Question

francium - 231 undergoes alpha decay. what is the resulting nuclide?
a ra - 227
b at - 235
c ra - 235
d at - 227

Explanation:

Step1: Recall alpha - decay formula

In alpha - decay, an atomic nucleus emits an alpha particle ($\ce{_2^4He}$). The mass number of the parent nucleus decreases by 4 and the atomic number decreases by 2.
The atomic number of Francium (Fr) is 87 and its mass number here is 231.

Step2: Calculate the mass number of the daughter nucleus

The mass number of the daughter nucleus = 231 - 4=227.

Step3: Calculate the atomic number of the daughter nucleus

The atomic number of the daughter nucleus = 87 - 2 = 85.
The element with atomic number 85 is Astatine (At).

Answer:

D. At - 227