1. Unit 1: Safety & Atomic Concepts
1.7 Isotopes
What did the scientist say when he saw two helium isotopes?
HeHe
Carle Place Middle Senior High School • Mrs. Nicosia • Chemistry

2. OBJECTIVE: BY THE END OF THIS VIDEO YOU WILL BE ABLE TO:
Differentiate between atomic number, mass number, and (average) atomic mass
Calculate the number of neutrons in an isotope
Calculate the (average) atomic mass for all isotopes of an element

3. ISOTOPES
Elements that have the same atomic number but different mass (different # of neutrons)

4. X ISOTOPE SYMBOLS Show the mass of isotope
Same atomic #, different mass #
Ex. isotope symbol of element X X
Mass #
Atomic #

5. COMMON ISOTOPES OF HYDROGEN
Name
Symbol #p #e #n
Mass
Protium H 1
Deuterium 2
Tritium 3 1 1 2 1 3 1

7. C EXAMPLE: The isotope symbol for Carbon-14
How many neutrons does it have?
14 – 6 = 8 14 6
Mass #
Proton #

8. EXAMPLE: WRITE THE ISOTOPE SYMBOL FOR:
Oxygen-17 O
How many neutrons does it have? 17 8

9. CHECK YOUR UNDERSTANDING:
Differentiate between atomic number, mass number, and (average) atomic mass
Calculate the number of neutrons in an isotope

10. STOP AND THINK
What is the correct Isotope Symbol for Chlorine-37

11. STOP AND THINK
How many neutrons are in the isotope
Boron-11

12. CALCULATING AVERAGE ATOMIC MASS

13. WHY IS ATOMIC MASS NOT A WHOLE NUMBER?
The atomic mass on the periodic table is a weighted average of the isotopes of the elements.
The weighted atomic mass takes into account the relative abundances (amounts) of all the naturally occurring isotopes.

14. EXAMPLE OF A GENERAL WEIGHTED AVERAGE
Your grade in chemistry
50% exams
10% quizzes
15% labs
25% HW/CW
(50 x 85) + (10 x 100.) + (15 x 95) + (25 x 80.) =
100
= 87 avg.

15. EXAMPLE: DETERMINE AVG ATOMIC MASS
Boron % amu
Boron % amu
Step 1: Multiply the mass of each isotope by its percent abundance then divide by 100
(19.78 x ) (80.22 x )
100
= amu

16. EXAMPLE Determine weighted atomic mass Potassium-39 93.12% 38.964 amu

17. CHECK YOUR UNDERSTANDING:
Calculate the (average) atomic mass for all isotopes of an element

18. STOP AND THINK
An element has two isotopes. 90% of the isotopes have a mass number of 20 amu, while 10% have a mass number of 22 amu. Calculate the atomic mass of the element.

19. YOU SHOULD BE ABLE TO:
Differentiate between atomic number, mass number, and (average) atomic mass
Calculate the number of neutrons in an isotope
Calculate the (average) atomic mass for all isotopes of an element