Difference Between Electron & Valence Shells - Explained

[Ratzinger]

I would like to get clarified the difference between electron shells and valence shells.
Electron shell is defined by quantum number n, the energy level. So there are 2e in n=1, 8e in n=2, 18e in n=3, 32e in n=4 and so on.
But what is the exact definition of valence shells, ecxept that the 1st takes 2, the 2th 8, the 3th 8, the 4th 18 electrons and so on?

Why does element 19 start with 4s^1 even though all the 3d orbitals are not filled yet? Why is not first one entire electron shell completed before starting to fill a new one?

thank you

 

[Astronuc]

The valence 'shells' or 'orbitals' are the outermost 'orbitals' and are available for chemical rections. Also remember that there are limited chemical reactions due to the limited number of non-metallic elements, e.g. halides, O, S, P, C, and so on. When metals are mixed, the result is an alloy, which is different than a chemical compound.

If one looks at the orbital wave equations for the 3d and 4s, the 4s would show lower energy levels than the 3d shells. Similarly for periods 5, 6 and 7, the s orbital fills before the d and f shells of the previous periods.

See the order in which shells are filled here -

http://en.wikipedia.org/wiki/Periodic_table#Explanation_of_the_structure_of_the_periodic_table

 

[Ratzinger]

thanks Astronuc

I also found a nice explanation to my problem in Linus Pauling "General Chemistry" on page 138 (Dover). Great book and a cheap buy.