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### 4s and 3d Orbitals

Posted: Mon Oct 22, 2018 1:47 pm
I remember from lecture that the 4s orbital has more energy than the 3d orbital, but if this is the case, why is the 4s orbital filled first?

### Re: 4s and 3d Orbitals  [ENDORSED]

Posted: Mon Oct 22, 2018 2:02 pm
According to Aufbau's principle, orbitals are filled in accordance to the lowest value of orbitals. For 4s, n+1=4+0=4, and for 3d, n+1=3+2=5
So, although 4s is higher in energy because it has a higher value of n, it is filled first because it has a lower value of orbitals.

### Re: 4s and 3d Orbitals

Posted: Mon Oct 22, 2018 3:05 pm
I forgot to add that for multi-electron atoms that come after atomic number 20, the 4s state is higher in energy than the 3d orbital, as you would expect. The 3d orbital is lower than the 4s only on the condition that the 4s orbital is occupied.

### Re: 4s and 3d Orbitals

Posted: Mon Oct 22, 2018 5:06 pm In some cases, the 4s orbital actually has LESS energy than the 3d orbital, and thus is filled first. This occurs for K and Ca. However, when an element's atomic number is greater than 20, e.g. Scandium (Sc) the 4s orbital has MORE energy than the 3d orbital, which is expected.