Name: ____________________________________________ Period: _____ Date: ____________________ Calculating Average Atomic Mass What is an isotope? _________________________________________________________________________ There are three isotopes of potassium that occur naturally: K – 39 has 20 neutrons 39 19 K K – 40 has 21 neutrons 40 19 K K – 41 has 22 neutrons 41 19 K Chemists have developed a method of measuring the mass of an atom called: ___________________________ abbreviated “___________” They did this because the masses of subatomic particles are ____________________________that even scientific notation makes it hard to calculate atomic masses of elements! The atomic mass that you see on the periodic table is the __________________________________ of the isotopes of that element. *The atomic mass you see on the periodic table is also the isotope that ________________________________ in nature! Question: There are three isotopes of potassium: K-39, K-40, and K-41. Which isotope is more abundant in nature? Answer: ______________ Question: Chlorine exists naturally as chlorine – 35 and chlorine – 37. The atomic mass of chlorine is 35.453 amu. Which isotope is more abundant in nature? Answer: ______________ Practice Problems **You can calculate the average atomic mass you see on the periodic table for any element! You need to know: 1. the masses of the isotopes 2. their percent abundance avg. atomic mass = mass (in amu) x percent abundance in decimal form 1. Magnesium has 3 isotopes. Calculate the atomic mass of magnesium if one isotope has a mass of 23.985 amu and a relative abundance of 78.99%, a second isotope has a mass of 24.986 amu with a relative abundance of 10.00%, and a third isotope has a mass of 25.982 amu with a relative abundance of 11.01%. 2. Find the average atomic mass of boron if 19.8% of the boron occurring in nature has a mass of 10.013 amu and 80.2% of boron atoms found in nature have a mass of 11.009 amu. 3. An element has two naturally occurring isotopes. Ag-107 has an abundance of 51.82% and a mass of 106.9 amu. Ag-109 has a relative abundance of 48.18% and a mass of 108.9 amu. Calculate the average atomic mass of silver. 4. Helium has two naturally occurring isotopes: helium-3 and helium-4. The atomic mass of helium is 4.003 amu. Which isotope is more abundant in nature? Explain why.