford hypothesized that the atoms must have a small positively charged core, the
nucleus, which is surrounded by mainly empty space, which contains the electrons. In 1914
Rutherford made up the word “proton,” which were subatomic particles that had
a positive charge. A student of Rutherford’s, a man named H. G. J. Moseley was the
one who gathered the empirical support for Rutherford’s work. In his experiments he
used X-rays to show that the positive charge in the nucleus grows by one, from each
element to the other. From this Moseley devised the concept of Atomic Number. In 1932,
James Chadwick established that the nucleus must contain heavy neutral particles as well
as positive ones, this was to explain the entire mass of the atom. He called the neutral
subatomic particles neutrons. I Danish scientist named Niels Bohr created a theory
explaining the periodic law. Bohr took the Quantum Theory of Energy, proposed by Max
Planck (in 1900), and the relationship between the sudden end of the periodic table. Using
this, periodic law, and some experimental evidence, Bohr hypothesized the following: -
Continues for 2 more pages >>