Manganese has five unpaired electrons, but Iron has four, then why is Iron ferromagnetic and Manganese paramagnetic? What's that property I'm missing?
Answer
I'm guessing you meant Manganese rather than Magnesium, since Manganese has five unpaired electrons but Magnesium doesn't.
The answer is that ferromagnetism is not simply a function of having unpaired electrons. The effect is far more subtle than that. You would expect that the unpaired electrons in any material would tend to align themselves in opposition to each other because this is usually the lowest energy configuration. However in addition to the usual charge and magnetic interactions there is an interaction called the exchange interaction (I've linked to the Wikipedia article, but this is a complex area and hard going for the beginner).
Whether a material is ferromagnetic depends on the relative strengths of the exchange interaction with the other interactions, and it's a fine balance. This means even small changes may change a material to ferromagnetic or back. For example, although Iron is the best know ferromagnet not all crystal forms of Iron are ferromagnetic. The austenitic form of Iron is paramagnetic not ferromagnetic, so just changing the crystal structure slightly can switch between ferromagnetism and paramagnetism.
In Manganese the balance of the interactions prevents ferromagnetism, though note Manganese alloys like Heusler alloy can be ferromagnetic.
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