Average Atomic Mass (review lesson)

• Chlorine has 2 stable isotopes

• Chlorine-35

• Chlorine-37

• A real-world sample that contains chlorine (ex: NaCl ) will involve billions and billions of chlorines.

• Some Cl-35 and some are Cl-37

35 based on Cl-35 ?
37 based on Cl-37 ?
or an average of them?

​

The mass of formula NaCl for table salt involves the mass of Na and the mass of Cl... but what number do we use for Cl?

That question was not fair!!

Indeed. You do not have complete information.

That is why this is only a participation grade.

35.45 u

number below element name in boxes of your Periodic Table.

(17 is Atomic Number)

Your course grade is also a weighted average

Not every grade category is given the same weight

​But... that assumes each category is given the same weight (which it is not)

EACH VALUE IS MULTIPLED BY ITS WEIGHT , AND THEN DIVIDED BY 100


You DO NOT divide by 3 at the end.

Example of element with 2 isotopes

​unit of mass for atoms is "u" ... unified atomic mass unit

So the mass of a magnesium-25 atom is approximately 25 u.

​weighted average calculation to get Atomic Mass

• Divide each percentage by 100 and simplify them... which switches from the values to decimal numbers before putting in the setup

• Use 0.10 instead of 10% ; Use 0.11 instead of 11% ; Use 0.79 instead of 79%

• (24 u x 0.79) + (25 u x 0.10) + (26 u x 0.11) = 24.305

• Every element has an average atomic mass and is listed on the Periodic Table

• It is a weighted average based on the percent abundances of its isotopes.

• This average is usually nearest its most abundant isotope's mass (weighted toward it)

• sometimes the term average is dropped and it is just called atomic mass.