Chemistry · Structure Of Atom · NEET
Each one gives a different piece of the electron's address. n (principal) tells you the shell number and the size of the orbital (bigger n = bigger, farther from nucleus). l (azimuthal) tells you the sub-shell and the shape (l = 0 is s, 1 is p, 2 is d, 3 is f). m (magnetic) tells you the orientation of the orbital in space. s (spin) tells you the spin direction of the electron, either +1/2 (up) or -1/2 (down). NEET 2024 asked exactly this as a match-the-column question, so memorise: n = size, l = shape, m = orientation of orbital, s = orientation of spin.
For a given n, the l values go from 0 up to (n - 1). So if n = 3, then l can be 0, 1 or 2. For a given l, the magnetic quantum number m goes from -l to +l including 0, which gives (2l + 1) values. For l = 2, that is -2, -1, 0, +1, +2, so 5 values. The spin s is always either +1/2 or -1/2, nothing else. A very common NEET trap is a set where m skips the value 0.
Go step by step. First, l must be between 0 and n - 1. Second, m must include every value from -l to +l, and it must include 0. Third, s must be +1/2 or -1/2. If any of these fails, the set is wrong. In NEET 2023 the wrong option had n = 5, l = 2 but m listed only -2, -1, +1, +2 (it dropped 0), so it was incomplete and therefore incorrect.
l tells you the shape of the orbital (the type of sub-shell: s, p, d, f). m tells you how that shape is turned in space (the orientation). For example, for a p sub-shell l = 1, and m = -1, 0, +1 gives the three p orbitals pointing along x, y and z. So l = which shape, m = which direction that shape points.
One orbital is fixed by three quantum numbers: n, l and m. Two electrons in that same orbital already share the same n, l and m. The only way they can differ is by spin (s). Since spin has only two values (+1/2 and -1/2), only two electrons can fit, and they must have opposite spins. This is the Pauli exclusion principle: no two electrons in an atom can have all four quantum numbers the same.
The number 3 is n, so n = 3. The letter p means l = 1. For l = 1, m can be -1, 0 or +1 (pick the orbital it sits in). Spin s is +1/2 or -1/2. So a valid set for a 3p electron is n = 3, l = 1, m = 0, s = +1/2. Note: an orbital needs only three numbers (n, l, m) to be named, but a single electron needs all four.
The incorrect set of quantum numbers from the following is:
Two electrons occupying the same orbital are distinguished by:
The relation between n_m (the number of permissible values of the magnetic quantum number m) for a given value of the azimuthal quantum number l is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Both are used. NCERT often writes it as m_s (spin magnetic quantum number), and many books write it simply as s. In NEET questions you may see either symbol; both mean the same thing and take values +1/2 or -1/2.
A shell with principal quantum number n has n^2 orbitals and can hold up to 2n^2 electrons. For example, n = 3 has 9 orbitals and holds 18 electrons.
Yes. For a given n, l can only take values from 0 up to (n - 1). So n = 1 allows only l = 0, while n = 4 allows l = 0, 1, 2, 3.
An orbital is fully described by three quantum numbers: n, l and m. A single electron needs all four (n, l, m and s) because two electrons can share the same orbital but must have opposite spins.
The azimuthal quantum number l decides the shape. l = 0 is spherical (s), l = 1 is dumbbell-shaped (p), l = 2 gives d shapes and l = 3 gives f shapes.
