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Atomic Structure (P. 544-553 and P.258-267) A. Atomic Structure » Atomic number » Mass Number » Protons, Neutrons, and Electrons B. Isotopes » Average atomic mass » Radioactive Decay A. Structure of an Atom The atom consists of -protons and neutrons concentrated in the nucleus -electrons constantly moving in a “cloud” around the nucleus Sub-atomic Particles Particle Mass(amu) Charge Proton 1.0 +1 Neutron 1.0 0 Electron 1/1836 -1 Location in Atom Nucleus Nucleus Cloud surrounding nucleus 1 atomic mass unit (amu) =1.673x10-24 g Defined to be 1/12 of the mass of a carbon atom containing 6 protons and 6 neutrons. Atomic Number • Atomic number = # protons in an atom • Uniquely identifies the element Atomic number Symbol Atomic mass 14 Si 28.086 Silicon Mass Number • Mass Number = # protons + # neutrons • Mass Number = atomic mass rounded to nearest integer • # neutrons = mass number – atomic number Protons, Neutrons, and Electrons 6 C 12.011 Carbon: 6 protons and 6 neutrons + + + + + 8 O 15.999 Oxygen: 8 protons and 8 neutrons 29 Cu 63.546 Copper: 29 protons and 35 neutrons Protons, Neutrons, and Electrons • In order to have neutrally charged atoms # protons = # electrons Bohr Model • Number of protons and neutrons in nucleus • Electrons in cloud Sulfur Energy level Maximum # electrons 1 2 2 8 3 18 Lewis Dot Diagram • Valence electrons: electrons in the outermost energy level • Use one dot to represent each valence electron Atomic Structure Summary • A=P=E M=A+N • Atomic # = # Protons = # Electrons • Mass # = # Protons + # Neutrons = Atomic # + # Neutrons B. Isotopes • Atoms of the same element with same number of protons but different numbers of neutrons. • Isotope symbol: Mass # Atomic # “Carbon-12” 12 6 C B. Isotopes © Addison-Wesley Publishing Company, Inc. B. Isotopes Hydrogen Isotopes B. Isotopes • How many protons, neutrons, and electrons are there in the Uranium isotopes U-235 and U-238? B. Isotopes • Average Atomic Mass – Reported on the Periodic Table – Weighted average of all naturally occurring isotopes of that element Avg. (mass # )(# of atoms) (mass # )(# of atoms) Atomic total # of atoms Mass B. Isotopes • About 8 out of 10 chlorine atoms are chlorine-35. Two out of 10 are chlorine-37. Avg. (35 amu)(8 atoms) (37 amu)(2 atoms) Atomic 10 atoms Mass Average atomic mass = 35.4 amu How many isotopes can an element have? Can an atom have any number of neutrons? • There are “preferred” combinations of neutrons and protons, at which the forces holding nuclei together seem to balance best. • Light elements tend to have about as many neutrons as protons. • Heavy elements need more neutrons than protons in order to stick together. B. Radioactive Decay • Atoms with too few or too many neutrons can sometimes exist for a while, but they are unstable. • Unstable atoms are radioactive: their nuclei change or decay by spitting out radiation. http://www.brainpop.com/science/energy/radioactivity/ http://www.brainpop.com/technology/scienceandindustry /carbondating/ • Nova Science Now – Island of Stability • http://www.pbs.org/wgbh/nova/physics/sta bility-elements.html