Monotopic elements

Elements that occur naturally with only one nuclide are monotopic and include phosphorus, 15P, and fluorine, 9F. Elements that exist as mixtures of isotopes include C ( eC and gC) and O ( gO, gO and gO). Since the atomic number is constant for a given element, isotopes are often distinguished only by stating the atomic masses, e.g. and C. 

Arsenic is monotopic. What does this statement mean Using Appendix 5, write down three other elements that are monotopic. 

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