Atomic Theory Timeline

1. Atomic Theory Timeline Brady Haering Mr. Carley Pd.1 10/22/12

2. 1803: John Dalton In 1803, John Dalton published his Atomic Theory. His theory states that all elements are made of microscopic particles called atoms and that all of the atoms of one particular element are always identical. Also, the theory said that the atoms of on element are completely different from the atoms of all other elements. His theory also related to compounds. He stated that atoms can combine to form compounds and that a given compound will always have the same relative number and types of atoms. Finally his theory detailed that an atom is indivisible chemically and is neither created nor destroyed in chemical reactions. Dalton had a very basic model of an atom, believing it to be an extremely tiny particle with a set mass, size, and behavior.

3. 1897: J.J. Thomson Many scientist believed that there were more than one component to an atom. This was proved by J.J. Thomson in his cathode ray experiment. By measuring the heat signature produced by cathode rays colliding with a thermal junction, he was able to determine a cathode ray’s mass and compared it to the way the ray was reflected electronically. He found that the mass of a cathode ray was always the same, and concluded that the atoms consisted of tiny negatively charged particles later referred to as “electrons”. In Thomson’s model of the atom, the atom is a spherical particle with negatively charged electrons spread throughout. This model is known as the “raisin bread model” or the “plum pudding model”.

4. 1911: Ernest Rutherford In 1911, Ernest Rutherford conducted his now famous gold foil experiment. In his experiment he directed a beam of particles through an extremely thin layer of gold foil. Around the foil was a detector that gave of flashes where particles hit it. Most of the particles traveled directly through the foil, while some were slightly deflected and others were reflected back completely. He was able to conclude from these results that the Plum Pudding atom model was incorrect, that an atom had a dense center (later called a nucleus), an atom is mostly empty space, and that atoms must contain positively charged particles to balance the negative electrons. In 1919, he called them protons. His nuclear atom model consisted of a dense nucleus surrounded by electrons traveling through the empty space around it.

5. 1913: Niels Bohr In 1913, Niels Bohr constructed his model of the atom. In Bohr’s model, the atom consists of a small nucleus consisting of positively charged particles. Orbiting around this nucleus were eletrons, which rotated stably, in very definite orbits, without giving off radiation as this would cause the atom to collapse in upon itself.

6. 1926: Erwin Schrödinger/ Werner Heisenberg The current model of the atom was created in 1926 by Erwin Schrodinger and Werner Heisenberg. In this model of the atom, the nucleus is surrounded not by individual orbiting electrons, but by electron clouds, or waves, that remain stationary rather than orbiting. The electrons may move within the clouds but do not directly orbit the nucleus. This model of the atom also shows that the nucleus contains both protons and neutrons, neutrons not having been discovered until after the development of Bohr’s atomic model by Ernest Rutherford and James Chadwick. This model is the currently accepted model of the atom.