http://www.ck12.org Chapter 5. Electrons in Atoms
second principal energy level consists of both the 2sand the 2psublevels, the answer is three. The configuration of
neon ends in 2s^22 p^6 , so it has eight valence electrons.
The magnetic properties of various elements are related to the number of unpaired electrons in each atom. For
example, hydrogen has only a single electron, so it is necessarily unpaired. Helium has no unpaired electrons
because both of its electrons are in the same orbital. To occupy the same orbital, they must have opposite spins, so
in terms of their magnetic properties, the two electrons cancel each other out. Proceeding across the second period,
we find the following numbers of unpaired electrons:
Li = 1, Be = 0, B = 1, C = 2, N = 3, O = 2, F = 1, Ne = 0In order to correctly identify the number of unpaired electrons, you may find it necessary to construct the orbital
filling diagram. Correctly following Hund’s rule will have an effect on the number of unpaired electrons. Oxygen’s
orbital filling diagram serves as an example:
The four electrons in the 2psublevel are ordered such that two are paired up in the first orbital, while the single
electrons in the second and third orbitals are unpaired.
Third Period Elements
Sodium, element number eleven, is the first element in the third period of the periodic table. Its electron configuration
is 1s^22 s^22 p^63 s^1. The first ten electrons of the sodium atom are the inner-shell electrons, and the configuration of just
those ten electrons is exactly the same as the configuration of the element, neon (Z= 10). This provides the basis for
a shorthand notation that is commonly used to abbreviate electron configurations for larger atoms. The elements that
are found in the last column of the periodic table are an important group of elements called the noble gases. They
include helium, neon, argon, krypton, xenon, and radon. The electron configuration of an atom can be abbreviated
by usingnoble gas notation,in which the elemental symbol of the last noble gas prior to that atom is written first,
followed by the configuration of the remaining electrons. For sodium, we can substitute [Ne] for the 1s^22 s^22 p^6 part
of the configuration. Sodium’s electron configuration can now be written [Ne]3s^1. The table below (Table5.4)
shows the electron configurations of the third period elements.
TABLE5.4: Electron Configurations of Third-Period Elements
Element Name Symbol Atomic Number Electron Configuration
Sodium Na 11 [Ne] 3 s^1
Magnesium Mg 12 [Ne] 3 s^2
Aluminum Al 13 [Ne] 3 s^23 p^1
Silicon Si 14 [Ne] 3 s^23 p^2
Phosphorus P 15 [Ne] 3 s^23 p^3
Sulfur S 16 [Ne] 3 s^23 p^4
Chlorine Cl 17 [Ne] 3 s^23 p^5
Argon Ar 18 [Ne] 3 s^23 p^6