The first electron configuration quantum number describes
a. the energy level.
b. the type of orbital (s, p,d, or
f.
c. the spin of each electron.
d. the number of electrons in the orbital.

Answer : The correct option is, (a) the energy level.

Explanation :

As we know that there are four quantum numbers :

Principle Quantum Numbers : It describes the size of the orbital and the energy level. It is represented by n. Where, n = 1,2,3,4.... Azimuthal Quantum Number : It describes the shape of the orbital. It is represented as 'l'. The value of l ranges from 0 to (n-1). For l = 0,1,2,3... the orbitals are s, p, d, f... Magnetic Quantum Number : It describes the orientation of the orbitals. It is represented as . The value of this quantum number ranges from . When l = 2, the value of will be -2, -1, 0, +1, +2. Spin Quantum number : It describes the direction of electron spin. This is represented as The value of this is for upward spin and for downward spin.

The first electron configuration quantum number describes the electron shell or the energy level of an atom. The value of 'n' ranges from 1 to the shell containing the outermost electron of that atom.

For example :

For :

      (Because it is in 1st shell)

        (Because it is in 's' orbital)

    (Because l = 0)

    (Because of the sign convention)

Hence, the correct option is, (a) the energy level.