Answer : The term used to label the energy levels of electrons is, Principle Quantum Numbers (n).
Explanation :
There are 4 quantum number.
Principle Quantum Numbers : It describes the size of the orbital or energy levels of electrons. It is represented by n. 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
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.
Hence, the term used to label the energy levels of electrons is, Principle Quantum Numbers (n).