The alpha decay of a radioactive nuclide (X) emits a He-4 nucleus and produces an isotope of Superscript 235 subscript 92 upper U ... What is X? Superscript 235 subscript 91 upper P a. Superscript 235 subscript 93 upper N p. Superscript 236 subscript 92 upper U. Superscript 239 subscript 94 upper P u.

Superscript 239 subscript 94 upper P u

(plutonium-239)

Explanation:

In alpha decay, the element subtracts 4 from its mass number and subtracts 2 from its atomic number.

So if we end with Uranium-235, which has an atomic number of 92:

235+4 = 239

92+2 = 94

Element 94 is Pu, plutonium, and its mass could very well be 239 if it's an isotope. So the answer is Superscript 239 subscript 94 upper P u.