Sub-Shell Question

[zbentinel]

Ok im studying just because I want to learn all about well everything,a nd Ive started with some chemistry. Im working through atomic structure at the moment, and i am really puzzled by one part of it, and after browsing the net for ages I cannot find an answer for this.

 

with sub shells I've noticed that the 4s energy level is filled before the 3d energy level, I want to know why this is, and also if there is a formula for working out which energy levels get filled before others. because Having 4s filled before 3d is confusing enough, but when I start getting into the f orbital equations and i think that 4f comes above 5s ect it all gets very tricky for me to work out what goes where. So apart from looking at the periodic table to see what order the shells should fill, are there any other ways I can do it?

 

Im sorry if this isnt clear but its a damn hard question to voice correctly.

[Klaynos]

The 4s energy levels are less energetic than the 3d, so they get filled first. I recall seeing, a few years ago, a graph which had all of the energy levels of all of the states plotted, so you'd have a line for '1' a line for '2' etc... so you could see where they cross and thus work out which one would be filled first.

 

There are certainly quantum mechanical methods of calculating the energy levels and there might be some other simpler method as well.

[zbentinel]

Hey thanks for the reply. Yeah I read that the 4s energy level is less energetic than the 3d, but 1 I dont understand why, and 2 if anyone knows a link to such a graph with all the energy levels in order thatd be great, because if i want to work out the electronic configuration for say Cerium, id never be able to do it in the correct order due to not knowing which energy levels come where.

[hermanntrude]

the rule for which energy level gets filled first is called the "n + l" rule. I think you know that the "n" is just the number of the shell. "l" is the second quantum number, related to the subshell... s subshells have l=0, p have l=1, d have l=2 and so on. For any subshell, add up the values of n and l,then put them in the order you see them listed and you will find they go in order:

 

1s : n = 1, l = 0, n+l = 1

2s : n = 2, l = 0, n+l = 2

2p : n = 2, l = 1, n+l = 3

3s : n = 3, l = 0, n+l = 3

 

Notice that the 3s and 2p subshells both have the same value of n+l, but the 2p goes first because it has a lower value of n. This rule is important as a tie-breaker when subshells have the same values of n+l.

 

3p: n = 3, l = 1, n+l = 4

4s: n = 4, l = 0, n+l = 4

3d: n = 3, l = 2, n+l = 5

4p: n = 4, l = 1, n+l = 5

5s: n = 5, l = 0, n+l = 5

4d: n = 4, l = 2, n+l = 6

5p: n = 5, l = 1, n+l = 6

6s: n = 6, l = 0, n+l = 6

4f: n = 4, l = 3, n+l = 7

5d: n = 5, l = 2, n+l = 7

6p: n = 6, l = 1, n+l = 7

7s: n = 7, l = 0, n+l = 7

5f: n = 5, l = 3, n+l = 8

6d: n = 6, l = 2, n+l = 8

7p: n = 7, l = 1, n+l = 8

8s: n = 8, l = 0, n+l = 8

5g: n = 5, l = 4, n+l = 9

 

etc etc etc

[zbentinel]

This is exactly what I wanted, Thank you very much, now I'll be able to figure things out a hell of a lot easier, and also thank you for pointing out the same value thing.

[hermanntrude]

weirdly, most teachers don't teach the n+l rule and just expect students to memorize the order of the subshells. It's a personal crusade of mine to show the n+l rule to the world.

[zbentinel]

Well I'm not actually in school or college I'm teaching myself from revision books ect out of interest mainly, and so I can go to uni in a few years time and do a degree. But this rule is extremely helpful, as I dont want to just memorise the order Id rather know why its in that order, and that formula explain it perfectly.

 

I have another question now. There are some questions and answers in the revision booklet and one of them is asking me to write down the electronic configuration for Iron.

 

So I put down:

 

Fe: 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d6.

 

The answer booklet told me it was this

 

Fe: 1s2, 2s2, 2p6, 3s2, 3p6, 3d6, 4s2.

 

Now I thought that 4s came before 3d, so why is the electronic configuration written out like this?

[hermanntrude]

Well I'm not actually in school or college I'm teaching myself from revision books ect out of interest mainly, and so I can go to uni in a few years time and do a degree. But this rule is extremely helpful, as I dont want to just memorise the order Id rather know why its in that order, and that formula explain it perfectly.

 

I have another question now. There are some questions and answers in the revision booklet and one of them is asking me to write down the electronic configuration for Iron.

 

So I put down:

 

Fe: 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d6.

 

The answer booklet told me it was this

 

Fe: 1s2, 2s2, 2p6, 3s2, 3p6, 3d6, 4s2.

 

Now I thought that 4s came before 3d, so why is the electronic configuration written out like this?

 

 

both answers are acceptable.

 

the first answer shows the electronic configuration in the order the shells are filled up. The second shows the configuration in order of the values of n and l (not n plus l), which in this case is also the same as the order in which iron LOSES electrons. Weirdly, once the 4s subshell is full it becomes the outer subshell and when transition metals ionize, they lose their s electrons first. It's probably best to use whichever one your book uses to avoid confusion.

[zbentinel]

thanks again hermanntrude, your answers have been the best explained and easiest to understand, I have tried giving you more rep but it wont let me until I spread the rep love around a bit more!

[BlondeEngineer]

Electron wants to have it's Proton... So the closer shells fill in first...

 

s shell is closer than d shell... etc...

 

However, some atoms support each other and create stable lattice structures, hence why the metals with f shells will act different than all the others... There is no real way to "predict" this behavior than by looking and understanding the periodic table and why it was situated in the way it was...

 

BTW, it really gets nasty when you come to the "artificial" atoms... Radioactive isotopes can really screw things over...

 

Best bet is to memorize the exceptions to the rule...

 

Have fun! Quantum Mechanics is a lovely sport...

 

I just can't seem to place that electron and clock it at the same time... YEESH!

[hermanntrude]

...There is no real way to "predict" this behavior...

 

did you even read the other posts? I just offered a perfectly good way of predicting this behaviour.