Thorium-232 ( Th) is an unstable nuclide that decays through a sequence of radioactive emissions to form a stable nuclide of lead. All of the emissions during this sequence are either alpha or beta (β–) particles. One of the intermediate nuclides, reached after four alpha and two beta decays, is a nuclide ofan element labelled X. What is the symbol for this nuclide of X?#

Answer:

15) C

During an alpha decay, the mass number (top number) decreases by 4 and atomic number (bottom number) decreases by 2. Four alpha decays occur.
So mass number decreases to: 232 – 4(4) = 232-16 = 216
Atomic number decreases to: 90-4(2) = 90-8 = 82
During a beta decay, the mass number (top number) does not decrease and atomic number
(bottom number) increases by 1. Two beta decays occur.
So mass number stays the same: 216
Atomic number increase to: 82 + 2(1) = 82 + 2 = 84