Paramagnetism of Transition Metal Salts

The salts of various first-row transition metals are weakly attracted to our "mondo" magnet because of unpaired electrons if they have weak-field ligands. This can be contrasted to the absence of paramagnetism in a complex salt with strong-field ligands. Samples of transition metal salts are placed in an apparatus that indicates the strength of the magnetic force that is exerted on them by measuring the angular displacement of a hanging sample from the vertical position.

Curriculum Notes 

This demo is usually used when discussing Hund's Rule and unpaired electrons. It is used in an inorganic chemistry course when discussing crystal field theory. Allow about 15 minutes for this demo.

Lead Time 
One day of lead time is required for this project.
Discussion 

Paramagnetism is when a substance is weakly attracted to a magnetic field. It occurs when there are unpaired electrons in the substance. Paramagnetism is most easily observed in the salts of some of the first row transition metals (manganese through nickel).These metal ions have unpaired electrons in degenerate d orbitals as predicted by Hund's rule and thus exhibit paramagnetism. But strong-field ligands can split the energy levels of the d orbitals so that they are no longer degenerate. Salts of these ions that have strong-field ligands are diamagnetic. 

Water is not attracted to the magnet poles; water is diamagnetic. 

There is no attraction of the sodium chloride to the magnet, even when the vial strikes the magnet. There are no unpaired electrons in sodium chloride and sodium chloride is diamagnetic. 

Manganese(II) sulfate monohydrate is strongly attracted by the magnet, which shows that it is paramagnetic. Manganese ions in manganese(II) sulfate monohydrate have five unpaired electrons. 

Iron(II) sulfate pentahydrate can be brought closer to the magnet than the manganese(II) sulfate monohydrate, but eventually it swings toward the magnet. This compound is not as strongly paramagnetic as the manganese(II) sulfate. Iron(II) ions have four unpaired electrons. 

Cobalt(II) chloride hexahydrate can be brought closer to the poles than the iron (II) sulfate or the manganese(II) sulfate before it swings toward the magnet. Cobalt(II) ions have three unpaired electrons. 

When nickel(II) sulfate hexahydrate is brought near the poles of the magnet, it appears to have about the same attraction as the cobalt(II) chloride, perhaps slightly less. Nickel(II) ions have two unpaired electrons. 

When zinc(II) sulfate heptahydrate is brought near the poles of the magnet there is no attraction. Since the d subshell in zinc(II) ion is full, there are no unpaired electrons and it is diamagnetic. 

Potassium hexacyanoferrate(II) is shown to be diamagnetic. The d orbitals in the iron(II) ions are split by the strong crystal field of the cyanide ligands, allowing all six electrons to pair. 

Hexaamminecobalt(III) chloride is not attracted to the poles of the magnet. As in the case of the hexacyanoferrate(II) ion, the strong field of the ammonia ligands splits the d orbitals and allows all electrons to pair.1

1Journal of Chemical Education, "Chemistry Comes Alive," > January 2006.

Materials 
  • computer
  • video projector
  • paramagnetic deflection apparatus
  • Vernier laboratory interface
  • sample vials containing compounds (water, sodium chloride, manganese(II) sulfate monohydrate, iron(II) sulfate heptahydrate, cobalt(II) chloride hexahydrate, nickel(II) sulfate hexahydrate, zinc(II) sulfate heptahydrate, potassium hexacyanoferrate(II) trihydrate, and hexamminocobalt(III) chloride)
Procedure 
  • Place each sample vial in the apparatus. The deflection in degrees will be projected.
  • Water and sodium chloride serve as controls. They are not attracted to the magnet.
  • Manganese(II) sulfate monohydrate, iron(II) sulfate heptahydrate, cobalt(II) chloride hexahydrate, and nickel(II) sulfate hexahydrate are weakly attracted to the magnet.
  • Zinc(II) sulfate heptahydrate, potassium hexacyanoferrate(II) trihydrate, and hexamminocobalt(III) chloride are not attracted to the magnet.
Safety Precautions 
  • The magnet is very heavy. Do not attempt to remove it from the cart.
  • It is also very strong. Be careful with iron and steel objects around it. It will pinch your finger badly if you aren't careful. You can carefully allow an iron or steel object to stick to it so that you can contrast paramagnetism and ferromagnetism, but make sure that it's big enough that you can get a grip to pull it loose. It ain't easy!
  • Don't let calculators, PDAs, computers, monitors, or credit cards get near the magnet.
  • Don't open the vials. Nickel(II) sulfate hexahydrate is a suspect carcinogen and is harmful if swallowed, inhaled, or absorbed through the skin. Zinc(II) sulfate heptahydrate is harmful if swallowed.
Prep. Notes 

Be careful with the "mondo" magnet. Besides being really strong (see above) it's also heavy !

© Copyright 2012 Email: Randy Sullivan, University of Oregon Chemistry Department and UO Libraries Interactive Media Group