1. Atomic Mass Mrs. Cook

2. Atomic Mass - The average relative mass of all naturally occurring isotopes of an element. relative mass – The mass of one object compared to a similar object.

3. The mass of atoms is compared to carbon 12. An atomic mass unit (amu) is defined as the mass equal to 1/12 the mass of a carbon 12 atom. An atom of carbon 12 then has a mass of exactly 12.00000 amu *Because 12C has 6p and 6n, the mass of a proton and a neutron are about 1 amu.

4. What is the mass of carbon on the periodic table? 12.011 amu It is not exactly 12, because carbon exists as three isotopes, carbon 12, carbon 13, and carbon 14. Carbon 13 and carbon 14 are heavier, so increase the average atomic mass.

5. Atomic Masses are a weighted average. That means that the more common isotopes are counted more in the average.

6. To calculate atomic mass: 1. Change the percent abundances to a decimal. (Divide by 100.) 2. Multiply the mass of each isotope by the decimal abundance of the isotope. 3. Add the results of step 2.

7. Remember: A single atom or isotope does not have the mass listed on the periodic table. The periodic table is the AVERAGE of all isotopes. We can use the average because very rarely do scientists have a sample of a single isotope.

8. Example: Magnesium occurs in nature in three isotopic forms: (78.70% abundance), (10.13% abundance), and (11.17% abundance). The relative masses of these three isotopes are 23.985, 24.986, and 25.983 amu, respectively. Calculate the atomic mass of magnesium for these data.

9. Solution: 1. Change the percent abundances to a decimal 78.70% = 0.7870 10.13% = 0.1013 11.17% = 0.1117 2. Multiply mass by decimal abundance 23.985 amu x 0.7870 = 18.8762 amu 24.986 amu x 0.1013 = 2.53108 amu 25.983 amu x 0.1117 = 2.9023 amu

10. 3. Add the results of Step 2 18.8762 amu 2.53108 amu + 2.9023 amu 24.30958 amu 24.31 amu Solution: *Notice that you keep all sig figs until the end*

11. Calculate the atomic mass for chlorine, given the following: Mass number Exact weight Percent abundance 3534.96885275.77% 3736.96590324.23%

12. Solution 34.968852 amu x 0.7577 = 26.4959 amu 36.965903 amu x 0.2423 = + 8.95684 amu 35.45274 amu 35.45 amu This is what “showing your work” looks like.

13. What is the atomic mass of an element if it has three naturally occurring isotopes. One has a mass of 27.976 amu and make up 92.23 % of the atoms. The second has a mass of 28.98 amu and a natural abundance of 4.67%. The third has a mass of 29.97 amu and makes up 3.10 % of the atoms. Which element is this?

14. 27.976 amu x 0.9223 = 25.80226 amu 28.98 amu x 0.0467 = 1.35337 amu 29.97 amu x 0.0310 = + 0.92907 amu 28.0847 amu It is Silicon! 28.1 amu Solution

15. Atomic Mass of Candium 1. Obtain a bag of candium. 2. Separate the “atoms” into different isotopes. 3. Count the number of each isotope present. 4. Calculate the percent of each isotope: % abundance = number of atom of the isotope X 100 Total # of atoms 5. Find the mass of each isotope by massing ten atoms and dividing the mass by 10. 6. Calculate the average “atomic” mass by multiplying the mass of each isotope by the percent abundance, as a decimal and adding the results.

16. Data: Total number of atoms:_________ Isotope“m”“s”“p” Number of atoms Percent abundance Mass of atoms Mass of the isotope The atomic mass of candium is______________. Show your work under the data table!