Atomic Theory-Democritus Greek philosopher, 400 BC Named the smallest piece of matter “atomos,” meaning “not to be cut.” Democritus’ atoms were small, hard particles that were all made of the same material but were different shapes and sizes. Atoms were infinite in number, always moving and capable of joining together.

Aristotle and Plato earth, fire, air and water Their ideas held sway because of their eminence as philosophers. The atomos idea was buried for approximately 2000 years.

John Dalton Early 1800s English Chemist Performed a number of experiments that eventually led to the acceptance of the idea of atoms. Became the foundation of modern chemistry! First theory based on scientific evidence.

Dalton’s Theory 1. Atoms cannot be divided! NO! Subatomic Particles 2. All atoms of a particular element are identical! NO! Isotopes 3. Different elements have different atoms YES! 4. Atoms combine in whole-number ratios only! YES! Called the Law of Definite Proportions 5. In a chemical reaction, atoms rearrange only. Atoms are not created or destroyed. YES! Called the Law of Conservation of Mass

J.J. Thomson 1897 English scientist Plum pudding model Atom is made of even smaller particles.

Thompson Model Cathode ray tube - Electric current through a gas Rays went from negative end to positive end – particles were NEGATIVE! Wheel inside spun – particles had MASS! Since atoms are neutral, there also must be positive subunits???

Ernest Rutherford 1908 English physicist Fired a stream of tiny positively charged particles at a thin sheet of gold foil (2000 atoms thick)

Gold Foil Experiment Most particles passed right through the gold atoms in the sheet of gold foil without changing course at all - Atom was mostly empty space. Some particles did bounce away from the gold sheet as if they had hit something solid - Atoms had a dense, positive center surrounded by negative charges.

Discovery of the Neutron Irene and Frederick Joloit-Curie Sir James Chadwick 1930’s Expanded Rutherford’s work

Niels Bohr In 1913 Danish scientist Theory placed each electron in a specific energy level around the nucleus, much like planets circle the sun. Orbits are located at certain distances from the nucleus.

The Quantum (Wave) Model Today’s atomic model Electrons do not move about an atom in a definite path, Impossible to determine the exact location of an electron. Probable location of an electron is based on how much energy the electron has. Atom has a small positively charged nucleus surrounded by a large region in which there are enough electrons to make an atom neutral.

Electron Cloud: A space in which electrons are likely to be found. Electrons whirl about the nucleus billions of times in one second They are not moving around in random patterns. Location of electrons depends upon how much energy the electron has. More energy = further from the nucleus

Atomic Scientist Eulogy Choose an atomic scientist from the list on the atomic timeline link (No Einstein) Write-up a eulogy that you will present next class period at the funeral 250 words max. Rubric for project on p16 of workbook Links to use for project http://atomictimeline.net/index.php www.eulogyspeech.net