electronic Arrangement??

j2u

hi i am studying chemistry again for the leaving cert,im having a little trouble with transition metals,the bother is really how much can they hold in their third shell,can all the elements hold more than 8 in the third shell and so fourth,i think that its like 2 8 18,am i right?that the max for the first shell is 2 and then 8 for the second and then 18 for the third.is this in all elements or just with transition elements?potassium and calcium though have 2,8,81 and 2,8,8,2 so whats with that?Scandium for example though is 2,8,9,2?if the third shell can hold 18 then why is there only 9 in the third and yet 2 in the fourth?and how much and the fourth shell hold and the fifth??i dont undestand why scandium 2 ,8,11?is this because it has to have some in the fourth shell and the minimum is 2,as you can see im confused and perhaps i have not been very clear about my questions so if aanybody can help please do ,and i apologize for the collage of questions

Son Goku

This is going to seem like the lamest answer in the world, but basically it's quantum mechanics.
Because of how wavefunctions operate, due to the Schrodinger Equation, that is how the energy levels stack up.

Sev

For the leaving cert, you dont need to worry about the real reasons why the electron shell structure is as it is. To understand it properly it requires a full many electron quantum mechanical treatment.. something which would be quite a bit beyond leaving cert level. For the moment be content with giving them whatever crappy reason they give in the Leaving Cert syllabus... because "blah blah" is a more "stable" orbit or some such.

I always felt that leaving cert chemistry was just a load of bullshít. After doing 2 years studying it for the leaving cert.. I came out of that last chemistry exam (which was my last exam of the leaving cert) realising that I hadnt actually really learned or applied anything of value, and what I mostly learned was fake, superficial psuedo-chemistry. I promptly forgot everything I knew about "chemistry" soon after.

Now maybe I'm quite naive to think there isnt actually any real content in the leaving cert chemistry course, I went on to study physics so my knowledge of Chemistry isnt so good. But Id be interested to see what other people's opinions are of leaving cert chemistry.

Don't let my opinions of the crapness of the leaving cert chemistry course blind you in anyway to the huge value of doing chemistry as a leaving cert subject. It is in my opinion the easiest leaving cert course (that I did), with a no nonsense exam. So despite that it might not be the most rigorous treatment of course content.. lap it up.. because its great for points.

Professor_Fink

j2u wrote:

can all the elements hold more than 8 in the third shell and so fourth,i think that its like 2 8 18,am i right?that the max for the first shell is 2 and then 8 for the second and then 18 for the third.is this in all elements or just with transition elements?potassium and calcium though have 2,8,81 and 2,8,8,2 so whats with that?Scandium for example though is 2,8,9,2?if the third shell can hold 18 then why is there only 9 in the third and yet 2 in the fourth?and how much and the fourth shell hold and the fifth??

Ok, the reason for all this is quantum mechanics, as Son Goku mentioned, but I thought I'd try to give you a more detailed explanation.

Each electron is in some state. Electrons are Fermions, which means that two or more Fermions can never be in the same state. The result of this is that each electron must occupy a seperate energy level. Electrons are spin-1/2 particles (and hence Fermions) which means they have two distinct internal states. Often we refer to these as spin up and spin down. The result of this is that each orbital can hold two electrons with different spins. The ground state of an atom occurs when the electrons occupy the lowest possible energy levels.

The reason that the third shell only partially fills before the fourth shell starts filling is that there is an overlap between the energy levels of the two shells, and it becomes energetically favourable to fill the next shell, than to keep filling the current shell.

Hope this explains it for you.

Wikipedia has more here:
http://en.wikipedia.org/wiki/Transition_metals#Electronic_configuration