To how many significant figures would you need to express the mass of an hydrogen-1 atom in amu to notice the change in mass that occurs when it gains an electron

Five significant figures

Mass of H-1 atom = 1.0078 u

Mass of electron = 0.0006 u

Total mass = 1.0084 u

You would have to express the mass of a hydrogen atom as 1.0078 u (five significant figures) to notice the change in mass as it gains an electron.

a polar bond is formed when electrons are shared unevenly between atoms