This worksheet is great to use as a practice tool for students to solidify they skills following a short briefing on characteristics of isotopes. It is used in conjunction with a model set allowing students to build an actual isotope in front of them (for example: to understand the difference between Carbon-12 and Carbon-14).
If you do not have wooden or plastic model sets for isotopes, make some out of paper or card stock. The students will strongly benefit from the hands-on experience while completing the worksheet.
Note this assignment is also best performed in groups, as students tend to learn better by collaborating.