Question

What is the daughter nucleus that is produced when Uranium-235 undergoes alpha decay?

Answer 1

`""_90^231Th`

Explanation:

Uranium-235 has an atomic number of `92`, the same as all uranium atoms, and a mass number of `235`, so is written as

`""_92^235U`

Alpha decay is when a particle consisting of two protons and two neutrons leaves the nucleus. This is a helium nucleus. This decreases the atomic number (proton number) by two and the overall mass by four, so

`""_92^235U -> ""_90^231X^(2-) + ""_2^4He^(2+)`

We generally don't write the charges, though, because we aren't looking at the electrons round the outside, but the nucleus inside, which is positive no matter what. I've written them simply to demonstrate that no electrons are lost in the decay.

We can find out what the element `X` is by looking at the periodic table and finding an element with an atomic number `90`.

This is thorium.

Therefore, the daughter nucleus is

`""_90^231Th`