# Diamagnetic vs Paramagnetic

As far as I know, to know whether a substance is paramagnetic or diamagnetic, we check if that particular species has got unpaired electron or not.

But my textbook lists-

Examples of Diamagnetic Materials - Copper,Lead,Sillicon,etc

Examples of Paramagnetic Materials - Aluminium,Sodium,Calcium etc

The valence shell configuration of Copper is 3d10 4s1 . So, shouldn't it be Paramagnetic?

The valence shell configuration of Calcium is 4s2 . So, shouldn't it be Diamagnetic?

You're right that Cu0 has one unpaired electron in the 4s orbital. If you were to observe a single Cu0 atom in the gas phase, you would observe some paramagnetic behavior.

However, in metallic copper, the valence orbitals are engaged in metal-to-metal bonding. The 3d and 4s orbitals interact in constructive overlap with neighboring copper atoms to produce bonding molecular orbitals.

When two 4s orbitals (each containing 1 unpaired electron) combine to form a sigma bonding MO, the electrons in that bonding MO become paired, per the image below.

Note: this MO diagram depicts the bonding in H2, but I'm just using it to illustrate the principle of partially filled atomic orbitals coming together to form a filled bonding MO

The process of deriving the MO diagram for a metallic solid is obviously more complicated than this, but the answer to your question is that a single Cu0 atom is paramagnetic, but Cu2 would be diamagnetic. By extension, metallic copper is effectively Cu$$_\infty$$ and also diamagnetic.

As you can tell, the process for determining whether an atom or ion is para/diamagnetic is a very different question than whether the bulk substance (metal) is para/diamagnetic. You can't do the latter by electron counting alone; you need to know more about bonding in the substance. As another example of this, you mention that lead is diamagnetic. Lead is another example of a paramagnetic atom (6p2; the 6p orbital contains two unpaired electrons) - but lead metal is diamagnetic because those electrons become paired in the bonding MOs.

• What about Sodium, Calcium and Aluminium? Shouldn't ALL these three be diamagnetic for the same reason i.e. they exist in crystals and so no electron is left unpaired? So, why are these paramagnetic? – Sristy Sep 23 '19 at 5:28
• Like I said in closing my answer, the (para)magnetic properties of bulk metals is far more complicated than my explanation and can't easily be predicted by electron counting. You get into metallic bonding, band structure, and a lot of solid-state physics that I don't understand well enough to explain. For example, look at the relative magnetic susceptabilities of Li and Na in this table: en.wikipedia.org/wiki/Paramagnetism#Delocalization – Greg D Sep 23 '19 at 15:15
• Why does Li have twice the magnetic susceptibility of Na, when they are both s1 in their valence shells? It has to do with the mechanism of Pauli paramagnetism, which, again, I can't explain. – Greg D Sep 23 '19 at 15:16
• So, basically I have to mug up the examples..but good explanation on your part although I don't know what Pauli Paramagnetism is . – Sristy Sep 23 '19 at 17:45