Isotopes — common mistakes

In calculations for $A_r$, always use the precise isotopic masses given in the question, not just the mass numbers. If isotopic masses are not provided, you may assume they are equal to the mass numbers (e.g., 35 and 37 for chlorine). Pay close attention to the number of decimal places or significant figures required in the final answer.

Because it is a weighted average of the masses of all the naturally occurring isotopes of that element. Since most elements have more than one isotope, and their masses are not simple integers, the average is almost always a decimal value.

Yes, all elements have isotopes. However, some elements have only one stable, naturally occurring isotope. These are called monoisotopic elements (e.g., fluorine, sodium, aluminium). Many other isotopes of these elements can be created artificially in labs.

Isotopes are difficult to separate because their chemical properties are identical. Separation must rely on their small mass difference. The most common method is using a mass spectrometer, which deflects ions based on their mass-to-charge ratio. Other methods, like gaseous diffusion (used for uranium enrichment), exploit the fact that lighter isotopes move slightly faster.