History Of AtomsBy: Abdulraheem Abu Issa

400 BC Democritus said that everything is

made out of atoms. Everything is made up of void and atoms. Depending on the arrangement, the density varies with less void being denser. His discovery has helped the development of the modern atomic theory.

1704Newton recognized that there are forces between atoms and that they affect the chemical properties of matter. He helped publicists understand that atoms affect the chemical properties of matter and helped us understand that atoms are what make everything work.

1803John Dalton was an English scientist who theorized that chemical element atoms have a unique atomic weight and can be placed in order on what is known today as the periodic table of elements.

1897Stoney suggested that a subatomic particle existed which was a particle of electricity held within the atom, named the electron.This helped us understand that within the atom, there are other particles and that atoms have a charge (as electricity is charged).

1900Planck discovered quantum theory, by studying the nature of light and other radiation. He hypothesised at very small scales, that matter was discrete rather than continuous.This means that we know that atoms behave differently to the human-scale world that we know and perceive.

1903 Nagaoka developed an early, incorrect "planetary model" of the atom which was based around an analogy to the explanation of the stability of Saturn’s rings. It had a massive nucleus, with electrons revolving around the nucleus, bound by electrostatic forces. However, this analogy was wrong although the electrons revolving around the nucleus was correct, and was abandoned in 1908.This model (although incorrect) helped develop the idea of the nucleus of an atom being orbited by electrons.

1909Millikan measured the charge on an electron with his oil-drop apparatus. He also proved that this quantity was a constant for all electrons (1910), thus demonstrating the atomic structure of electricity. This means that we know that all the electrons in every atom has the same charge.

1911Rutherford formulated a model of the atom, which was like this: a very small positively charged nucleus, orbited by electrons. He also speculated on the existence of ‘neutrons’, which could somehow compensate for the repelling effect of the positive charges of protons by causing an attractive nuclear force and thus keeping the nuclei from breaking apart.Rutherford discovered the basic form of an atom regarding protons and electrons.

1944Moseley determined the charge of most nuclei using X-rays. He also discovered that an element's atomic number and number of protons were equal, leading to the Periodic Table of Elements being arranged by atomic number instead of atomic mass, which helped organise it much better.It meant that all atoms of different elements have different weights.

1922He suggested that when an atom is heated, electrons can jump to higher levels & when the atom cools, they drop down to lower levels. He predicted the levels (or shells) had a maximum capacity of electrons each, the innermost being 2.This helps us understand why atoms release light and energy at certain frequencies.

1932James Chadwick, an English scientist, used alpah particles in an experiment and discovered a neutral atomic particle with a mass close to a proton. Thus the neutron was discovered.

Bibliography http://www.timetoast.com/timelines/84510