The d- and f-Block Elements

Chemistry · Class 12

Simulation · Chemistry · Class 12

Counting unpaired electrons with a magnet

"Counting unpaired electrons with a magnet" is a free, interactive Class 12 Chemistry simulation for NEET and JEE, built on the lesson "Magnetic properties" in The d- and f-Block Elements. The slider walks from Sc³⁺ (3d⁰) to Zn²⁺ (3d¹⁰). The five boxes fill by Hund's rule: one electron in each box first, then pairing starts. Watch the unpaired count n and the spin-only moment μ = √[n(n + 2)] BM rise to a peak at d⁵ and fall again. Μ = √[n(n + 2)] BM: 1.73, 2.83, 3.87, 4.90 and 5.92 BM for n = 1 to 5 (NCERT's table rounds n = 2 to 2.84). d⁰ and d¹⁰ ions have n = 0 and are diamagnetic; d⁵ ions have the largest moment. JEE Main asked 61 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).

From the lesson Magnetic properties in The d- and f-Block Elements. Change the values and watch what happens.

Counting unpaired electrons with a magnetChemistry · Class 12

In JEE Main

42 shifts analysed

61 questions from The d- and f-Block Elements across the 42 JEE Main shifts Lumi has analysed (2017–2026).

See the past questions

The idea behind it

NCERT §4.3.9

  • A diamagnetic substance is pushed out of a magnetic field; a paramagnetic one is pulled in. A ferromagnetic substance is pulled in very strongly, and ferromagnetism is an extreme form of paramagnetism.
  • Paramagnetism comes from unpaired electrons: each has a magnetic moment from its spin and its orbital motion. In first-row compounds the orbital part is effectively quenched, so only spin counts.
  • Spin-only formula: μ = √[n(n + 2)] BM, where n is the number of unpaired electrons and BM is the Bohr magneton. One unpaired electron gives 1.73 BM.
  • The moment rises with n: n = 1, 2, 3, 4, 5 give 1.73, 2.83, 3.87, 4.90 and 5.92 BM (√8 = 2.83; NCERT's Table 4.7 prints 2.84 for the n = 2 ions Ti²⁺ and Ni²⁺). A measured moment therefore tells you how many electrons are unpaired.
  • Worked case: a divalent ion of Z = 25 is Mn²⁺, 3d⁵, with five unpaired electrons, so μ = √35 = 5.92 BM. For Z = 27, Co²⁺ is 3d⁷ with three unpaired electrons: μ = √15 = 3.87 BM.
  • d⁰ and d¹⁰ ions (Sc³⁺, Ti⁴⁺, Zn²⁺) have no unpaired electrons and are diamagnetic.
  • Measured moments of hydrated ions match the spin-only values closely for Ti³⁺ to Mn²⁺ (Mn²⁺: 5.92 calculated, 5.96 observed) but run higher for Fe²⁺ (5.3-5.5 against 4.90), Co²⁺ (4.4-5.2 against 3.87) and Cu²⁺ (1.8-2.2 against 1.73).

Questions

3 answers

What does the "Counting unpaired electrons with a magnet" simulation show?
The slider walks from Sc³⁺ (3d⁰) to Zn²⁺ (3d¹⁰). The five boxes fill by Hund's rule: one electron in each box first, then pairing starts. Watch the unpaired count n and the spin-only moment μ = √[n(n + 2)] BM rise to a peak at d⁵ and fall again.
Does JEE Main ask questions from The d- and f-Block Elements?
Yes. JEE Main asked 61 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
Is this simulation free?
Yes. It is free and runs in the browser on a phone or a laptop. It belongs to the lesson "Magnetic properties" in The d- and f-Block Elements (Class 12 Chemistry).

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