A radioactive metal atom decays $($goes to another kind of atom$)$ by emitting an alpha particle $($He$2+$ ion$).$ The alpha particles are collected as helium gas. A sample of helium with a volume of $12\mathrm{.}10$ mL was obtained at $771$ mmHg and $23\backslash$deg C$.$ How many atoms decayed during the period of the experiment?