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Counting Atoms

Counting Atoms. 3.3. Counting Atoms. Very difficult to count Atomic Number – # of p + of each atom of that element Whole numbers Elements arranged by atomic # Identifies the element. Isotopes. Isotope – an atom of the same element with different # of neutrons

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Counting Atoms

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  1. Counting Atoms 3.3

  2. Counting Atoms • Very difficult to count • Atomic Number – # of p+ of each atom of that element • Whole numbers • Elements arranged by atomic # • Identifies the element

  3. Isotopes • Isotope – an atom of the same element with different # of neutrons • Elements are mix of isotopes • Hydrogen has 3 • Mass Number – total # of protons and neutrons that make up the nucleus of an isotope

  4. Designating Isotopes • Hyphen notation = Hydrogen – 3 • Nuclear symbol- U • 92 is atomic # (# of protons) • # of n0 = mass # - atomic # • How many p+,n0,e- r in Cl-37, Br-80? • What’s the notation for an isotope with 15 e- and 15 n0? 238 92

  5. Relative Atomic Mass • Atomic Mass Standard is C-12 • Atomic Mass Unit – (amu) exactly 1/12 the mass of a carbon-12 atom • Masses are relative to C-12 atoms. • All masses are averages of amu’s b/c of isotopes

  6. Average Atomic Mass • AAM – the weighted average of the atomic masses of the naturally occurring isotopes of an element • The mass of Cu-63 is 62.94 amu and the mass of Cu-65 is 64.93 amu. The abundance of Cu-63 is 69.17% and Cu-65 is 30.83%. Calculate the relative atomic mass of copper.

  7. R.A.M. Problem • Chlorine exists as Cl-35 which has a mass of 34.969 amu and makes up 75.8 % of Cl atoms. The rest of naturally occurring chlorine is Cl-37 with a mass of 36.996 amu. What is the average mass of chlorine?

  8. R.A.M. Problem • Uranium-234 makes up 0.005% of uranium atoms and has a mass of 234.041 amu. Uranium-235 makes up 0.720% and has a mass of 235.044 amu. Uranium-238 has a mass of 238.051 amu and makes up 99.275 %. What is the average atomic mass of Uranium?

  9. R.A.M. Problem • Carbon-12 makes up 98.90% of existing carbon. Carbon-13, with a mass of 13.003 amu, makes up 1.10%. Traces of carbon-14 also exist. What is the average atomic mass of carbon?

  10. Relating Mass # to Atoms • Mole – Amount of a substance that contains as many particles as there are atoms in exactly 12 g of C-12. • Way of counting (like 1 dozen = 12) • Avogadro’s number – the # of particles in 1 mole = 6.022 x 1023

  11. Molar Mass • Molar Mass – mass of 1 mole of a substance • Expressed as g/mol • For an element it’s = to atomic mass • What’s the molar mass of 1 mole of Li, Hg, C, He, and Cl

  12. Conversions with Molar Mass • Convert … 2 mole of He to grams of He 3.5 mole of Cu to grams of Cu 100 grams of C to mole of C 250 grams of Br to mole of Br

  13. Conversions with Avogadro’s # • How many moles are in 3.01 x 1022 atoms of Ag? • How many moles are in 4.97 x 1020 atoms of Cu? • How many atoms are in 15 moles of C? • How many atoms are in 4 moles of S

  14. Practice • Complete the section review on page 87 and do numbers 2-7.

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