MadSci Network: Chemistry

Re: Is the arrangement of energy shell 2,8,8,more than 8 or 2,8,18,32?

Date: Mon Apr 6 07:30:04 2009
Posted By: Calvin Cole, Faculty, Engineering Physics, Northeastern State University
Area of science: Chemistry
ID: 1237689360.Ch

We may have a bit of terminology issue here.  First, when we speak of an
“Energy Shell” in an atom we usually mean the electrons associated with the
same primary quantum number, usually designated with the letter “n”.  After
the first of these, n=1, however there is a secondary quantum number,
usually “l” that subdivides theses upper levels into an increasingly larger
number of divisions because as n increases there are multiple values of l
for each new n.  Fairly soon the higher values of l actually have energies
that are nearly the same as, or even larger than,  the next biggest value
of n than the one with which they are associated.  Second, we also often
speak of the “Outer Shell” of electrons of an atom as the ones that mostly
determine its chemical properties.  What happens is that for all but the
lightest elements this outer shell is composed of electrons associated with
the largest primary quantum number “n” and those few of the next lower
primary quantum number, n less one, that happen to have about the same
energies as the electrons with the higher primary quantum number.  The net
result of all this is as follows:
	The number of electrons associated with quantum number n=1 (the first
“Energy Shell”) is 2,  with n=2 (the second “Energy Shell”) it is 8, with
n=3 it is 18 and with n=4 it is 32 hence 2,8,18,32 as in your study guide.
 This order reflects the numbers associated with the solution to a
mathematical formula (the wave equation).
	The number of electrons in the “Outer Shells” of actual atoms is reflected
in the structure of the periodic table itself.  Because of the way
sublevels can overlap we actually see 2, 8, 8, more than 8 as your teacher
expressed it as the grouping of electrons in energy levels of real atoms. 
Of course the electrons occupying these outer shells do not all have the
same primary quantum number “n” but they do have similar energies.  This
sequence of numbers reflects the order in which electrons stack up in
energy and position in actual atoms.  

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