REMINDERS &

DAILY SCHEDULE

Element Quiz #2 tomorrow Atomic Structure Test Friday

Review Isotope/Different Element Worksheet Remind 101 Isotope Abundance/Average Atomic Mass

Notes Isotope Practice Packet

ISOTOPES:Average Atomic Mass & Abundance

AVERAGE ATOMIC MASS

The decimal number on periodic table

Weighted average of all isotopes of an element

Depends on percent (relative) abundance and mass of each isotope

Measured in “atomic mass units” (amu)

EXAMPLE

Element X has two isotopes

#1: Mass=6 amu Relative Abundance= 7.5%

#2 Mass=7 amu Relative Abundance=92.5%

What element is it?

TO SOLVE EXAMPLE 1

Average atomic mass=[(mass #1)x(abundance #1)] + [(mass

#2)x(abundance #2)]

(6 x .075) + (7 x .925)=

0.45 + 6.475=

6.925 amu

Closest to ?Lithium 6.94

FINDING PERCENT ABUNDANCE

There are two stable isotopes of Chlorine:

Chlorine-35 (which weighs 34.97 amu) and

Chlorine-37 (which weighs 36.97 amu).

If the relative atomic mass of Chlorine is 35.45 amu, what is the percent

abundance of each isotope?

Chlorine-35 (which weighs 34.97 amu) &Chlorine-37 (which weighs 36.97 amu).

Cl-35= ? %

Cl-37= ? %

X

Y ?

• Having 2 variables would be much harder.

• We want to express Cl-37 abundance using X

60%

40% Cl-35

Cl-3780%

20%

Cl-35Cl-37

.6 & .4 .8 & .2

Chlorine-35 (which weighs 34.97 amu) & Chlorine-37 (which weighs 36.97 amu). Chlorine relative atomic mass 35.45 amu.

Cl-35= ? %

Cl-37= ? %

X

1-X

• Cl-35 = 34.97 amu

• Cl-37 = 36.97 amu

34.97 X 36.97 1-X

(34.97X) + (36.97)-(36.97X) = 35.45

35.45

-2.00X + 36.97 = 35.45

-2.00X = -1.52

2.00X = 1.52

X= 1.52/2.00

X = 0.76

Chlorine-35 (which weighs 34.97 amu) & Chlorine-37 (which

weighs 36.97 amu). Chlorine relative atomic mass 35.45 amu.

Cl-35 = X = .76 = 76%

Cl-37 = (1-X) = (1-.76) = .24 = 24%

76%

24%

Chlorine Isotopes Abundance

Cl-35Cl-37

ISOTOPE PRACTICE PACKET

Turn in if finished today—if not it is homework due at the beginning of class tomorrow

Don’t forget to sign up for and tell your parents about remind 101