Uranium and thorium are the radioactive elements found in some abundance in the Earth’s crust. As each isotope of these elements decays, new radioactive elements are created that have much shorter half-lives than uranium or thorium. A series of alpha and beta decays occurs that leads to a stable isotope of lead (Pb). One such series begins with the isotope thorium-232 and proceeds through these elements: Th ⇒ Ra ⇒ Ac ⇒ Th ⇒ Ra ⇒ Rn ⇒ Po ⇒ Pb ⇒ Bi ⇒ Po ⇒ Pb 
a. Using the periodic table, find the atomic numbers for all of these elements.
b. Identify which of the reactions in this series involve alpha decay and which involve beta decay. (The change in atomic number provides all the information you need.)
c. Write the reaction equations for the first three decays in this series.
d. Fill in the mass numbers for all of the isotopes in this series.