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HISTORY OF THE ATOM ATOMISM 5th century BC, Ancient Greece – Leucippus, Democritus Both from the Ionian school of naturalistic philosophy The earliest proponents of the concept of atomism “All matter is composed of indivisible particles called atoms.” Proposed the earliest views on the shapes and connectivity of atoms. “They reasoned that the solidness of the material corresponded to the shape of the atoms involved. Thus, iron atoms are solid and strong with hooks that lock them into a solid; water atoms are smooth and slippery; salt atoms, because of their taste, are sharp and pointed; and air atoms are light and whirling, pervading all other materials.” BILLIARD BALL MODEL – “CHEMICAL ATOMISM” ~1800, Great Britain – John Dalton Elements are made of tiny particles (small solid spheres) called atoms. All atoms of a given element are identical. Compounds are composed of atoms in specific ratios. Chemical reactions are rearrangements of atoms (mass is conserved). DISCOVERY OF CATHODE RAYS ~1860, Germany A heated cathode of an electric circuit in a vacuum with a large potential difference applied between the electrodes emitted a strange beam that was thought to be an electromagnetic wave. DISCOVERY OF THE ELECTRON ~1900, Great Britain – J. J. Thomson Investigated cathode rays. Can be deflected in their path by magnetic and electric fields → carry a charge. Concluded that these rays were actually particles, later called electrons. Qe Able to measure their charge-to-mass ratio . me THOMSON’S PLUM PUDDING MODEL The atoms consist of a number of negatively electrified corpuscles enclosed in a sphere of uniform positive electrification. Atoms are neutral → the total neg. charge = the total pos. charge. MILLIKAN’S OIL DROP EXPERIMENT ~1910, USA – R. A. Millikan Charged droplets of oil between two metal electrodes State of mechanical equilibrium between the downward gravitational force and the upward electric force The charges were all multiples of a fundamental value, calculated to be 1.602×10−19 C → charge is quantized. RUTHERFORD’S GOLD FOIL EXPERIMENT ~1910, New Zealand/Great Britain – E. Rutherford Beam of alpha particles (helium atoms without electrons) was directed onto a sheet of very thin gold foil. RUTHERFORD’S GOLD FOIL EXPERIMENT Expected Results Observed Results Expected Results Observed Results Thomson’s Model Rutherford’s Model Conclusion: The atom is mostly empty space with a small, dense, positively charged nucleus surrounded by negative electrons. DISCOVERY OF THE PROTON ~1920, E. Rutherford Alpha particles were shot into nitrogen gas → scintillation detectors showed the signatures of hydrogen nuclei. This hydrogen could only have come from the nitrogen → nitrogen must contain hydrogen nuclei. о The hydrogen nucleus is present in other nuclei as an elementary particle named proton. (Greek πρτον = first) DISCOVERY OF THE NEUTRON ~1930, Great Britain – James Chadwick Bombarded beryllium with alpha particles. Radiation emitted by beryllium stroke into paraffin wax. Protons shot out from the paraffin wax. What was in the “beryllium radiation”? DISCOVERY OF THE NEUTRON The radiation not affected by electric or magnetic field → neutral. Photons would be too “weak” to shoot out protons. These new neutral particles have approximately the mass of protons. Named neutrons. ATOMIC NUCLEUS – SUMMARY a very dense region at the center of an atom consisting of nucleons: protons and neutrons accounts for almost all the mass of an atom ~10-15 m in diameter A Z Atomic (Proton) Number Z The number of protons X Mass (Nucleon) Number A in an atomic nucleus identical to the charge in an atomic nucleus number of the nucleus uniquely identifies a chemical element in a neutral atom, equal to the number of electrons The number of nucleons different for each different isotope of a chemical element e.g., common 126C has 6 n0 while radioactive 146C has 8 n0 Chemical Element Is composed of atoms with the same Z Naturally a mixture of different isotopes Nuclide Is composed of atoms with the same Z and A PERIODIC TABLE OF THE ELEMENTS ELEMENTARY (?) PARTICLES Electron is not known to be made up of smaller particles Mass me 9.11031 kg, charge Qe 1.6 1019 C BUT Protons and neutrons ARE known to be made up of QUARKS (though these do not exist separated or isolated) Mass m p mn 1800 me , charge Qp 1.6 1019C QUESTIONS 1) What is the charge of an electron? 2) What did Millikan find out in his experiment? 3) How was an atomic nucleus discovered? 4) What is the charge of a proton? 5) What is the charge of a neutron? 6) What is the difference between a chemical element and a nuclide? 7) 3 4 2 14 What particles are 1 H , 2 He , 6 C, 8) What do you know about quarks? 235 92 U atoms composed of?
