4.2 Periodic Table Squares and Average Atomic Mass

Element Name Atomic Number Chemical Symbol Atomic Mass Atomic Number: Number of Protons Atomic Mass: Average mass based on the abundance of an element’s isotopes Chemical Symbol Atomic Mass

Mass Number = # of protons + # of neutrons Learning Check Based on the atomic number and mass number, find the number of neutrons in an oxygen atom. (Hint: assume the atomic mass is 16) Answer: 8 neutrons

Isotopes When the NUMBER OF NEUTRONS changes in an atom.

Average Atomic Mass Represents the average mass of an element based on its isotopes. UNITS are amu (atomic mass units) Average Atomic Mass = (% abundance x mass) + (% abundance x mass) + (% abundance x mass) …..

Let’s try this together Lithium’s isotopes have an abundance ratio of 7.5% of Lithium-6, and 92.5% of Lithium-7. What is Lithium’s average atomic mass? Answer: (% abundance x mass) + (% abundance x mass) = average atomic mass (0.075 x 6) + (0.925 x 7) = 6.925 amu

Try this on your own Iron’s isotopes have an abundance ratio of 15% Fe-55, and 85% Fe-56. What is iron’s average atomic mass? (.15 x 55) + (.85 x 56) = 55.85 amu

One more for good luck Chlorine’s isotopes have an abundance ratio of 75.78% Cl-35 and 24.22% of Cl-37. What is the average atomic mass of Chlorine? (.7578 x 35) + (.2422 x 37) = 35.48 amu