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 Most atom diameters are between 1 x 10 10 m 100 pm and 5 x 10 10 m 500pm or 1 5 Angstrom SI unit of length for atomic dimensions o 1 1 x 10 10 m Electrons attracted to protons of nucleus by electrostatic force between particles of opp electrical charge Atomic mass unit amu 1 amu 1 66054 x 10 24 g Diameter of atomic nucleus 10 4 Atomic Numbers Mass Numbers and Isotopes Atoms of each element have characteristic number of protons Atomic number Number of protons of element Mass number Number of protons plus neutrons in atom Isotopes Atoms with identical atomic numbers but different mass numbers same protons different neutrons 2 4 Atomic Weights The Atomic Mass Scale Hydrogen lightest atom arbitrarily assigned relative mass of 1 12 C has exactly 12 amu Atomic Weight Atomic weight Average atomic mass of element The Mass Spectrometer Mass spectrometer Most accurate means of determining atomic weights Mass spectrum Gives masses of ions and relative abundances 2 5 The Periodic Table Periodic table The arrangement of elements in order of increasing atomic number with elements having similar properties placed in vertical columns Periods Horizontal rows of periodic tables o First has just H and He o 7 periods Groups Vertical columns o 1 18 o Elements in group have similarities in physical and chemical properties o Alkali metals 1 alkaline earth metals 2 chalcogens 16 halogens 17 noble gases or rare gases 18 o Metallic elements metals Left and middle of periodic table luster high electrical and heat conductivity solid at room temp except Hg o Nonmetallic elements nonmetals Separated from metals by line from B to At varying state of matter o Metalloids Elements with properties of metals and nonmetals 2 6 Molecules and Molecular Compounds Only noble gases are normally found as isolated atoms Most matter is composed of molecules or ions Molecules and Chemical Formulas Many elements found in molecular form two or more of same type of atom bonded together Chemical formula Ex O2 Diatomic molecule Molecule made up of two atoms o H O N F Cl Br and I elements form diatomic molecules Oxygen O2 and ozone O3 exhibit different chemical and physical properties Molecular compounds Compounds composed of molecules containing more than one type of atom Most molecular substances contain only nonmetals Molecular and Empirical Formulas Molecular formulas Chemical formulas that indicate the actual numbers of atoms in a molecule Empirical formulas Chemical formulas that give only the relative number of atoms of each type in a molecule o Subscripts are always smallest possible whole number ratios Ex H2O2 can be written as HO Molecular formula and empirical formula can be identical Empirical formula can be determined from molecular but not the other way Picturing Molecules Structural formula Formula that shows which atoms are attached to which o