Radon-210 (21086Rn) decays by alpha decay to an isotope of polonium (Po) with a mass number of .

The question seems to be incomplete. However, I can explain the process of alpha decay using Radon-210 as an example.

In alpha decay, an unstable parent nucleus emits an alpha particle, which consists of 2 protons and 2 neutrons (a helium-4 nucleus). The parent nucleus loses 2 protons and 2 neutrons, resulting in a new daughter nucleus.

For Radon-210 (21086Rn), the alpha decay process would look like this:

21086Rn → 42He + 20684Po

Here, Radon-210 decays into an alpha particle (42He) and a new element, Polonium-206 (20684Po). The superscripts represent the mass number (number of protons + neutrons), and the subscripts represent the atomic number (number of protons).

In this decay process, the mass number decreases by 4 (from 210 to 206) and the atomic number decreases by 2 (from 86 to 84), which is characteristic of alpha decay.