FREE ESSAY ON ATOMIC THEORY

ATOMIC THEORY

Chemistry:
A Brief History of Atomic Theory February 28, 1999 
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: - Each electron has a fixed
quantity of energy related to the circular orbit in which the election is found. -
Electrons cannot exist between orbits, but they can move to unfilled orbits if a quantum
of energy is absorbed or released. - The higher the energy level of an electron, the
further it is from the nucleus. - The maximum number of electrons in the first three
energy levels is 2, 8, and 8. - An atom with a maximum number of electrons in its
outermost level is stable, that is, it is unreactive. Bohr's theory was developed
mathematically, so as to explain the visible spectrum of hydrogen gas, as well as to
predict other lines of ultraviolet and infrared light. One of the great things about
Bohr's theory is that it explains periodic law. The theory states that properties of
elements can be explained by the way that their electrons are arranged. Due to the fact
that orbits can only contain certain amounts of electons. Quantum mechanics is a highly
mathematical theory developed in the 1920s that describes the positioning of electrons as
patterns of probability, instead of distinct orbits. It explains the positioning of
protons, and atomic numbers. It describes the energy levels of electrons outside the
nucleus, stability of atoms, and the amounts of electrons that can exist at certain
levels. It is a very complex theory which explains much about atomic theory.