Ch 2 Atoms Molecules and ions 2 1 Atomic Theory of Matter Democritus and other Greek philosophers described indivisible particles called atomos o Plato and Aristotle established there are no indivisible particles Dalton s Atomic Theory 1803 1807 o Each element is composed of extremely small particles called atoms o All atoms of a given element are identical but the atoms of one element are different from the atoms of all other elements o Atoms of one element cannot be changed into atoms of a diff element by chemical rxns atoms are neither created nor destroyed in rxns Law of conservation of mass Total mass of materials present after chemical rxn is same as total mass present before rxn o Cmpds are formed when atoms of more than one element combine a given cmpd always has the same relative number and kind of atoms Law of constant composition In given cmpd relative numbers and kinds of atoms are constant Dalton s theory used to deduce law of multiple proportions If two elements A and B combine to form more than one cmpd masses of B that can combine with given mass of A are in ration of small whole numbers 2 2 The Discovery of Atomic Structure Atoms is composed of subatomic particles Particles with same charge repel one another particles with unlike charges attract Cathode Rays and Electrons High voltage applied to electrodes in tube without air causing production of radiation which were cathode rays Cathode rays moved from neg electrode to pos electrode Rays detected by their causing certain materials to fluoresce or give off light Cathode rays deflected by electric or magnetic fields consistent with their stream of neg electrical charge J J Thomson 1856 1940 o Cathode rays are same regardless of cathode material type o Described them as streams of negatively charged particles o Paper is discovery of the electron o Used the cathode ray tube with hole in anode o Calculated 1 76 x 108 C g for ratio of electron s electrical charge to its mass Robert Millikan 1868 1953 o Millikan oil drop experiment o Measured charge of electron 1 602 x 10 19 C o With Thomson s ratio calculated electron mass as 9 10 x 10 28 g Radioactivity Henri Becquerel 1852 1908 o 1896 discovered uranium spontaneously emits high energy radiation o Radioactivity Spontaneous emission of radiation Ernest Rutherford o Discovered types of radiation alpha beta and gamma o Paths of and radiation bent by electric field in opp directions but radiation unaffected o Showed and rays have fast moving particles o particles have 2 charge attracted to neg plate o particles have 1 charge attracted to pos plate o Gamma has no particles and no charge The Nuclear Model of the Atom Plum pudding model Thomson s proposal that atom consists of a uniform positive sphere of matter in which mass is evenly distributed and in which electrons are embedded 1910 Rutherford studied how particles were deflected or scattered through gold foil o Most particles passed through o Some scattering at large angles o Nuclear model Most of mass of each atom and positive charge reside in dense region called nucleus o Empty space in atom electrons moving around nucleus o Particles were only deflected if in contact with the small positive nucleus Eventual discovery of positive particles protons by Rutherford and neutral particles neutrons by Chadwick 2 3 The Modern View of Atomic Structure Charge of electron is 1 602 x 10 19 C and is positive for protons Electronic charge Quantity 1 602 x 10 19 C Expressed in multiples electron is 1 and proton is 1 neutrons are neutral All atoms have an equal number of electrons and protons so have no net electrical charge