Chemistry · D And F Block Elements · NEET
n is the number of UNPAIRED electrons in the d-orbitals of the ION (not the atom). It is NOT the number of d-electrons and NOT the charge. Example: Cr2+ is 3d4, so it has 4 unpaired electrons, meaning n = 4. Always write the electron configuration of the ion first, fill the 5 d-boxes by Hund's rule (one electron each before pairing), then count the single arrows.
Step 1: Find the ion's oxidation state and remove that many electrons — remove from 4s FIRST, then 3d. Step 2: Write the 3d^x configuration. Step 3: Draw 5 d-boxes and fill by Hund's rule (spread out singly first). Step 4: Count boxes with only one arrow. Example for Fe3+: Fe is [Ar]3d6 4s2; remove 3 electrons (2 from 4s, 1 from 3d) → 3d5 → all 5 boxes have one electron → n = 5.
For the first row (3d) transition metals, the magnetic moment comes almost entirely from the SPIN of unpaired electrons. NCERT says the orbital angular momentum contribution is 'effectively quenched' (cancelled by the surroundings), so we use the spin-only formula. The s electrons are already paired in ions or removed, so they do not add magnetism. That is why we only count unpaired d-electrons.
BM means Bohr Magneton, the unit of magnetic moment. One single unpaired electron gives about 1.73 BM. The more unpaired electrons an ion has, the larger its magnetic moment, so it is attracted more strongly to a magnet. If n = 0, μ = 0 and the ion is diamagnetic (not attracted).
Just take the square root. For n = 1: √3 = 1.73 BM. n = 2: √8 = 2.84 BM. n = 3: √15 = 3.87 BM. n = 4: √24 = 4.90 BM. n = 5: √35 = 5.92 BM. NEET often gives the answer as √24 OR as 4.90 BM — they mean the same thing, so learn both forms.
Yes. Any two ions with the SAME number of unpaired electrons have the same spin-only moment, even if they are different metals. Example: Cr2+ (3d4, n=4) and Fe2+ (3d6, n=4) both give μ = √24 = 4.90 BM. NEET loves this trap, so always compare n, not the metal.
The calculated spin-only magnetic moment of Cr2+ ion is:
'Spin only' magnetic moment is same for which of the following ions? A. Ti3+ B. Cr2+ C. Mn2+ D. Fe2+ E. Sc3+
Match the metal ions (Column I) with their spin magnetic moments (Column II): (a) Co3+ (b) Cr3+ (c) Fe3+ (d) Ni2+ ;; (i) √8 BM (ii) √35 BM (iii) √3 BM (iv) √24 BM (v) √15 BM
Try the real previous-year questions from this chapter — each with the answer and a full solution.
μ = √n(n+2) Bohr Magnetons (BM), where n is the number of unpaired electrons. It ignores orbital motion because for 3d metals the orbital contribution is quenched.
μ = √5(5+2) = √35 = 5.92 BM. This is the maximum for a first-row transition ion, seen in Mn2+ and Fe3+ (both d5).
No. If there are zero unpaired electrons, μ = 0, so the ion is diamagnetic and is repelled by a magnetic field. Examples: Sc3+ (3d0) and Zn2+ (3d10).
No, not reliably. For 4f lanthanoid ions the orbital angular momentum is NOT quenched, so their real moments differ from the spin-only value. The spin-only formula is meant for first-row (3d) transition ions.
Put n = 1 into the formula: √1(1+2) = √3 = 1.73 BM. NCERT states a single unpaired electron has a moment of 1.73 BM.
