Question

(1): Do you expect the mass of 12C and 13C the same? if not, how do we report "average atomic mass" for carbon? what additional information we need to get the average atomic mass for carbon?
(1b) If an element has two isotopes, for example 12C, and 13C, list what they have in common and list what they differ.

Answer 1

1b) Iso means same and topes means number of protons (atomic number)

Isotope represent the atoms with same atomic number (number of protons ) and different mass number (number of protons+number of neutrons).

Hence, they belong to the same element but they have different number of neutrons.

Example 12C has 6 protons and 6 neutrons.

13 C has 6 protons and 7 neutrons.

1a) The mass of 12 C and 13 C is not the same.

It depends on the natural abundance of the isotope of the element.

For 12 C , natural abundance is 98.9% whereas the natural abundance of 13 C is only 1.1%

Average atomic mass of an element = sum of (mass of each isotope x its natural abundance)

Hence , average atomic mass of Carbon = (12 amu x 0.989) +(13.003 amu x 0.0110)

= 11.868 amu + 0.143033 amu

= 12.011 amu