Physics 2000 Science Trek Elements as Atoms

Electron Configurations and the Periodic Table II

I've never come right out and said this, but I'm sure you've noticed that the energy levels get filled in order from lowest energy to highest; when you add a new electron, it goes into the lowest-energy state that's available.

Sure, that's pretty obvious. That's why I thought the next electron for potassium would go into a d orbital instead of up to a whole new row--you said that a higher primary level always means a higher energy.

I said usually, not always. The energy levels aren't always so well-behaved as one might like; it sometimes happens that the first orbitals in a "higher" primary level actually have less energy than the top orbitals of the level below.
Click on the advanced button to find out why this occurs.

So in potassium, the 4s orbitals end up with less energy than the 3d states--that's why potassium starts a new row in the periodic table. I bet calcium (Ca) puts an electron in that second 4s orbital...yup, it does.

Okay, now click on scandium (Sc).

Now the 3d orbitals are getting some attention! All those elements from scandium through zinc (Zn) are just filling the ten green d spaces. Then gallium (Ga) goes back up and starts the 4p orbital.

This periodic table ends at krypton, when the 4p orbitals are filled--but of course one can keep going. Click here to open an extended version of the periodic table, which shows the electron configurations for the elements up to 103. I'll let you play around with this and figure out the order in which the various orbitals are filled.

A note about the arrangement of elements: barium (Ba) is element number 56; element 57, lanthanum (La), begins that first separate row at the bottom, which continues up to number 71, lutetium (Lu). Number 72, hafnium (Hf), is up next to barium again. A similar thing happens in the seventh row of the table. Look at the electron configurations and see if you can tell why these elements are arranged in this way.


Go to quantum numbers

> 18819th