[tex]\;_{\phantom{2}85}^{217}\; \text{At} \to \;_\text{\phantom{2}83}^{213}\; \text{Bi} + \;_2^{4} \; \text{He}[/tex]
Astatine has atomic number 85. "217" is the mass number of this isotope of At.
The mass number of an atom decreases by 4 as it undergoes alpha decay. Its atomic number decreases by 2. It emits an alpha particle, which is a helium-4 nucleus, in this process. The alpha particle has mass number 4 and atomic number 2.
The daughter nucleus has mass number 217 - 4 = 213 and atomic number 85 - 2 = 83. The atomic number of 83 corresponds to bismuth. Therefore the daughter nucleus is an atom of bismuth-213.
By convention, write the mass number A on the top-left corner of an element. Write its atomic number Z on the bottom-left corner. Note that A is always greater than Z for an isotope.
