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History of the Atomic Theory Objectives • Understand the “evolution” of atomic theory. • Discuss how old accepted scientific thought gives way to new ideas. • New technology leads the way to new ideas. Democritus • 400 BC Philosopher • Matter could not be infinitely divided. • A smallest unit would be discovered. • This atomos would be the smallest possible piece of matter. 300 BC Egyptian alchemists... •believed all metals on earth sought to become more and more perfect, thus gradually changing to gold. This really caught on by 100 AD 13th & 14th c. Alchemists sought To transmute metals to gold Atomic Theory • John Dalton - Atomic Theory – All matter is composed of indivisible atoms – All atoms of a given element are identical – Atoms of different elements physically mix, or chemically combine in simple, whole number ratios to form compounds – Reactions occur when atoms combine, separate, or rearrange Sub-Atomic Particles • It turns out, atoms are divisible. • J.J. Thomson in 1897 investigating electricity with his cathode ray tube, discovered negatively charged electrons. • He tested many elements in his Cathode ray tube, and found that all cathode rays had the same charge and mass. He reasoned that all elements have these particles. • Since atoms are normally neutral, Thomson proposed the electrons where embedded in a positively charged ball The Cathode Ray Experiment Plum-Pudding Model Electrons Positive ball This model didn’t last long….. Characteristics of the electron • Robert Millikan conducted his famous “Oil Drop” experiment to determine the charge to mass ratio of an electron. • He found that the charge is equal and opposite to a proton. Defined as 1• The mass is much smaller than a proton and equal to 1/1840 of a proton. Ernest Rutherford • Experiments with radio active elements such as radium and uranium showed that these emit high energy particles called alpha particles. • Alpha particles were known to have a positive charge. • They could be used as projectiles to fire at atoms to probe their composition. Ernest Rutherford’s Experiment Rutherford’s Model Rutherford Concluded... • There was a localized positive charge (nucleus) • Nucleus must have a large mass • Lots of empty space • Protons are + charged particle Imagine a marble on the 50 yd line….If the entire atom is the stadium, then the marble is the size of the nucleus Atomic Mass • Although atoms are very small, they have measurable mass. • With Rutherford’s proton and electron model, there was still unaccounted for mass!?! Irene Joliot-Curie & James Chadwick accounted for the extra mass in a particle called the neutron which did not have a charge. Neutrons are located in the nucleus with the protons. What does an atom look like? • It’s agreed that the protons and neutrons make up a nucleus. • Rutherford suggested electrons orbit the nucleus like planets orbit the sun. • But orbits would decay Quantum Theory states... • Electrons are in orbitals (cloudlike) which are where you can expect to find electrons The Atom, finally... Nucleus protons neutrons Electrons in orbitals (cloud like) http://www.youtube.com/watch?v=vUzTQWn-wfEatom song