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Magnetic properties
Most of the transition elements and their compounds show paramagnetism. The paramagnetism first increases in any transition element series, and then decreases. The maximum paramagnetism is seen around the middle of the series. The paramagnetism is described in Bohr Magneton (BM) units. The paramagnetic moments of some common ions of first transition series are given below in Table
Explanation :A substance which is attracted by magnetic filed is called paramagnetic substance. The substances which are repelled by magnetic filed are, called diamagnetic substances. Paramagnetism is due to the presence of unpaired electrons in atoms, ions or molecules.
The magnetic moment of any transition element or its compound/ion is given by (assuming no contribution from the orbital magnetic moment).
μs = √4S(S+1) BM = √n(n+2) BM
where, S is the total spin (n
×s) n is the number of unpaired electrons and s is equal to ½ (representing the spin of an unpaired electron).
From the equation given above, the magnetic moment
(μs) increases with an increase in the number of unpaired electrons.
Magnetic moments of some ions of the 3d-series elements
Ion |
Outer configuration |
No. of unpaired electrons |
Magnetic moment (BM) |
|
|
|
Calculated |
observed |
Sc3+ |
3d0 |
0 |
0 |
0 |
Ti3+ |
3d1 |
1 |
1.73 |
1.75 |
Ti2+ |
3d2 |
2 |
2.84 |
2.86 |
V2+ |
3d3 |
3 |
3.87 |
3.86 |
Cr2+ |
3d4 |
4 |
4.90 |
4.80 |
Mn2+ |
3d5 |
5 |
5.92 |
5.95 |
Fe2+ |
3d6 |
4 |
4.90 |
5.0-5.5 |
Co2+ |
3d7 |
3 |
3.87 |
4.4-5.2 |
Ni2+ |
3d8 |
2 |
2.84 |
2.9-3.4 |
Cu2+ |
3d9 |
1 |
1.73 |
1.4-2.2 |
Zn2+ |
3d10 |
0 |
0 |
0 |
In d-obitals belonging to a particular energy level, there can be at the maximum five unpaired electrons in d5 cases. Therefore, paramagnetism in any transition series first increases, reaches a maximum value for d5 cases and then decreases thereafter.
Example 1: Which ion has maximum magnetic moment
(a) V+3
(b) Mn+3
(c) Fe+3
(d) Cu+2
Ans: c
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