Chemistry · Structure Of Atom · NEET
An electron spins around its own axis, like the Earth spins while going around the Sun. This spinning can be in two directions: clockwise or anticlockwise. The spin quantum number m_s just tells which direction. It does not tell size, shape, or position of the orbital. It only describes the electron's own spin. It is the 4th quantum number, added after n, l, and m_l.
Only two values are allowed: +1/2 and -1/2. Nothing else. +1/2 is called spin up (up arrow) and -1/2 is called spin down (down arrow). NEET often tries to trick you with values like +1 or 0 for m_s. Those are always WRONG. If you see any value other than +1/2 or -1/2 for m_s, that quantum number set is invalid.
No. This is important. The values of n, l, and m_l are linked to each other (for example, l depends on n). But m_s is INDEPENDENT. It is always just +1/2 or -1/2, no matter what n, l, or m_l are. So an electron in a 1s orbital and an electron in a 4f orbital both have the same two spin choices.
This is the Pauli Exclusion Principle. It says no two electrons in an atom can have the same set of all four quantum numbers. Two electrons in the same orbital already share the same n, l, and m_l. So the only way to make them different is to give them different m_s. One gets +1/2 and the other gets -1/2. That is why an orbital holds a maximum of 2 electrons, with opposite (anti-parallel) spins.
n = size of orbital (which shell). l = shape of orbital (s, p, d, f). m_l = orientation of orbital in space (which direction it points). m_s = orientation of the electron's spin. NEET 2024 asked exactly this match. Remember: m_s is the only one about the ELECTRON itself, not about the orbital.
No, but they are connected. m_s (+1/2 or -1/2) describes one electron's spin direction. The spin-only magnetic moment is calculated for an atom or ion using the number of UNPAIRED electrons with the formula mu = sqrt(n(n+2)) BM, where n is the number of unpaired electrons. Unpaired electrons cause magnetism because their spins are not cancelled by a partner.
Two electrons occupying the same orbital are distinguished by:
Match List-I (Quantum Number) with List-II (Information provided): (A) m_l (B) m_s (C) l (D) n; (I) shape of orbital (II) size of orbital (III) orientation of orbital (IV) orientation of spin of electrons.
How many electrons can fit in the orbital for which n=3 and l=1?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Exactly two: +1/2 and -1/2. This is fixed and never changes, no matter which orbital the electron is in.
George Uhlenbeck and Samuel Goudsmit proposed the electron spin quantum number in 1925 to explain the extra closely-spaced lines (doublets, triplets) seen in the spectra of multi-electron atoms.
Two. Both electrons share the same n, l, and m_l, but they must have opposite spins (+1/2 and -1/2). This comes from the Pauli Exclusion Principle.
By convention +1/2 is spin up (up arrow) and -1/2 is spin down (down arrow). NEET does not usually punish you for choosing the reverse convention, but the two values themselves must always be +1/2 and -1/2.
The first three quantum numbers (n, l, m_l) define an orbital's energy, shape and orientation, but they cannot explain the split spectral lines of multi-electron atoms. The spin quantum number was added as the fourth number to explain this.
