|MadSci Network: Chemistry|
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.
Try the links in the MadSci Library for more information on Chemistry.