1. Isotopes and Calculating Atomic Mass

2. Elements are made of billions of atoms
Remember: an element is made of billions of tiny atoms.
They all look alike (same #p)
They do not weigh alike (different mass#)
Element:

3. 12 12 14 8 12 12 12 12 14 8 12 12 12

4. Isotopes – Multiple atoms of the same element that have different mass numbers.
Carbon – Carbon – 14
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5. Isotopes
All elements have isotopes meaning all elements are made of tiny atoms that have different masses.

6. 12 12 14 8 12 12 12 12 14 8 12 12
98% Carbon-12
2% Carbon-14 12

7. What is Atomic Mass?
Atomic mass: the weighted average mass of ALL the isotopes of that element.
This number is located on the bottom of each block on the periodic table
Not a whole number
Different from the Mass #

8. Mass # vs. Average Atomic Mass
Mass # = Individual grade in class - you have many (quiz, test, classwork, etc)
Average Atomic Mass = Average grade in May - you have one grade for Chemistry
Average Grade HW
Quiz
Test
Lab
Quiz

9. Average Atomic Mass
This is a piece of carbon that contains hundreds of billions of atoms, all with different masses.
Carbon-12 is found in 98% of the sample.
Carbon-14 is found in 2% of the sample.
What is the average atomic mass of carbon?

10. Steps for Calculating Atomic Mass
Change percent to decimal
Multiply the decimal by the mass # of that isotope
Then add the two sums together.
Answer must have units
98% = .98
2% = .02
.98 x 12 =
.02 x 14 = 0.28 =
12.04 amu

11. Determine the atomic mass of the following mixture of isotopes
80% I, 17% 126 I, 3% 128 I
50% 197Au, 50% 198 Au
99% 1 H, 0.8% 2 H, 0.2% 3 H
95% 14 N, 3% 15 N, 2% 16 N

12. Practice – always use mass with more sig figs
*The actual mass of a proton and neutron 1.008
Isotope
Mass (amu)
Neon – 20
19.992
Neon – 21
20.994
Neon - 22
21.991
Practice – always use mass with more sig figs
35 atoms of neon were individually weighed – the following information was found. What is the average atomic mass of Neon?
Isotope
Quantity
Mass (amu)
Neon – 20 31
19.992
Neon – 21 1
20.994
Neon - 22 3
21.991