Question

which equation summarizes the alpha decay of $_{85}^{210}at$?
a. $_{85}^{210}at
ightarrow_{83}^{206}bi + _{2}^{4}he$
b. $_{85}^{210}at
ightarrow_{83}^{206}bi + _{-1}^{0}e$
c. $_{85}^{210}at
ightarrow_{86}^{210}rn + _{2}^{4}he$
d. $_{85}^{210}at
ightarrow_{86}^{210}rn + _{-1}^{0}e$

Explanation:

In alpha - decay, an atomic nucleus emits an alpha particle ($\ce{^4_2He}$). The mass number of the parent nucleus decreases by 4 and the atomic number decreases by 2. For $\ce{^{210}_{85}At}$, after alpha - decay, the new element will have a mass number of $210 - 4=206$ and an atomic number of $85 - 2 = 83$. The element with atomic number 83 is $\ce{Bi}$ (bismuth).

Answer:

A. $\ce{^{210}_{85}At
ightarrow^{206}_{83}Bi + ^{4}_{2}He}$