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### Why is 4s before 3d?

Posted: Sun Oct 20, 2019 9:03 pm
Hello, I was wondering why electrons fill 4s before 3d? Is this an important topic to know for the midterm?

### Re: Why is 4s before 3d?

Posted: Sun Oct 20, 2019 9:06 pm
This would be an important topic to know for the midterm! So when you look at the periodic table, electrons fill orbitals in that order. So since the elements that have 4s orbitals are in front of those that have valence electrons in the 3d state, they fill the 4s first. :) Over all, it's important to know that the electrons follow the periodic trends. I hope this helps!

### Re: Why is 4s before 3d?

Posted: Sun Oct 20, 2019 9:07 pm
Because 4s has lower energy than 3d. There are some exceptions like this where higher energy level subshells are actually filled first.

### Re: Why is 4s before 3d?

Posted: Sun Oct 20, 2019 9:07 pm
The 4s sublevel is filled before the 3d sublevel because the 4s is slightly lower in energy; therefore, easier to fill. An easy way to remember this is by looking from left to right on the periodic table.

### Re: Why is 4s before 3d?

Posted: Sun Oct 20, 2019 10:32 pm
charleejohnson3L wrote:This would be an important topic to know for the midterm! So when you look at the periodic table, electrons fill orbitals in that order. So since the elements that have 4s orbitals are in front of those that have valence electrons in the 3d state, they fill the 4s first. :) Over all, it's important to know that the electrons follow the periodic trends. I hope this helps!

The numbers 3 and 4 are used in respect to their energy levels, but do not necessarily have to do with the order in which the orbitals are filled.

### Re: Why is 4s before 3d?

Posted: Sun Oct 20, 2019 10:47 pm
This is because 4s has a lower energy and is easier to fill up. This will very likely be on the midterm.

### Re: Why is 4s before 3d?

Posted: Sun Oct 20, 2019 11:07 pm
because 4s is more easily filled up

### Re: Why is 4s before 3d?

Posted: Mon Oct 21, 2019 12:28 am
Hi,

For the 4s orbital n+ l = 4 + 0 = 4. For the 3d n+l = 3 + 2 = 5. So 4s is lower in energy so fills first.After calcium the 3d orbital falls below the 4s in terms of energy. This means the 4s electrons are lost first in chemical reactions and define the atomic radius of the atom. In summary, repulsive forces in the 3d sub shells tend to push electrons up into the larger 4s where the repulsion is less. Does this make sense?

### Re: Why is 4s before 3d?

Posted: Mon Oct 21, 2019 9:35 am
As mentioned by previous posters, the 4s orbital is filled before the 3d orbitals because it actually has lower energy than the 3d orbitals before they are populated. Note however that this reverses as soon as electrons begin to fill the 3d orbitals, at which point the electrons in the 4s orbital will be at a higher relative energy level than the electron(s) in the 3d orbitals. This is why the transition metals will lose electrons from their 4s orbital first when ionized.

I know you didn't ask about this but it is just as important to note that there are several exceptions to the rule described above. The ones that Lavelle would expect us to know and remember are Cr and Cu, which have electronic configurations of [Ar] 3d5 4s1 and [Ar] 3d10 4s1, respectively. Here, the 4s orbital is left only half-filled because singly- (in the case of Cr) or doubly-occupying (in the case of Cu) all of the orbitals in the 3d subshell results in greater stability than if the 4s orbital were fully filled and the 3d subshell was one electron short of the aforementioned configurations.

### Re: Why is 4s before 3d?

Posted: Mon Oct 21, 2019 3:29 pm
The 4s orbital initially has lower energy than the 3d orbital so according to the Aufbau principle it would fill first. However, as the 3d orbital is filled it begins to have less energy than the 4d orbital so electrons are removed from the 4d orbital.