An atom of uranium-238 undergoes radioactive decay to form an atom of thorium-234. which type of nuclear decay has occurred?

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Alpha decay

Explanation:-

Uranium 238 has atomic number 92 and mass number 238.

Thorium 234 has atomic number 90 and mass number 234.

So, the change in atomic number as uranium 238 disintegrates into thorium234 = 92 – 90 = 2

So, the change in mass number as uranium 238 disintegrates into thorium234= 238 – 234 = 4

We know that when an alpha particle is emitted, the mass number decreases by 4 and the atomic number decreases by 2.

So when an atom of uranium 238 undergoes radioactive decay to form an atom of thorium-234, alpha decay has occurred.