Transcript Average atomic mass

TAKE A ….
• GUIDED NOTES PAGE
• ATOMIC MASS W.S.
….OFF MY DESK
Average atomic mass
How we get those weird decimal
numbers on the periodic table
Whole numbers
• If protons and neutrons
weigh 1 amu each, why is
the atomic mass on the
periodic table a decimal
number?
• Answer: The atomic
weights given on your
table are “weighted
averages” of the different
isotopes of that element.
Quick review of percentages
• A percentage is equal to that number divided by 100.
• So, 25% = 25/100
• When you divide a number by 100, just move the
decimal point two places to the left.
• Ex. 25% = .25
• 30% = .30
• 2% = _____
• 1.25% = ______
• .05% = _______
• 125% = ________
Example of how to find the average
atomic mass
• Let’s say that Rb has two isotopes, Rb-85 and Rb-87.
72% of all Rubidium is Rb-85. 28% of all Rb is Rb-87.
What is the atomic mass of Rb?
• Answer:
0.72 x 85 amu= 61.2
0.28 x 87 amu = 24.32
Next add the two numbers you get together:
(61.2 + 24.32 = 85.56)
**This is slightly different than what you will see on the
periodic table because I rounded the percentages a bit
Example 2:
• 92.5 % of all Lithium atoms are Li-7. 7.5%
are Li-6. What is the average atomic
mass?
.925 x 7 amu = 6.475
.075 x 6 amu= 0.45
6.925 amu