The potential energy of an electron in an orbit of hydrogen atom is -6.8 eV . The de-Broglie wavelength of the electron in this orbit is

( $r_o$ is Bohr radius)

If a radioactive substance decays $10 \%$ in every 16 hours, then the percentage of the radioactive substance that remains after 2 days is

If a nucleus $P$ converts into a nucleus $Q$ by the decay of one alpha particle and two $\beta^{-}$particles, then the nuclei $P$ and $Q$ are

The phenomenon of physics that deals with the constitution and structure of matter at the minute scales of atoms and nuclei is