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 n 0 = mass # - atomic # How many p +,n 0,e - r in Cl-37, Br-80? What’s the notation for an isotope with 15 e - and 15 n 0 ? 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 10 23

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 10 22 atoms of Ag? How many moles are in 4.97 x 10 20 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.