Ask A Scientist Chemistry Archive

name         Pat F.
status       educator
age          50s

Question -   Dear Sir:  I'm teaching 8th graders that the number of
electrons in energy levels is 2, 8, 8, 18, 18, 32, etc.  This is the way
I learned, but now my textbook says the amount of electrons in the
energy levels are 2,8,18,32, etc,on these amounts I cannot teach about valance
electrons or draw configurations that match the Periodic Table.  Can you
help?       Pat
------------------------------------------------
I may be in the minority, but I believe that we "over-teach" electron
configuration(s). They work OK so long as we confine the discussion to the
first two rows of the periodic table (more or less) where energy levels are
widely separated, but once we get into the transition elements and begin
filling 'd' orbitals, we have to start doing a shell game (pun intended).

Even in the first row, when we get to carbon, we have to invoke the sacred
"electron promotion energy" and "hybridization" to "explain" the four
equivalent bonds of carbon, and save our electron configuration dogma.
Consider: We start off with H-like atomic orbitals and before we get to the
sixth element, we already have to "make excuses"!!!

Is it any wonder that things get confusing when we get to the transition
elements, when we have to say, "Oh! Yeah, the order of filling is 's', then
'p',  but when from Ca, [Ar]4s2, gee whiz the d-orbitals fill before the p's
and the next elements Sc, Ti,... , fill [Ar]4s2,3d1,3d2 and after we fill
all the d's then we go back and fill the p's that we said filled just after
the 's' orbitals.

But have no fear,students, when we go from Tc [Kr]5s2,4d5 to Ru [Kr],5s,4d7
the 'd' and 's' orbitals reverse! Sorry 'bout that.

Don't even mention what happens further down in the periodic table when the
'f' orbitals start and they fill after the 's' and before the 'd', which
fill before the 'p'.

My point is that we are describing a very complicated process and try to
make a simplistic picture do too much -- too much is expected from a very
approximate model. We should admit up front that the model is not all that
good but it provides an approximate picture so long as we do not expect too
much from it.

Vince Calder
=========================================================
Hi, Pat !!

Do you know the sketch for fulfilling the
atomic levels that follows ?
1s2
2s2  2p6
3s2  3p6  3d10
4s2  3p6  3d10  3f14
etc
The sum of electrons in each level reaches :
2
2  +  6  =  8
2  +  6  +  10  =  18
2  +  6  +  10  +  14  =  32
etc.
of course, there is a rule for the fulfillment, difficult
to explain here, but easily found on text books.
The noble elements follow such numbers, what
corresponds to the most stable configuration.
Best Regards

Alcir Grohmann
=========================================================
The new text book has just reorganized the electrons. The concept of valence
is still possible to teach:
The first shell 1, has only two s electrons                     -1s(2)
The 2nd shell   2, has two s electrons and six p electrons      -2s(2),2p(6)
So far everything is the same.
The 3rd shell 3, has two s electrons, six p electrons and
         ten d electrons
-3s(2),3p(6), and 3d(10).
Essentially examining each shell individually provides the concept of
valence.
For example, within the 3rd shell with the 18 electrons, one readily explain
valence.
=========================================================