The nucleus of the element X with mass number 81 contains 31% more neutrons as compared to protons. Then the relationship between the nuclei of element X with nuclei of element $$^{79}\text{As}$$ is that they are

Solution

If X has p protons, it has $$1.31p$$ neutrons, so $$2.31p = 81$$ and $$p \approx 35$$, giving $$n = 46$$ (X is $$^{81}\text{Br}$$). Arsenic has $$Z = 33$$, so $$^{79}\text{As}$$ has $$79 - 33 = 46$$ neutrons. Both nuclei have 46 neutrons, so they are isotones.

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