Woah, why is the floor moving? Oh wait, it's just the D orbital...Why is the D orbital moving? Before we can answer that, we need to find out what electron configuration is.
Electron configuration is another way to identify an element based on its electrons. If you recall from before, every element has a distinct number of electrons. These electrons are found in energy levels around the nucleus. These electrons are further divided into different orbitals.
There are rules that define how an electron fills up each energy level, which is how we can predict the electron configuration. The rules are as followed:
1)Aufbau Principle-electrons occupy orbitals in order of increasing energy.
2)Hund's Rule-Electrons fill in the same direction until there are no spaces left, and then they fill in the opposite direction
3)Pauli-Exclusion Principle-Electrons fill in pairs.
As we learned before, orbitals can only hold a certain amount of electrons. We also learned that they have a certain number of different shapes. For simplicity sake in understanding such a complex concept, we will say that each shape can only hold two electrons, (which is true, just not a very good way of putting it) which spin in opposite directions. Hence, s orbitals have one shape and can only hold 2 electrons, p orbitals with 3 shapes holding 6 electrons in total, etc.
Now, let's go into the actual figuring out of an electron configuration. You really only need to use Aufbau Principle to figure out most configurations, but some exceptions occur which we will talk about later that require other rules. Aufbau Principle states that electrons fill in increasing energy. We denote orbitals first by the energy level it's in and then the name of the orbital. So, the lowest is 1s, then 2s, then 2p, 3s, and you follow down the periodic table. You then, through superscript, denote how many electrons are in that orbital. So, Boron would be 1s22s22p1.
Now, going back to why the D orbital is moving and to some exceptions in the table. Surprisingly, 3d orbital has more energy than 4s orbital, which means electrons will fill in the 4s orbital first, then 3d. Hence, for visuality sake, the D orbital is moving to denote this exception. It is as if the D orbital shifted up and is in the 3rd energy level, but we know that in reality it is actually in the 4th energy level.
The exceptions are also in the D orbital. This brings us back to the other rules. Hund's rule states that electrons will fill in one direction until the orbital is filled before filling in the reverse direction. Electrons like to be able to fill half or a full orbital. Hence, in groups 6, the element will prefer to have a half filled S and D orbital rather than a filled S orbital and almost half filled D orbital. Same with group 11, where the element would rather have a fully filled D orbital and half filled S orbital than a filled S orbital and almost filled D orbital.
Exceptions in the F orbital exist as well, but most of the time, in a classroom setting, knowing the electron configuration of the F orbital is not required. It is listed just for reference.