1. Daily Science Who discovered the electron and how did he discover it? What was his model called? What tells you the number of p + an element has? Write Manganese in short hand notation. Element Symbol Atomic #ProtonsNeutronsElectronsMass #Atomic Mass Cr28 33

2. Atomic Mass and isotopes Pg. 31

3. Isotopes An isotope is an element that has the same number of protons but a different number of neutrons. Ex. Carbon has 2 common isotopes: – Carbon-12 Carbon-14 The numbers represent the mass number

4. Atomic Mass Atomic mass units (amu) are used to express an atoms mass Atomic mass units are based off the element Carbon (amu= 1/12 the mass of Carbon)

5. Average atomic mass Average atomic mass is a weighted average of the isotopes of a given element. More common isotopes have a greater effect on the average atomic mass than do less common isotopes. Ex. Chlorine- 37 and Chlorine- 35 (average atomic mass is 35.45)

6. Percent Abundance The atomic mass can be calculated by summing the products of each isotope’s percent abundance and that isotope’s atomic mass. Mass contribution tells how much of each isotope contributes to the total atomic mass. Mass contribution = (% abundance written as a decimal) x (atomic mass)

7. Average atomic mass Aam = [(% abundance)(atomic mass)] 1 + [(% abundance)(atomic mass)] 2 + …..

8. Percent Abundance example Chlorine 35 has an atomic mass of 34.969 amu and its percent abundance 75.770%. What is its mass contribution? Chlorine 37 has an atomic mass of 36.966 amu and its percent abundance is 24.230%. What is its mass contribution. Using the two previous answers, what is the average atomic mass of Chlorine?