Copper has two naturally occurring isotopes, 63 cu and 65 cu. if that is the case, why is the atomic mass of the copper listed as 63.55 amu on the periodic table

Explanation: There are 2 given isotopes of Copper.

Mass number of Isotope 1 : 63amu

Mass number of Isotope 2: 65 amu

Let the fractional abundance of isotope 1 be 'x', so the fractional abundance for Isotope 2 will be '1-x'

The average atomic number of Copper given is 63.55 amu.

So, the formula for average atomic mass is given by:

Putting values in above equation, we get:

The fractional abundance of Isotope 1 = 0.725

The fractional abundance of Isotope 2 = 0.275

So, the natural percentage abundance for isotope 1 = 72.5%

The natural percentage abundance for isotope 2 = 27.5%

Whenever more number of isotopes are present of an element are present, we take the average atomic mass of that element.

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