1. Subatomic Heavyweights
Isotopes of atoms

2. Isotopes???
A chemist investigating a sample of lithium found that lithium atoms have different mass numbers. The chemist determined that the structures of the three types of atoms would be similar to the three drawings below.
What is different about the three atoms?
What is the atomic number of each atom?
What is the mass number of each atom?
Do you think they are all lithium atoms? Explain
Write your answers on the back of this paper

3. What is an isotope?
Isotope – atoms with same protons but different numbers of neutrons
3 p+
3 e-
3 n
3 p+
3 e-
4 n
3 p+
3 e-
5 n

4. How do we represent isotopes?
lithium-6
lithium-7
lithium-8 Li 3 6 Li 3 7 Li 3 8

5. How do isotopes differ? Physical properties
Differ only in mass and related properties
Density
Chemically almost identical

6. Famous isotopes H 1 3

7. Famous isotopes
uranium - 235

8. C 6 14
Famous isotopes Generated from N-14 in atmosphere by cosmic rays Absorbed in plants C-14 ½ life = 5800 yrs Date up < 60,000 yrs

9. How do they determine % of isotopes?
Mass spectrometer – separates atoms on the basis of mass (and charge)

10. Back to lithium… Atomic weight vs. mass number
What is the atomic mass of lithium listed on your periodic table?
Do you think the scientist found more lithium with three, four, or five neutrons?
Why do you think the atomic mass of lithium is not a whole number?
93% 7%
<0.1%

11. The Big Question
How do isotopes of an atom account for the atomic mass of an element?
Goal - Predict the isotopes of an element.

12. Which balls are protons, electrons and neutrons?
boron atom 1 2 3 4 5 6 7 8 9 10
# protons
# neutrons
# electrons
mass number

13. (cont.) Element Chemical Symbol Atomic Number Atomic Mass # of protons
# of electrons
# of neutrons
Boron B
5 or 6
Chlorine 17
Lithium
6.94
Vanadium V 23
Nitrogen 7
Magnesium
Argon Ar
39.9
or 22

14. Making Sense
How are all the atoms of an element the same? What doesn’t change?
How many protons and neutrons in lanthanum-57?
Explain why the atomic masses listed in the periodic table are not usually whole numbers.
Is the atomic mass of carbon always the same everywhere in the universe?
Do the physical and chemical properties of carbon isotopes differ?

15. stuff to know Vocabulary Mass number Isotope Atomic mass Skills
Know two ways to represent isotopes
Determine dominant isotope from atomic mass
Calculate atomic weight from isotope distribution
Determine # of neutrons from mass and atomic #’s
Concepts
What is a weighted average?
How do isotopes account for the atomic weight?
How do isotope phys. and chem. properties differ?
stuff to know