Question

Thorium is an element that occurs naturally in the earth's crust. The only naturally occurring isotope of thorium is 232Th and it is unstable and radioactive. The first step in the Thorium decay chain is the emission of an alpha particle. Which of these nuclei emerges from the decay?
A) 22888Ra
B) 23086Rn
C) 23289Ac
D) 23892U

Answer 1

A, radium-228. Alpha radiation is essentially just two protons and two neutrons leaving the atom, causing the atomic number to drop by two (since the protons left) and the mass number to drop by four (since two neutrons and protons left).

Answer 2

A.228_88Ra

In alpha decay a helium nucleus, 42He, is emitted. Since the total number of nucleons must be conserved, the atomic number of the parent nucleus must decrease by two, and the mass number must decrease by four. Here, the new atomic number is 90 - 2 = 88, and new mass number is 232 - 4 = 228.

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