What is the number of unpaired electrons present in ground state of Silicon and Chromium, respectively ?

What is the energy of an electron in a hydrogen atom in a stationary state corresponding to n = 2 ?

If the velocity of the electron in Bohr's first orbit is $2.19 \times 10^6\ \text{Ms}^{-1}$, Calculate the de Broglie wavelength associated with it.
$[h = 6.626 \times 10^{-34}\ \text{J s}$ & Mass of electrons $= 9.10938 \times 10^{-31}$ kg $]$

Calculate the de Broglie wavelength of an electron in the first Bohr orbit of a hydrogen atom if the velocity of an electron in the first orbit is $2.2 \times 10^6 \text{ ms}^{-1}$. [mass of electron $= 9.1 \times 10^{-31}$ kg, plank's constant (h) $= 6.626 \times 10^{-34}$ J s]