1. What do you know
about the
Periodic Table?

2. Atomic Number

3. Mass Number
Mass Number - the number of protons plus neutrons in the nucleus of an atom.

4. Look at a periodic table.
Are there any atomic masses that are whole numbers?
Can you give an explanation for your observation?

5. Look at the periodic table
Look at the periodic table. Why do all the atomic masses on the periodic table include decimal points instead of just whole numbers?

6. Number of protons + neutrons
Name: sodium
Number of protons
Atomic number: 11
Atomic weight:
Mass Number
Number of protons + neutrons

7. Number of protons + neutrons
Name: sodium
Mass Number
Atomic weight:
Number of protons + neutrons
Since the mass number is the number of protons plus neutrons, you would expect it to be a whole number. However, most periodic tables indicate mass number with a small decimal component. This is the result of the occurrence of small numbers of isotopes in any sample.

8. In this collection of atoms, are all the atoms of a given element exactly the same?
Isotopes of an element have the same chemical properties but different weights.
Cow milk (whole) Nutritional value per 100 g (3.5 oz)
Energy 60 kcal   250 kJ
Carbohydrates     5.2 g
- Sugars  5.2 g  
- Lactose 5.2 g  
Fat 3.25 g
– saturated  1.9 g
- monounsaturated  0.8 g  
- polyunsaturated  0.2 g
Protein 3.2 g
Water 88 g
Vitamin A equiv.  28 μg  %
Thiamin (Vit. B1)  0.04 mg   % Riboflavin (Vit. B2)  0.18mg   12%
Vitamin B12  0.44 μg   %
Vitamin D  40 IU % Calcium  113 mg % Magnesium  10 mg % 
Potassium  143 mg   %
100 ml corresponds to 103 g.[21] Percentages are relative to US recommendations for adults. Source: USDA Nutrient database

9. In 1913, T. W. Richards found two atomic masses for lead.
In 1919, F. W. Aston separated neon atoms into two different isotopes, after he invented the mass spectrograph.
Since that time, many isotopes of the elements have been discovered.

10. ALL atoms are isotopes.
Naturally occurring chemical elements are usually mixtures of isotopes, and so their atomic masses are weighted averages of the masses of the isotopes in the mixture.

11. Isotopes
Isotopes - Different forms of an element having the same number of protons but different numbers of neutrons (and therefore different atomic weights).

12. Isotopes are identified by their Mass Number
Mass Number = Protons + Neutrons
Average Atomic Weight - Average weight of an element based on the naturally occurring isotopes and the relative abundance of these isotopes on Earth.

13. All isotopes of an element have the same chemical properties but different weights.
Naturally occurring chemical elements are usually mixtures of isotopes, and so their atomic masses are weighted averages of the masses of the isotopes in the mixture.

14. Isotopes of Pennies
In this activity, you will find weighted averages of the masses of two kinds of pennies. Then you will find the number of each type of penny in a mystery sample, using the concepts you developed during the activity.

15. Complete parts A and B of packet
On the first page of the lab packet, define the following terms in your own words:
Isotopes
mass number
atomic number
atomic mass
Complete parts A and B of packet