Question

The molecular mass of copper is 63.5g, whereas the mass number is always an integer. The reason for such variation is:
A.Presence of impurities
B.Presence of isobars
C.Presence of isotopes
D.All of the above

Answer 1

Hint:
The atomic number of an element is the number of protons which is equal to the number of electrons in a neutral atom. The mass number of an element is the sum of the number of protons and number of neutrons in an atom.

Step-by-step explanation
Step 1:
The atomic number of an element is specific for a given element. For example, the atomic number of sodium is always 11 and does not vary under any circumstances.
Step 2:
When two or more atoms having the same atomic number differ in their mass number, they are said to be isotopes. Their difference is due to the difference in their number of neutrons. For example, hydrogen has three isotopes: $_1^1P,_1^2D,_1^3T$.These three isotopes differ in their mass number due to the increasing number of neutrons in them. As protons and neutrons are discrete numbers, so their sum which is the mass number is always an integer.
Step 3:
As we have seen that the isotopes of an element possess different relative atomic masses, so in order to assign a particular value for the relative atomic mass of an element we calculate the weighted average of their atomic mass in the ratio of their occurrence in nature. For example: for isotopes A and B for a given element of atomic masses ${M_1}\& {M_2}$ respectively and their natural occurrence to be ${C_1}\& {C_2}\%$respectively will have an average atomic mass of = $\dfrac{{{M_1} \times {C_1} + {M_2} \times {C_2}}}{{{C_1} + {C_2}}}$
Step 4: Copper has two stable isotopes $_{29}^{63}Cu\& _{29}^{65}Cu$ having mass number 63 and 65 and a natural abundance of 69.15% and 30.85%.
So, the average isotopic mass of copper is $= \dfrac{{63 \times 69.15 + 65 \times 30.85}}{{69.15 + 30.85}} = 63.5a.m.u$

Hence, the reason for the variation is due to the presence of isotopes and Option (C ) is the correct option.

Note: The mass number is the sum of the number of protons and neutrons whereas the average atomic mass is the weighted average of their isotopic mass.