Q. How to calculate atomic mass of an element?

Copper survive as a mixture of two isotopes. The lighter isotope (Cu-63) amid 29 protons and 34 neutrons makes up 69.17% of copper atoms. The heavier isotope (Cu-65) with 29 protons and 36 neutrons constitutes the remaining 30.83% of copper atoms. The atomic mass of Cu-63 is 62.930 amu, as well as the atomic mass of Cu-65 is 64.928 amu. Use the data above to calculate the atomic mass of copper.

For a trouble that has several numerical values it is wise to organize the data into a table.

First compute the contribution of each isotope to the average atomic mass being sure to convert each percent to a fractional abundance.

Mass contribution = mass of isotope *   abundance of isotope

For Cu-63

Mass contribution = 62.930 amu * 0.6917 = 43.53 amu

For Cu-65

Mass contribution = 64.928 amu * 0.3083 = 20.02 amu

The average atomic mass of the element is the total of the mass contributions of each isotope.

Atomic mass Cu = mass contribution Cu-63+ mass contribution Cu-65

Atomic mass Cu = 43.529 amu + 20.017 amu = 63.546 amu