A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Why don't magnets stick to aluminum?

February 22, 1991

Dear Cecil:

Why don't magnets stick to aluminum?

Cecil replies:

It all has to do with electron shells. In a column of general circulation, however, it's always risky to jump straight into a discussion of electron shells. Better we should edge into this.

First some facts. Fact #1: magnets only stick to other magnets. Fact #2: big magnets are made up of jillions of tiny magnets. Fact #3: so are the metals the magnets stick to, notably iron, nickel, and cobalt, which are called ferromagnetic materials. The difference is that in the big magnets the tiny magnets are organized, i.e., they're all lined up with their north poles in one direction and their south poles in the opposite direction. In an ordinary ferromagnetic material, the tiny magnets are scattered every which way, and their magnetic fields cancel each other out, so no magnetism overall.

But suppose we enterprisingly place a ferromagnetic material in a strong magnetic field. Voilà, the formerly scrambled atoms line up parallel with one another. The material as a whole becomes magnetized and sticks to the magnet. Aluminum doesn't contain tiny magnets, so there's nothing to get organized and nothing for the big magnet to stick to.

Certain restless intellects out there may now be wondering: what's with this tiny magnet crap, anyway? That's where the electron shells come in. As you may have guessed by now, the tiny magnets we're talking about are individual atoms. Some atoms, such as those in iron, have individual magnetic fields, while others, such as those in aluminum, do not. It all has to do with the electrons.

Electrons may be thought of as spinning, much as the earth does. They spin one way, they develop a magnetic field with north on top and south on the bottom; they spin the opposite way, they develop a magnetic field with north on the bottom and south on top. For convenience, we call the two directions of spin positive and negative.

Most atoms, such as those in aluminum, have half their electrons spinning in one direction and half in the opposite direction. That means the magnetic fields of the individual electrons cancel each other out. But in the ferromagnetic materials things are different. Take a gander at the third subshell of the M shell of iron, for example. (A shell is an electron's orbit. Electrons are rigidly organized into layers of shells, with so many electrons per shell.) What a wacky sight! We find five electrons with a positive spin and one with a negative spin. This gives the iron atom a pronounced magnetic field. You get those iron atoms lined up, you've got yourself a magnet.

Then we get into a little matter requiring a discussion of quantum mechanics.   (What's that, you're sorry you asked? Too late now.) Certain non-ferromagnetic materials, such as chromium and manganese, also have uneven numbers of positive- and negative-spinning electrons in their inner electron shells.  Each atom of these substances is magnetic, but the substance as a whole is not.  Why?  Well, in chromium and manganese, each atom with "up" magnetism is paired with an atom of "down" magnetism, cancelling out the magnetism of the substance as a whole.   In iron, however, all the atomic magnets point in the same direction, so it does (or can) have magnetism overall.

What keeps all the iron atoms pointed in the same direction? It's a quantum mechanical effect known as "exchange interaction." The details of this are still being debated but one plausible interpretation goes like this: Let's say the inner shell or "local" electrons of iron Atom A are spinning in such a way that they have "up" magnetism. The local electrons cause the nearby loose electrons floating around in the metal (the "conduction" electrons) to have opposite or "down" magnetism. The conduction electrons in turn cause the local electrons of neighboring iron Atom B to have "up" magnetism. Result: all the atomic magnets point up and the iron is potentially magnetic.

So why are chromium, manganese, et al different? It turns out manganese and chromium atoms are so close together that the local electrons of Atom A force the local electrons of neighboring Atom B to orient themselves in the opposite direction, without any intervening conduction electrons entering into the picture. Thus each "up" atom is paired with a "down" atom, and the material has no magnetism overall.

That's all pretty clear, right?  Well, maybe not.  But it's about as clear as stuff like this ever gets.

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