The number of the correct reaction(s) among the following is _______.

The de-Broglie's wavelength of an electron in the $$4^{\text {th }}$$ orbit is ________ $$\pi \mathrm{a}_0$$. ($$\mathrm{a}_0=$$ Bohr's radius)

The number of different chain isomers for C$$_7$$H$$_{16}$$ is __________.

Number of molecules/species from the following having one unpaired electron is ________.

$$\mathrm{O}_2, \mathrm{O}_2^{-1}, \mathrm{NO}, \mathrm{CN}^{-1}, \mathrm{O}_2^{2-}$$