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The Atom • Smallest particle of an element • All matter is created of atoms • Comes from the Greek word atomos • “indivisible” Democritus (460 – 370 B.C.E.) • Greek philosopher • First to propose the idea of the atom’s existence • Theorized that the atom is indivisible and indestructible, but had no experimental evidence John Dalton (1766-1844) • English chemist and teacher • Pioneered the modern discovery process about atoms • Transformed Democritus’s theories on atoms into an actual scientific theory How did Dalton create his theory? • Studied the way elements combined in chemical reactions and the ratio in which they combined • Ex: Liquid mercury • All drops have the same properties • All drops are made of the same atoms Dmitri Mendeleev • A Russian chemist and teacher • Arranged the then 60 known elements in order by increasing atomic mass • Created the first periodic table in 1869 J. J. Thomson (1856-1940) • Discovered the electron in 1897 • Passed electric current through a tube filled with gas and observed a glowing beam known as a cathode ray • Concluded all atoms contained electrons Robert A Milikan (1868-1953) • U.S. Physicist • Calculated the charge of an electron in 1909 • 1.60 x 10-19 Coulomb • Also calculated the mass of the electron • 1/1840 the mass of a hydrogen atom Eugen Goldstein (1850-1930) • Discovered positively charged subatomic particles called protons in 1886 • Used a variation of the cathode ray • Calculated the mass of the proton • 1840 times the mass of the electron James Chadwick (1891-1974) • Confirmed the existence of neutrally charged particles, the neutron, in 1932 • Calculated that neutrons had a mass equal to the mass of a proton Ernest Rutherford (1871-1937) • Former student of J. J. Thomson • Devised the gold-foil experiment in 1911 • Created a new atomic model based on his findings Niels Bohr (1885-1962) • Danish student of Rutherford • In 1913 he proposed that an electron is found only in specific circular paths known as orbits around the nucleus Louis de Broglie (1892 – 1987) • French Physicist • In 1923, he proposed that all moving particles, including electrons, had properties of waves • Influenced Schrödinger's work Erwin Schrödinger (1887-1961) • Austrian physicist • Devised and solved a mathematical equation to describe the true behavior of electrons in atoms in 1926 • Known as the quantum mechanical model Modern Day Model of the Atom
