The Atomic Theory

The Atomic Theory

            In the earlier centuries, many had tried to come up with an explanation of what things were made up. First theories were traced from Heraclitus whose theory is that things are in a state of flux, where everything changes. However, Paramenides said that if a thing has to change, it should make a transition from that state to the nothing state before it could be transformed, making the theory of Heraclitus logically impossible.

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            Then came Democritus who initialized the Atomic Theory. According to Democritus, all things are made up of indestructible units called atoms. Democritus had found no way to prove it but many adopted this theory and assumed that the atoms are the tiniest material that can no longer be divided or destroyed further. According to Democritus, the atoms combine to come up with macroscopic matter that we see around us (Green Planet Solar, 2009).

            John Dalton came up with an atomic theory in which he mentioned that everything is composed of atoms and that each element is made of different atoms but atoms of elements are the same(De Leon, nd). Dalton had also observed that elements combine to form a compound and this compound formation comes in proportion of an element with another element. According to Dalton, a chemical reaction takes place only when atoms rearrange (Sensese, 2005).

            For Democritus and Dalton, atom is the tiniest matter and atom can never be destroyed. However, modern atomic theory says that atoms can break and this was the result of the nuclear atomic theory. J.J. Thomson found out the presence of electrons in an atom when the light passing through his cathode ray tube deflected when he applied an electric field. Thomson had thought that this bending of light was due to the negative charges of the light giving a reason that light also has also a particle nature aside from behaving as a wave. These negative charges are from the atoms that made up the light and he called it corpuscles, which was later named electrons. Thomson then came up with the structure of the atom with his plum pudding model wherein the electrons are scattered in a mass of a positively charged soup.

            Rutherford made a follow-up with Thomson’s model of the atom. When he did an experiment with a alpha particles, he found out that atom is most likely an empty space and the positive charges on the atom is concentrated only at a certain region and this concentrated mass of positive charges is the nucleus. He made this conclusion when he shone alpha particles through the gold foil and the result was that not all alpha particles were able to pass through, some were deflected and some were reflected. In his model, electrons move freely around the atom and the nucleus is at the center.

However, when electrons move, energy is required, which means, time will come that the electrons would have to stop from moving. But if it remains immobile, the nucleus could attract the negative charges. Bohr came up with the planetary model of the atom in which the electrons are allowed to move in orbits and only on these orbits.

            Further studies showed that around the nucleus is an electron cloud by which there is a probability of finding an electron. The electrons move freely within these orbital, not necessarily around the nucleus like in the planetary model, depending on its energy (de Leon, nd).

Works Cited

De Leon, N. n.d. “Modern Atomic Theory.” 22 July 2009 <>

_____________.”Dalton’s Atomic Theory.” 22 July 2009 <>

Senese, Fred. 25 July 2005. “Dalton’s Atomic Theory.” 22 July 2009 <>

“History of the Atom Part 1: Democritus to Dalton” 22 July 22, 2009 <>

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