There are two tricksy little elements that don’t follow the rules for electron configuration, so… this is what they do. If you are not familiar with how to fill in electron configurations looking like this, I suggest you go and check out my other video on how to do that before you do this one. Because for this one we’re doing exceptions to rules, and it’s better to know the rules properly before I teach you the exceptions to them. So here we have Chromium: it has 24 electrons. I’m going to start filling at the bottom – I’m going to start filling singly and each electron’s going to have opposite spins. So here we go. One, two, three, four, five, six, seven, (filling singly before I fill doubly), eight, nine, ten, eleven, twelve, thirteen, fourteen, fifteen, sixteen, seventeen, eighteen, nineteen, moving to the 4s orbital because it has a lower energy than the 3d orbital and we need to start with the lowest energy levels & then fill upwards. But this is where things start to get a bit odd. One, two, three, four, five – you can see here I have now filled this 3d orbital even though this 4s orbital isn’t completely full. Which goes against the rule that you must fill the lowest orbital before you fill the next orbital up. The reason for this is – filling it this way gives every orbital in the 3d subshell an electron. And having a fully half-filled – and I know that sounds weird, but if you notice they’ve all got the same spin, so every spin in the orbital is filled even though it’s only filled with one electron. So having that half filled is more stable than the alternative configuration, which would be having two electrons in here and four electrons in here. So the reason this happens is because it’s a more stable configuration. We can write this out as – 1s²2s²2p⁶3s²3p⁶3d⁵4s¹ . Copper has 29 electrons – one, two, three, four, five, six, seven, eight, nine, ten, eleven, twelve, thirteen, fourteen, fifteen, sixteen, seventeen, eighteen – moving to 4s because it’s got a lower energy level – nineteen, and then I’m going to leave that half filled, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29. Now even though, like chromium, our 4s subshell isn’t fully filled, I have moved up and filled up the 3d subshell, because this is going to give Copper a more stable electronic configuration – if this was full and this only had 9 electrons in it. So those are the exceptions to the rules, the reason they do it is to become more stable. 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4s¹ .