Essay, Research Paper: A Brief History Of Atomic Theory

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.