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Democritus He said you cannot cut a piece of matter infinitely, but at some point you would get the smallest piece of matter possible. All matter is composed of tiny, indivisible particles called “atomos”. 450 BC Greek Philosopher

John Dalton 1803-School teacher Studied the ratios in which elements combine in chemical reactions Formulated first Atomic theory of matter: Each element is composed of extremely small particles called atoms. All atoms of a given element are identical, but they differ from those of any other element Atoms are neither created nor destroyed in any chemical reaction. A given compound always has the same composition or ratio of each atom

JJ Thomson England 1897 He discovered the electron while experimenting with cathode rays. He determined that the cathode ray was made of negatively charged particles – electrons. He also was able to estimate that the mass of the electron was equal to about 1/1800 of the mass of a hydrogen atom. His discovery of the electron won the Nobel Prize in 1906. He showed that the production of the cathode ray was not dependent on the type of gas in the tube, or the type of metal used for the electrodes. He concluded that these particles were part of every atom. He created the “plum pudding model” of the atom.

Cathode Ray Tube Experiment Performed by JJ Thomson Showed the existence of the electron. The ray is negative and composed of electrons. The negative end of a magnet attracts the beam.

Thomson’s Model of the Atom Called the “plum pudding” model The atom was comprised of positively charged matter with negative electrons “stuck” in the positive “pudding”

Ernest Rutherford Born in New Zealand 1871-1937 Tested Thomson’s theory of atomic structure with the “gold foil” experiment in 1910. Bombarded thin gold foil with a beam of ‘alpha’ particles. If the positive charge was evenly spread out, the beam should have easily passed through. All of the positive charge and most of the mass of an atom are concentrated in a small core, called the nucleus. Gold Foil Experiment (alpha scattering) he determined that an atom’s positive charge and most of its mass was concentrated in the core (most of the atom is empty space) he named the core “the nucleus”

Gold Foil Experiment Performed by Ernest Rutherford Positive alpha particles were shot at a thin sheet of gold foil The deflection of the beam indicated the existence of a small, positively charged nucleus.

Rutherford’s Model of the Atom Contained a positive nucleus Electrons were around the outside of the nucleus

James Chadwick Neutrons- Discovered in 1932 (England 1891-1974). He bombarded Beryllium with alpha particles. Neutrons were emitted and in turn ejected protons from paraffin.

Robert Millikan measured the charge of an electron using the oil drop experiment. x-rays gave the oil a negative electron 1.60x10^-19 coulomb is the charge of an electron using Thomson’s charge to mass ratio, he determined the mass of the electron is 9.11x10^-28g

Oil Drop Experiment Performed by Robert Millikan Determined the charge of the electron. 1909

Bohr Model of the Atom Nucleus in center with protons and neutrons Electrons are orbiting the nucleus in a set pattern Each orbital has a unique amount of energy

Quantum Model of the Atom (Schrodinger) Contains a nucleus with protons and neutrons Electrons move very quickly around the nucleus, not in a circle or orbit, but randomly within an electron cloud.

Extra Scientist Information Joseph Proust (1799) Law of Definite Proportions His work helped to inform Dalton’s Atomic Theory Michael Faraday (1832) Electromagnetism Discovered Electrolysis – splitting molecules with an electrical current

Extra Scientist Information Cont. Henri Becquerel (1896) Worked with x-rays and photographic film and discovered the idea of “radioactive decay” Did not have a name for radioactivity Marie Curie (1898) Coined term radioactivity Worked with uranium and thorium Discovered Polonium and Radium Hans Geiger (1906) Developed Geiger Counter to detect radioactivity

Extra Scientist Information Cont. Albert Einstein(1905) Wrote a paper convincing people atoms exist Put pollen in water Observed the pollen “dancing” and being “bumped” around by water molecules Louis de Broglie (1924) Posed the question, “If light can be viewed in terms of both “wave” and “particle” properties, why can’t particles of matter, like electrons, be treated the same way?” His question/idea was later proved to be correct