Why is the atomic mass of many elements not a whole number? a. elements are unstable. b. an element may have isotopes with different numbers of neutrons. c. the atomic mass includes all the electrons. d. isotopes are always expressed in decimals.

Answer : The correct option is, (b) an element may have isotopes with different numbers of neutrons.

Explanation :

Isotope : It is defined as the element that have the same number of protons but have the different number of neutrons of each of the atom.

Atomic number is defined as the number of protons or number of electrons.

Atomic number = number of protons = number of electrons

Mass number is defined as the sum of number of protons and number of neutrons.

Number of neutrons = Mass number - Number of protons

Average atomic mass of an element is defined as the sum of masses of each isotope each multiplied by their natural fractional abundance.

From this we conclude that the atomic mass of many elements is not a whole number because an element have isotopes with different numbers of neutrons.

Hence, the correct option is, (b)