Since they can still be unstable...nuetral atoms have the same amount of protons to electrons but to be stable they need to fill up there outer shell by gaining or losing electrons
Amount of Niobium-91 initially
= 300/91 =3.2967mol
2040 years = 3 ×680 = 3 half-lives
therefore, amount left = 0.4121mol
mass of Niobium-91 remaining = 0.4121 ×91 =37.5g
The mass in a chemical reaction remains (mostly) the same.
(except for radiation/nuclear fission, in which mass gets converted into energy)
