The History of the Atom

Learning Target: Identify major contributions to the modern theory of atomic structure brought forth by John Dalton, J.J. Thomson, Robert Millikan, Ernest Rutherford, Niels Bohr, and James Chadwick.

John Dalton Born 1766 Did most work from 1808-1827 England First to develop a theory for the structure of an atom - Based on five major principles http://www.uh.edu/engines/epi1411.htm

Dalton’s Atomic Theory Five Major Principles: Elements are made of indivisible atoms All atoms of the same element have the same properties and the same mass Compounds are made of atoms of different elements combined together Chemical reactions involve the reorganization of those atoms Atoms cannot be created nor destroyed http://uh.edu/engines/jdsymbols.jpg

Dalton’s Atomic Theory The first and second principles: Elements are made of indivisible atoms AND All atoms of the same element have the same properties and the same mass “Atomos” His thought was that each element existed as a simple spherical substance, different in physical and chemical properties from any other element This is his MODEL of the ATOM http://uh.edu/engines/jdsymbols.jpg

Dalton’s Atomic Theory The third principle: Compounds are made of atoms of different elements combined together When Dalton was able to separate a substance, he called it a compound. Remember, elements could not be separated into smaller parts. Unlike a mixture, compounds are chemically combined and can not be separated without a chemical change. http://uh.edu/engines/jdsymbols.jpg

Dalton’s Atomic Theory The third principle… what else does it tell us? When a particular compound is created, Dalton stated that it must be created the exact same way each time. Compound #31 must always be made of three empty spheres (oxygen) with one sphere with a cross (sulphur). This idea is called the Law of Definite Proportions – when elements combine, they do so in a ratio of whole numbers. http://uh.edu/engines/jdsymbols.jpg

Dalton’s Atomic Theory The third principle… can different compounds be made of the same elements? Yes Take a look at compounds #26 and #27  One represents what Dalton believed to be nitrous oxide and the other nitric acid. These are an excellent example of the Law of Multiple Proportions, which states that if two elements form more than one compound between them, then the ratios of the different compounds must be different from each other. http://uh.edu/engines/jdsymbols.jpg

Dalton’s Atomic Theory The fourth principle: Chemical reactions involve the reorganization of those atoms

Dalton’s Atomic Theory The fifth principle: Atoms cannot be created nor destroyed In all chemical reactions the total mass of the original chemicals equals the total mass of the created chemicals. This is known as the Law of Conservation of Mass.

Dalton’s Atomic Theory It’s flawed! Are atoms only spheres? Or, is there something else inside?

Dalton’s Atomic Theory Another flaw – Look at compound #21, Dalton’s version of water; that’s one oxygen with one hydrogen. Isn’t water made with two hydrogen atoms?

Dalton’s Atomic Theory So why is it still studied? Despite these major flaws, Dalton opened the door to enormous understandings of atoms, compounds, and reactions that we still use today. And, three major laws that are also still important – Law of Definite Proportions, Law of Multiple Proportions, and Law of Conservation of Mass.

Lived 1856 – 1940 England Did work from 1884 - 1914 J.J. Thomson Lived 1856 – 1940 England Did work from 1884 - 1914 http://upload.wikimedia.org/wikipedia/commons/c/c1/J.J_Thomson.jpg

Thomson’s Experiment Thomson was fascinated by the idea that electricity could cause a trapped gas to glow. He called his device a cathode ray tube, or Crooke’s tube; these are still used today as standard fluorescent bulbs, neon lights, and spectroscope tubes, to name a few current models.

Thomson’s Experiment It’s not the cathode ray tube that’s fascinating, or useful to the development of the atom, however. It’s the fact that magnets held against the tube could deflect (or attract) the light beam inside.

Thomson’s Atomic Model The Plum Pudding Model The electrons were thought to be positioned throughout the atom, but the atom was also was said to have had a "cloud" of positive charge. Thomson won the Nobel prize for the discovery of the electron in 1906.

Robert Millikan Lived 1868 - 1953 University of Chicago! Experimenting with the input of Harvey Fletcher

Millikan’s Experiment of 1909

Millikan’s Results Millikan’s Oil Drop Experiment, published in 1913, helped the atom in two ways.

Millikan’s Results First, it showed the charge on the electron… 1.6x10-19 coulombs. And second, it showed the mass of the electron was 1/2000th of a proton.

Millikan’s Results And second, it showed the mass of the electron was 1/2000th of a proton. Thomson had already determined the charge-to-mass ratio for the electron, but the actual values were unknown. Millikan was able to calculate it.

Ernest Rutherford Lived 1871 - 1937 New Zealand origin, England lab One of Thomson’s students

Rutherford’s Experiment Alpha Particle 4 +2 2 He Rutherford’s Gold Foil Experiment, in 1909-1911, predicted that all particles would go straight through the atom described by the Plum Pudding Model.

Rutherford’s Experiment That’s not what happened, though! Some particles went through the foil while others were deflected and still others bounced back.

Rutherford’s Atomic Model The Planetary Model Rutherford proved that there was a positively charged, very dense, very heavy center, called the nucleus. While the rest of the atom was empty space. He is credited with the discovery of the proton.

Niels Bohr Lived 1885 - 1962 1913 published theory on atomic structure Denmark

Bohr’s Experiment Bohr’s Hydrogen Energy Level Experiment used a spectroscope to separate the colors of a heated hydrogen gas. What could cause specific colors of light to be created, rather than a full continuous spectrum?

Bohr’s Experiment Specific colors of light have specific wavelengths of specific energy. This led Bohr to theorize that there are levels, called energy levels, around the atom’s nucleus for electrons to travel between

Bohr’s Experiment Bohr realized that electrons must absorb energy as the hydrogen gas is heated. When the electron absorbs the energy, it moves to a higher energy level. The electron eventually releases that energy, as a wavelength of color, falling to a lower energy level.

Bohr’s Atomic Model The Orbital Model Electrons travel in orbits, now called energy levels, around the atom's nucleus. The variable “n” represents the energy levels. http://www1.assumption.edu/users/bniece/spectra/tutorials/as_bohr.html

Bohr’s Atomic Model Bohr’s Orbital Model is still not the current model of the atom. It did, however, determine some important information about atoms. The chemical properties of each element can largely be determined by the number of electrons in the outer orbits of its atoms. Bohr’s work became the basis for quantum mechanics which is the current model of the atom.

James Chadwick Lived 1891 - 1974 Major discovery: 1932 England

Chadwick’s Experiment Alpha Particle Bombardment Alpha particles (the same particles that Rutherford used) were used to bombard a sample and it was discovered that neutral particles were released that changed the mass of the atom.

Chadwick’s Atomic Model The neutron was the last subatomic particle discovered. Chadwick won the Nobel Prize for his discovery of the neutron in 1935.

Erwin Schrödinger Lived 1887 - 1961 Germany

Schrödinger’s Atomic Model Louis de Broglie had determined that electrons moved in wave-like motion.

Schrödinger’s Atomic Model This meant that electrons were not moving around the nucleus in orbits, as described by Bohr, but, rather, in clouds. The Cloud Model

Schrödinger’s Atomic Model Joining this idea with the theories of the scientists before him, Schrödinger developed the Schrödinger equation. The Schrödinger equation and it’s use of quantum mechanics is the current model of the atom.