Why is the relative atomic mass of many elements not whole numbers? 

Because of the existence of isotopes. 

The correct answer is option 3. Because of the existence of isotopes.

The relative atomic mass of many elements is not whole numbers primarily due to the existence of isotopes.

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. This means isotopes of an element have different atomic masses.

Since most elements have more than one naturally occurring isotope, the relative abundance of each isotope contributes to the overall atomic mass of the element.

The relative atomic mass reported on the periodic table is an average of the masses of all naturally occurring isotopes, weighted by their abundance.

Because isotopes have different masses, the average atomic mass is usually not a whole number.

While other factors such as impurities or ionization during determination might affect the accuracy of atomic mass measurements, they are not the primary reason why atomic masses are not whole numbers. It's the existence of isotopes that primarily leads to fractional atomic masses.