The number that describes the main energy
level of an electron in an atom is

1. the magnetic quantum number mℓ
2. the mass number a.
3. the angular momentum (or subsidiary)
quantum number ℓ.
4. the principal quantum number n.
5. the atomic number z.

4. the principal quantum number n.

Explanation:

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.

Mass number is defined as the sum of number of protons and neutrons that are present in an atom. It is represented by A.

Azimuthal or angular momentum 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...

Principle Quantum Number : It describes the size of the orbital and the main energy level of an electron in an atom. It is represented by n. Where, n = 1,2,3,4....

Atomic number is defined as the number of protons or number of electrons that are present in an atom. It is represented by Z.