Mullis1 Discovery of Atomic Structure By 1850 scientists knew that atoms were composed of charged particles. Electrostatic attraction: Like charges repel.

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Presentation transcript:

Mullis1 Discovery of Atomic Structure By 1850 scientists knew that atoms were composed of charged particles. Electrostatic attraction: Like charges repel Opposites attract

Mullis2 Cathode Rays and Electrons C.R. 1 st discovered in mid-1980s from studies of electrical discharge thru partially evacuated tubes (CRTs) Cathode rays = radiation produced when high voltage is applied across the tube. The voltage causes negative particles to move from the negative electrode (cathode) to the positive electrode (anode). The path of electrons can be altered by the presence of a magnetic field.

Mullis3 Consider cathode rays leaving the positive electrode through a small hole…. If they interact with a magnetic field perpendicular to an applied electric field, then the cathode rays can be deflected by different amounts. Amount of deflection depends on applied magnetic and electric fields. Deflection also depends on the charge-to-mass ratio of an electron. Thomson determined the charge-to-mass ratio of an electron in Charge-to-mass ratio = 1.76 x 10 8 C/g C: Coulomb, SI unit of electric charge

Mullis4 Millikan Oil-Drop Experiment Sprayed oil drops over the hole in a positively charged plate and measured the electrostatic force of attraction. Found the charge on the electron to determine its mass Concluded the charge on the electron must be 1.60 x C Mass of electron = 1.60 x C = 9.10 x g 1.76 x 10 8 C/g

Mullis5 Radioactivity (Spontaneous emission of radiation) TypeSymbolChargeMass (amu) Alpha particle He Beta particle β Positron β Gamma ray γ 00

Mullis6 A Positively Charged Nucleus Rutherford shot alpha particles though a thin piece of gold foil. Some of these particles were deflected instead of passing straight through Recall “like repels like.” When a + alpha particle encountered a nucleus of a gold atom, it was deflected by the dense positively charged nucleus.

Mullis7 Scientist Contributions Thomson: “Discovered” electron (1897) Cathode ray experiments “Plum pudding” atomic model Millikan: Mass of electron Oil-drop experiment (1909) Rutherford: Positively charged nucleus (1911) Gold foil experiments Discovered proton (1919) Chadwick: Discovered neutron (1932)

Mullis8 Small Numbers Electronic Charge: x C Charge on an electron: x C Charge on a proton: x C Atomic Mass Unit (amu): x g Proton mass: amu Neutron mass: amu Electron mass: x amu Unit of length used to note atomic dimensions: 1 Angstrom (Å) = 1x m

Mullis9 Atomic Number Number of protons or electrons in an element Identifies the element Atomic Mass Nucleus contains most of the mass of an atom. Protons and neutrons are each ~ 1.67 x g. Electrons are each ~ 9.11 x g. Use atomic mass unit (amu) instead of gram. The mass of one proton is ~ 1 amu. Mass Number The sum of the number of protons and number of neutrons in the nucleus Is approximately equal to the average atomic mass shown on periodic table. Number of neutrons = mass number – atomic number

Mullis10 Isotopes Atoms of the same element with different numbers of neutrons Have the same number of protons Example: Carbon-12 and Carbon-14 Radioactive Isotopes Unstable in nature Can be used to date fossils and rocks The time it takes for half of the radioactive atoms in a piece of the fossil to change to another element is its half-life.

Mullis11 Isotopes A X Isotopes have the same Z, but different A. Isotopes have different numbers of neutrons. An atom of a specific isotope is called a nuclide. Nuclides of hydrogen include: 1 H = hydrogen (protium) 2 H = deuterium (heavy hydrogen) 3 H = tritium ( 3 H is radioactive.) Z