How to calculate it! Average Atomic Mass.

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Presentation transcript:

How to calculate it! Average Atomic Mass

Quiz! Write the number of protons, neutrons and electrons for each isotope: 1. B-11 2. C-14 3. H-2 4. O-18 5. Na-23

Average Atomic Mass Weighted average of all isotopes’ masses for an element Calculate: Percent of isotope one * mass of that isotope PLUS Percent of isotope two * mass of that isotope Percent of isotope three * mass of that isotope And so on!

Example Naturally occurring copper is 69.17% copper-63 with a mass of 62.929 amu and 30.83% copper-65 with a mass of 64.928 amu. What is the average atomic mass of copper? What we know: Cu-63 69.17% 62.929 amu Cu-65 30.83% 64.928 amu Equation: %*mass + %*mass = average mass

Example continued Substitute: Solve! (0.6917)(62.929) + (0.3083)(64.929) = avg mass Solve! 43.528 amu + 20.018 amu = 63.546 amu

Radioactive Isotopes Some isotopes have too many particles in the nucleus Radioactive decay: nucleus breaks, forming a new element May take many decay steps to reach a stable isotope Time it takes half the atoms to decay is called half life

Problem solving activity p 552 Radioactive Element Changes to Element Half-life U-238 Pb-206 4 460 my K-40 Ar-40 1 260 my Rb-87 Sr-87 48 800 my C-14 N-14 5 730 years How many years would it take for half the Rb-87 atoms in a piece of rock to decay? How long would it take for 75% of the atoms to decay?

Problem solving activity p 552 Radioactive Element Changes to Element Half-life U-238 Pb-206 4 460 my K-40 Ar-40 1 260 my Rb-87 Sr-87 48 800 my C-14 N-14 5 730 years After a long time only 25% of the atoms of U-238 remain. How old would you predict the rock is? What element are the other 75% of the atoms now?