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Structure of the atom The ancient Greeks were the first to postulate that matter consists of indivisible constituents. Later scientists realized that the atom consisted of charged entities. EARLY 1900s CRT (Cathode Ray Tubes) JJ Thompson 1. Rays are same no material what material is used to generate them. 2. Rays are stream of negatively charged particles. Charge of electron determined later by Milliken Structure of atom Studies of radioactivity: matter is composed of negative, positive and neutral material J.J. Thompson’s interpretation: cookie dough model Structure of Atom • ~1910 Earnest Rutherford tested Thompson’s Model Nuclear Atom Rutherford concludes: massive nucleus with electrons orbiting around it. Modern View of atom • An atom is almost entirely empty space. 1nm = 10-9 m 10Å = 1nm Å= angstrom Atom Diameter = 0.1-0.5 nm Nuclear diameter = 10-5 nm Ratio of nuclear size to atomic radius: 10-4 Carbon atom diameter = 0.154 nm Modern View of Atom COMPONENTS OF ATOMS mass charge (a.m.u.) (a.u.) proton p electron e neutron n ~1 ~0 ~1 Neutral 19 9 Ion 39 19 F K + +1 −1 0 ISOTOPES • Number of protons = atomic number • DEFINES identity of element For a given element, number of protons is fixed, but number of neutrons can vary. 35 17 Cl 37 17 Cl Isotopes: atoms of a given element with a different number of neutrons. The Atomic Mass Scale • We define: mass of 12C = exactly 12 amu. 1 amu = 1.66054 x 10-24 g 1 g = 6.02214 x 1023 amu • Using atomic mass units: 1H weighs 1.6735 x 10-24 g 16O 2.6560 x 10-23 g. Average Atomic Masses Relative atomic mass: average masses of isotopes: Naturally occurring C: 98.892 % 12C + 1.108 % 13C. Average mass of C: (0.98892)(12 amu) + (0.0108)(13.00335) = 12.011 amu. Atomic weight (AW) is also known as average atomic mass (atomic weight). Atomic weights are listed on the periodic table. The Mole and Avogadro’s Number Mole: connection between microscopic (what we don’t see: atoms) and macroscopic (what we see) 1 mole of anything (atoms, molecules, green peas) = Avogadro’s number of them 1 mole 12C atoms has 6.02 × 1023 12C atoms. 1 mole CO2 molecules = 6.02 × 1023 CO2 molecules 1 mole peas = 6.02 × 1023 peas Experimentally, 1 mole of 12C has a mass of 12 g. CRT Cathode Rays and Electrons • A cathode ray tube (CRT) is a hollow vessel with an electrode at either end. • A high voltage is applied across the electrodes. • The voltage causes negative particles to move from the negative electrode to the positive electrode. • The path of the electrons can be altered by the presence of a magnetic field. CRT Consider cathode rays leaving the positive electrode through a small hole. • If they interact with a magnetic field perpendicular to an applied electric field, the cathode rays can be deflected by different amounts. • The amount of deflection of the cathode rays depends on the applied magnetic and electric fields. • In turn, the amount of deflection also depends on the charge to mass ratio of the electron.