Chemistry · Structure Of Atom · NEET
It tells you the ORIENTATION of an orbital, that is, the direction the orbital points in space. The azimuthal number l fixes the SHAPE (s, p, d, f), but many orbitals of that same shape can point in different directions. m_l labels each of those directions. For NEET, remember: n = size, l = shape, m_l = orientation, m_s = spin.
For a fixed l, m_l is allowed to take every whole number from -l up to +l, and it must include 0. Count them: from -l to +l there are l values below zero, l values above zero, and 1 zero. That is l + l + 1 = 2l + 1. So a subshell always holds (2l+1) orbitals.
Yes. Zero is always one of the allowed values. This is the single most common NEET mistake. For l = 2 (d subshell), the correct set is -2, -1, 0, +1, +2 which is 5 values. If you drop the 0 and write only 4 values, the set becomes WRONG, which is exactly how NTA sets traps (see NEET 2023 below).
Use 2l+1. s (l=0): 2(0)+1 = 1 orbital. p (l=1): 2(1)+1 = 3 orbitals. d (l=2): 2(2)+1 = 5 orbitals. f (l=3): 2(3)+1 = 7 orbitals. Each orbital can hold 2 electrons, so max electrons are s=2, p=6, d=10, f=14.
For any d subshell l = 2, so m_l = -2, -1, 0, +1, +2. That is 5 orbitals. The label 3d just means n = 3 with l = 2; the m_l values do NOT depend on n, only on l. So 3d, 4d and 5d all have the same five m_l values.
l decides the shape and the subshell (l = 0 is s, 1 is p, 2 is d, 3 is f). m_l decides the orientation within that subshell and counts how many orbitals there are, which is 2l+1. In short: l = which subshell, m_l = which orbital inside it.
m_l can be negative (that is normal, it runs from -l upward). But its size can never be bigger than l. The rule is -l is less than or equal to m_l is less than or equal to +l. So for l = 1, m_l = +2 is FORBIDDEN, but m_l = -1 is perfectly allowed.
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:
The incorrect set of quantum numbers from the following is:
Match List-I (Quantum Number) with List-II (Information provided): (A) m_l (B) m_s (C) l (D) n ... (I) shape (II) size (III) orientation of orbital (IV) orientation of spin
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Three. For p, l = 1, so number of orbitals = 2l+1 = 2(1)+1 = 3. They are p_x, p_y and p_z, corresponding to m_l = -1, 0, +1.
No. m_l depends only on l. It runs from -l to +l no matter what n is. That is why 2p, 3p and 4p all have the same three m_l values (-1, 0, +1).
m_l gives the number of orbitals (2l+1), and each orbital holds 2 electrons (from spin). So the max electrons in a subshell is 2 x (2l+1): s=2, p=6, d=10, f=14.
It is one allowed orientation of the orbital, not 'no orbital'. For example, in the p subshell m_l = 0 corresponds to the p_z orbital. Zero is a real, valid orbital and must always be counted.
Because these orbital orientations only become separate energy levels when the atom is placed in a magnetic field (the Zeeman effect). Without a field the (2l+1) orbitals have the same energy (they are degenerate).
