CA StandardsStudents know how to relate the position ofan element in the periodic table to itsatomic number and atomic mass.Students know the nucleus of the atom ismuch smaller than the atom yet containsmost of its mass.
Modern Atomic Theory All matter is composed of atoms Atoms cannot be subdivided, created, ordestroyed in ordinary chemical reactions.However, these changes CAN occur innuclear reactions!Atoms of an element have a characteristicaverage mass which is unique to thatelement.Atoms of any one element differ inproperties from atoms of another element
Discovery of the ElectronIn 1897, J.J. Thomson used a cathode ray tubeto deduce the presence of a negatively chargedparticle.Cathode ray tubes pass electricity through a gasthat is contained at a very low pressure.
Conclusions from the Study of the Electron Cathode rays have identical propertiesregardless of the element used to producethem. All elements must contain identicallycharged electrons.Atoms are neutral, so there must bepositive particles in the atom to balance thenegative charge of the electrons Electrons have so little mass that atomsmust contain other particles that accountfor most of the mass
Thomson’s Atomic ModelThomson believed that the electrons were likeplums embedded in a positively charged“pudding,” thus it was called the “plum pudding”model.
Rutherford’s Gold Foil Experiment Alpha () particles are helium nuclei Particles were fired at a thin sheet of gold foil Particle hits on the detecting screen (film) arerecorded
Rutherford’s Findings Most of the particles passed right through A few particles were deflected VERY FEW were greatly deflected “Like howitzer shells bouncing off of tissue paper!” Conclusions: The nucleus is small The nucleus is dense The nucleus is positively charged
Atomic NumberAtomic number (Z) of an elementis the number of protons in thenucleus of each atom of thatelement. Element # of protons Atomic # (Z)Carbon 6 6Phosphorus 15 15Gold 79 79
Mass NumberMass number is the number of protons andneutrons in the nucleus of an isotope. Mass # = p+ + n0 18 8 8 18Arsenic 75 33 75Phosphorus 16 15 31
IsotopesIsotopes are atoms of the same element havingdifferent masses due to varying numbers ofneutrons. Isotope Protons Electrons Neutrons Nucleus Hydrogen–1 1 1 0 (protium) Hydrogen-2 1 1 1 (deuterium) Hydrogen-3 1 1 2 (tritium)
Atomic MassesAtomic mass is the average of all thenaturally occurring isotopes of that element. Isotope Symbol Composition of % in nature the nucleus Carbon-12 12C 6 protons 98.89% 6 neutrons Carbon-13 13C 6 protons 1.11% 7 neutrons Carbon-14 14C 6 protons <0.01% 8 neutrons Carbon = 12.011