October 2019 by Andrew Robinson
I am very disappointed in this company. In their text book on chemistry, unit 2, lessons 1, exploration 3, at the end of the lesson they say that you can round the average atomic mass to find the most common isotope of an element. However, if you look at Copper and it's isotopes, the average atomic mass of Copper is 63.55, meaning that it's most common isotope should be Copper-64, however Copper-64 does not exist in nature. I will fix this review once the online textbook is fixed.
