Copper exists in nature as two isotopes. The atomic masses and relative abundance of these isotopes is given in the table. What is the average

atomic mass of copper?

Isotope

Cu-63

Cu-65

Relative Abundance (%)

69.17

30.83

Atomic Mass (am)

62.93

64.94

A. 62.93 amu

B. 63 55 amu

C. 63.935 amu

D. 64.94 amu

E. 127.87 amu

The average atomic mass of the copper from the two given copper isotopes is 63.55

Explanation:

The average atomic masses of the isotopes of an element is calculated by the addition of the masses of the two isotopes in proportionality to their percentage of natural occurrence.

Cu-63 relative abundance 69.17 Atomic mass 62.93

Cu-65 relative abundance 30.83 Atomic mass 64.94

Average atomic mass = Cu 63 Atomic mass*relative occurrence percentage + Cu 65 Atomic mass*relative occurrence percentage

62.93*0.692+64.94*0.308

= 63.546 amu is the average atomic mass of copper.