#: Class: Date: Isotopes and Average Atomic Mass 

Name: KEY #: _____ Class: _____ Date: _________________

Isotopes and Average Atomic Mass 

Use the information given about each element to determine if the pair of elements are isotopes or different elements.

1. 
   Element D has 6 protons and 7 neutrons.
   

Element F has 7 protons and 7 neutrons. Different elements

2. 
   Element J has 27 protons and 32 neutrons.
   

Element L has 27 protons and 33 neutrons. Isotopes

3. 
   Element X has 17 protons and 18 neutrons.
   

Element Y has 18 protons and 17 neutrons. Different elements

4. 
   Element Q has 56 protons and 81 neutrons.
   

Element F has 56 protons and 82 neutrons. Isotopes

5. 
   Element T has an atomic number 20 and mass number 40.
   

Element Z has an atomic number 20 and mass number 41. Isotopes

6. 
   Element P has an atomic number 92 and mass number 238. Isotopes
   

Element S has 92 protons and 143 neutrons.
The average atomic mass is the weighted average of all the isotopes of an element. Determine the average atomic mass.

7. 
   Rubidium is a soft, silvery-white metal that has two common isotopes, ⁸⁵Rb and ⁸⁷Rb. If the abundance of ⁸⁵Rb is 72.2% and the abundance of ⁸⁷Rb is 27.8%, what is the average atomic mass of rubidium?
   

⁸⁵Rb 0.722 x 85= 61.37
⁸⁷Rb 0.278 x 87= + 24.186
86.556 amu

8. 
   Naturally occurring chlorine that is put in pools is 75.53% ³⁵Cl and 24.47% ³⁷Cl. Calculate the average atomic mass.
   

³⁵Cl 0.7553 x 35= 25.4355
³⁷Cl 0.2447 x 37= +9.0539
34.4894 amu

9. 
   Copper used in electric wires comes in two isotopes: ⁶³Cu and ⁶⁵Cu. ⁶³Cu has an abundance of 69.09%. The other isotope, ⁶⁵Cu, has an abundance of 30.91%. Calculate the average atomic mass of copper.
   ⁶³Cu 0.6909 x 63= 43.5267
   ⁶⁵Cu 0.3091 x 65= +20.0915
   63.6182 amu
   
10. 
    Hydrogen has three common isotopes: ¹H (99%), ²H (0.8%), and ³H (0.2%). What is the average atomic mass of hydrogen?
    

¹H 0.99 x 1= 0.99
²H 0.008 x 2 0.016
³H 0.002 x 3= + 0.006
1.012 amu