Why do chemistry books lie to us about how many electrons fit on the shells ?

Question

I'm pissed off as I discovered at 19 years old that the third shell holds 18 electrons not 8 like we have been taught in those books throughout our school life. Now it's annoying to continue to study from these same sources seeing them revising the info about 2,8,8 etc. and knowing it should be 2,8,18 etc.! Why do they do this? At this age, believe me, we already know how to count to 18, and guess what, even further than that! It just makes you wonder whether the other info you're learning now is also lies of some sort... :(

Answer 1

Some authors think that it's better to state for the time being that n=3 holds 8 electrons because the next two electrons go to the 4s orbitals. And some intro texts really don't discuss electron configuration past element no. 20 (Ca) so they don't get into why the 3d orbitals fill next.

Answer 2

They do that because elements start filling up their fourth shell before they've finished filling the third.

The first two elements have electrons in the 1s sub-shell only. The next eight go on to use 2s and 2p. The next eight use 3s and 3p, but leave 3d empty. Then the next two use s4, then the first row of transition metals fill up 3d, then the next six elements fill up 4p.

Therefore, while the third energy level CAN hold 18 electrons, it only starts using more than 8 after the next energy level up has filled its s sub-shell.

Just saying 2, 8, 8 is easier, since it doesn't require the long, and more indepth explanation.

This page has a diagramatic illustration of the order subshells are filled:

http://www.green-planet-solar-energy.com...

Answer 3

18? maybe it was a typo never heard of it.

can u site ur source?

Answer 4

Your confusion comes from two different ways of looking at 'shells'.  But first, there ARE truly errors that show up in textbooks.  This does not mean that the author is lying to you.  Think about it, there are probably a few errors in what you've written, too.  But in this case, the confusion is yours.  

When you define a shell as the quantum number associated with it, then you're right about the 18.  The 3d is the first d, giving the '3-shell' 18 electrons.  

But the shells don't fill in a nice quantum-number order, as you probably know.  So, 4s fills before 3d.  On the periodic table, then, row 3, starting with the 3s alkali metals and ending with the 3p noble gasses, contains only 8, making 8 the right answer.  Since the natural periodicity caused by the noble gasses and alkalai metals is a prominent feature, and the electron shells actually fill according to the rows of the periodic table, that's a better way to define the shells.  

So that you don't get all excited again when you discover the next 'lie', the same thing happens again with the rare earths (the f shells).  

The moral of the story is that when you find an error in the textbook, think real hard about where your own error might be, and then talk about the error.  

Answer 5

i guess it is just old information

like it might have been thought correct at the time of publish and your school hasn't updated their library

Answer 6

If you are a girl as your pic indicates, then you are of the new breed: getting "pissed off" at your elders! Don't give up being skeptical; that is what scientists are SUPPOSED to do and some of those "old" ideas need to be rethought!

If you want some more problems with electron distribution, consider that 2 electrons go into each orbital without any e-e repulsion!

A different way of looking at e-distribution is the MCAS model where only ONE electron goes into each orbital. Orbitals are pointed uniformly outward from the nucleus. 8 e's to the four corners of a cube; 18 to the edges and centers of the faces; and 24 to the faces in "squares" around the center. These distributions allow the electrons to be as far removed from one another as possible and still be associated with the nucleus.

The current spdf model PILES up electron density along the 3 major axes with those spherical orbitals intersecting them all!

Ever thought about how 7-f orbitals can be uniformly distributed? the 5-d's were done by having a dx2 (degenerate) orbital when 6 should have occured. In other words, the model was forced to fit the number of electrons whether the electron spatial distribution was correct or not.

All required "hybridization" when modeling molecules!

To your "problem": the numbers refer to the filling of the outermost orbitals: Thus: He 2, Ne (2)8, Ar (2)(8)8, Kr (2)(8)(18)8, etc. So in a realistic way, both ARE correct. They refer to different aspects of the same topic.