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The Birth of Atomic Theory What is an atom like? What are atoms like? • Atomic theory – A theory about what atoms are like – Scientific theories • The best explanation possible using what the information that is known. Theories can be updated if new information is discovered. • Many scientists have made Atomic Theories over the years… Democritus- the Greek Philosopher (~400 B.C.) • Said the universe is made of super small particles called atoms • Said atoms cannot be divided into smaller parts • Movements of atoms cause all changes we can see John Dalton, the English chemist (1808 A.D.) • All matter is made of atoms, which cannot be divided. • Atoms of the same element are exactly the same • Atoms of different elements can join to form molecules. • A chemical reaction is a rearrangement of atoms. J.J. Thomson (1891) • Used cathode ray tubes to discover the electron. • An electron’s mass is 1,836 times smaller than a proton • Proved atoms are made up of smaller parts. • Created the “plum pudding” model of the atom. Ernest Rutherford (1911) • The gold foil experiment showed the Plum Pudding model was incorrect. • Rutherford discovered the nucleus Niels Bohr, the Danish scientist (1913) • Said there are different energy levels that describe how far an electron is from the nucleus • The further away from the nucleus, the higher the energy level. • Electrons can only orbit in certain locations. Schrödinger’s Model (1926) • Electrons do not move around the nucleus like planets around the sun • The exact location of electrons cannot be determined • Electrons are found in orbitals, or locations where they are most likely to be. Image Sources • Crookes tube- two views – by D.Kuru via Wikimedia Commons • J.J. Thomson portrait – via Wikimedia Commons • Plum pudding – by Musical Linguist via Wikimedia Commons • blueberry muffin – by mars! via Flickr • Plum pudding atom – by FastFission via Wikimedia Commons • Ernest Rutherford (Nobel) – via Wikimedia Commons • Rutherford atom – by cburnett via Wikimedia Commons • Orbital | 3, 2, 0 > – via The Physics Hypertextbook