21.Half life of a radioactive material is 1 day. If starting material is 16 mg, how much of it will be left after 4 days

22.An electron moves around a nucleus (Z = 6), then energy required to move the electron from third Both s orbit to fourth Bohr s orbit will be

23.Atomic number of an atom is Z and its atomic weight is M. Then number of neutrons in the nucleus will be

24.Half life of a radioactive material is 30 minutes. If the radiations emitted by it have intensity 32 times the permissible safe level. Minimum time after which one can work with the material safely is

25.When an electron jumps from the orbit n = 2 to n = 4, then wavelength of the radiations absorbed will be where R is Rydberg s constant