The magnetic moment of electron due to orbital motion is proportional to
($n$ = principal quantum number)

A radioactive element has rate of disintegration $16{,}000$ disintegrations per minute at a particular instant. After four minutes, it becomes $2000$ disintegrations per minute. The decay constant per minute is

The shortest wavelength in the Balmer series of hydrogen atom is equal to the shortest wavelength in the Brackett series of a hydrogen like atom of atomic number 'Z'. The value of 'Z' is

A radioactive sample has half-life of $5$ years. The percentage of fraction decayed in $10$ years will be