Leucippus, Democritus certa 450 B.C.
-The idea of the atom was first introduced by Leucippus of Miletus around 450 B.C.. Leucippus and his pupil, Democritus, theorized that all matter was composed of atoms, or bits of matter too small to be seen. The word "atom" comes from a Greek word, "atomos", meaning uncuttable, because Leucippus believed that atoms were the building blocks of all materials and therefore could not be broken down any further.
Aristotle-Aristotle didn't like Leucippus and Democritus idea about the atom . He claimed that there was no smallest part of matter and that different substances were made up of proportions of fire, air, earth, and water. As there were of course no experimental means available to test either view, Aristotle's prevailed mainly because people liked his philosophy better.
John Dalton-In 1803 Dalton came up with these theories1. Matter is made up of indivisible atoms.
2. All atoms of an element are identical.
Periodic table of elements, black&white
Atom radii in the periodic table of elements
The Periodic Table (Simon Basher book)3. Atoms are neither created nor destroyed.
4. Atoms of different elements have different weights and chemical properties.
5. Atoms of different elements combine in simple whole numbers to form compounds.
-It was not until 1850 that another atomic theory was proposed. Dalton stated that all matter is made of indivisible and indestructible atoms, which differ from element to element.The atom, it was assumed, was of uniform density and constitution.
William CrookesHis most important work was in the investigation of the conduction of electricity in gases. He developed the Crookes tube and in it produced cathode rays for the first time. He also invented the radiometer and the spinthariscope, a particle detector.
Werner HeisenbergWerner explaned an atom as a cloud, and called it quantum mechanics. Therefore, in quantum mechanics, there is not an electron circling aroung the atomic nucleus, but rather an electron...