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Chapter 3.3 : Counting Atoms
 

Chapter 3.3 : Counting Atoms

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    Chapter 3.3 : Counting Atoms Chapter 3.3 : Counting Atoms Presentation Transcript

    • Counting Atoms
      Chapter 3.3
    • Objectives
      Explain what isotopes are
      Define atomic number and mass number, and describe how they apply to isotopes.
      Given the identity of a nuclide, determine its number of protons, neutrons, and electrons.
      Define mole in terms of Avogadro’s number, and define molar mass.
      Solve problems involving mass in grams, amount in moles, and number of atoms of an element
    • Atomic Number
      Symbol – Z
      Number of protons in nucleus of atom
      Determines the identity of the atom!!!!!
      C - 6
      He - ___
      F - ___
      Pb - ___
      Use periodic table to find these!
      Since atoms are neutral atomic number is also number of electrons
      So C also has 6 electrons
      He - ____ electrons
    • Isotopes
      All Hydrogen atoms contain 1 proton
      Many naturally occurring elements can contain different numbers of neutrons.
      Isotope – at0ms of the same element that have different masses.
      n
      n
      n
      Protium
      99.985 %
      Deuterium
      0.015 %
      Tritium
      Trace
      Three isotopes of Hydrogen
    • Mass Number
      Mass number – total number of protons and neutrons in the nucleus of an isotope
      Symbol – A
      Protium has 1 proton and 0 neutrons
      Mass number = #p + #n
      = 1 + 0
      = 1
      • What is the mass number deuterium and tritium?
      • Find on periodic table
    • Designating Isotopes
      Hyphen Notation
      Name of atom – mass number
      Uranium - 235
      • Nuclear symbol
      • A X A : mass number, Z : atomic number, X : symbol of element
      • 235U
      Z
      92
      • Number of protons Neutrons Electrons
      92 143 92
      • Nuclide – general term for any isotope of any element
    • Relative Atomic Mass
      Oxygen – 16 , mass of 2.657 x 10-23 g
      Easier to use relative scale
      Need arbitrarily chosen standard
      All others are compared to carbon - 12
      • One atomic mass unit (1 amu) – exactly 1/12 the mass of a carbon – 12 atom.
      • All others are compared to carbon -12
      • Hydrogen – 1 , about 1/12 the mass of carbon -12
      • Precise value is 1.007825 amu
      • Magnesium – 24 , Slightly less than twice that of carbon -12
      • Precise value is 23.985042 amu
    • Average Atomic Mass of Elements
      Most elements occur naturally as mixtures of isotopes
      Average atomic mass : weighted average of the atomic masses of the naturally occurring isotopes of an element
      Ex. of weighted average
      Box contains two types of marbles, 25% have mass of 2.00g, and 75% have a mass of 3.00 g.
      25 marbles x 2.00 g = 50 g
      75 marbles x 3.oo g = 225 g
      so 50 g + 225 g = 275 g (TOTAL MASS)
      275 g ÷ 100 = 2.75 g average marble mass
    • Calculating average atomic mass
      Or
      (2.00 g x 0.25) + (3.00 g x .75) = 2.75 g
      • Calculate the average atomic mass of copper
      • Use table 3-4 pg. 80 in text
      • Copper - 63 69.17% with mass of 62.929599 amu
      Copper – 65 30.83% with mass of 64.927793 amu
      (0.6917 x 62.929599 amu) + (0.3083 x 64.927793 amu)
      =63.55
      Match with atomic mass on periodic table
      Round to two decimal places in calculations
    • Relating Mass to Numbers of Atoms
      The MOLE
      SI base unit for amount of a substance
      Abbreviated as mol
      Defined as : amount of substance that contains as many particles as there are atoms in exactly 12 g of carbon – 12
      Counting unit just like a dozen
      We don’t order 12 or 24 eggs, we order 1 or 2 dozen
      In same way, a chemist may want 1 mol of carbon or 2 mol of iron.
      63.55 g of copper or 1 mol of copper
    • Avogadro’s Number
      The number of particles in a mole – AVOGADRO’S NUMBER
      6.022 x 1023particles in exactly one mole of a pure substance
      • How big is this number?
      • If every person on earth (5 billion people) counted 1 atom per second, it would take 4 million years to count all the atoms.
    • Molar Mass
      Molar mass – mass of one mole of a pure substance
      Units : g/mol
      Numerically equal to atomic mass of element
      Mass of 6.02 x 1023 atoms of element
      One mole He 6.02 x 1023atoms 4.oo g
      One mole Li 6.02 x 1023atoms6.94 g
      One mole Cu 6.02 x 1023atoms63.5o g
      One mole Fe 6.02 x 1023 atoms 55.78 g
    • Gram/Mole Conversions
      Mass
      In grams
      Moles
      In mol
      Atoms
      In atoms
      Avogadro’s Number
      Molar mass
      What is the mass in grams of 3.50 mol of the element copper, Cu?
      A chemist produced 11.9 g of Aluminum, Al. How many moles of aluminum were produced?
      How many moles of silver, Ag, are in 3.01 x 1023 atoms of silver?
      What is the mass in grams of 1.20 x 1023 atoms of copper, Cu?
    • The Ten Commolements
      (found at http://members.tripod.com/~smalls42/commolements.html)
      Thou shalt not use 6.02 x 1023 in vain.
      Thou shalt not use the term mole if thou has no true knowledge of the term mole.
      Thou shalt not kill a mole.
      Thou shalt not covet your neighbor's mole.
      Thou shall always remember to celebrate Mole Day.
      Thou shalt not disparage Mole Day.
      Thou shalt not use a mole out of season.
      Thou shalt always honor the one who introduced thou to Mole Day.
      You shalt always keep sacred 10/23.
      Thou shalt always remember these commolments or thou will never properly celebrate Mole Day.