Certainly, it is true that calculations have proven to be the Achilles' heel for a significant proportion of students. No investigation appears to have been conducted which might cast light on the reasons for their difficulties, but one can hazard the suggestion that such students may have deferred the execution of calculations until late in their course.
Isotopes are atoms of the same element that contain different numbers of neutrons.
The careful study of the Table below should prove rewarding. The seventh bear is, presumably, little more than a bit of fluff. In Nature, elements usually exist as a mixture of isotopes.