Unit 3: Atomic Structure Atomic Mass Units (amu) and Calculating Atomic Mass.

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Presentation transcript:

Unit 3: Atomic Structure Atomic Mass Units (amu) and Calculating Atomic Mass

Remember??? Mass number = protons + neutrons Isotopes= atoms with different # of neutrons Atomic masses occur with decimal points because they are an average of different isotopes

Atomic Mass Units Defined as 1/12 the mass of a Carbon-12 atom. The way that atomic mass is measured in the Periodic Table Abbreviated “amu”

To find the atomic mass, you need to know: Number of stable isotopes Mass of each isotope Natural % abundance of each isotope

Real-World Application: Weighted Average Let’s just say that the weights that make up your grade are as follows: Tests – 70% of grade Quizzes – 20 % of grade Homework –10% of grade You earn the following grades: test – 80, quiz – 100, homework – 100

To Find Weighted Average… Final Grade = (.7)(80)+(.2)(100)+(.1)(100) = 86% Don’t forget! All percentages must = 100% All decimals = 1

To Find Atomic Mass : Atomic mass of element = A.M. isotope 1 *(% 1 ) + A.M. of isotope 2 *(% 2 )

Example - Chlorine Calculate the atomic mass of chlorine if the two common isotopes of chlorine have masses of amu (75.00% abundance) and amu (25.00% abundance).

Atomic Mass of Chlorine (35.45amu)(.7500) + (37.29amu)(.2500) amu amu Atomic mass of Chlorine= amu a.m.=37.29amu % 2 = 25.00% a.m.= 35.45amu % 1 = 75.00%

What if you don’t have the percentage or the atomic mass? Calculate the atomic mass of one of indium’s isotopes (there are two) if the atomic mass of indium is amu and one isotope of indium has a mass of amu and an abundance of %.

amu = ( amu)*( )+(x)*( ) amu = (x)*( ) = x a.m.= ? % 2 = ? a.m.= amu % 1 = % a.m. of indium= amu = 4.205% Atomic mass of Indium= 113 amu