5.3. Electron Arrangement in Atoms http://www.ck12.org
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Click image to the left for more content.- Electron Configurations 2 at http://www.youtube.com/watch?v=YURReI6OJsg (10:18)
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Click image to the left for more content.Second Period Elements
Periods refer to the horizontal rows of the periodic table. Looking at a periodic table, you will see that the first period
contains only the elements hydrogen and helium. This is because the first principal energy level consists of a single
sorbital, so only two electrons are required to fill the entire level. Each time a new principal energy level begins
to hold electrons, a new period is started on the periodic table. As one moves across the second period, electrons
are successively added to orbitals in the second principal energy level. After beryllium (Z= 4), the 2ssublevel is
complete, and the 2psublevel begins to fill starting with boron (Z= 5). Since there are three 2porbitals and each
orbital holds two electrons, the 2psublevel is filled after six elements. Listed below (Table5.3) are the electron
configurations of the elements in the second period.
TABLE5.3: Electron Configurations of Second-Period Elements
Element Name Symbol Atomic Number Electron Configuration
Lithium Li 3 1 s^22 s^1
Beryllium Be 4 1 s^22 s^2
Boron B 5 1 s^22 s^22 p^1
Carbon C 6 1 s^22 s^22 p^2
Nitrogen N 7 1 s^22 s^22 p^3
Oxygen O 8 1 s^22 s^22 p^4
Fluorine F 9 1 s^22 s^22 p^5
Neon Ne 10 1 s^22 s^22 p^6Upon reaching the element neon, a 6thelectron is added to the 2psublevel, and the second principal energy level is
now filled. We will see that the electrons in the outermost principal energy level are very important for predicting and
explaining chemical reactivity, so they are given a special name.Valence electronsare the electrons in the highest
occupied principal energy level of an atom.In the second period elements listed above, the two electrons in the 1s
sublevel are called inner-shell electrons, and they are not directly involved in the atom’s reactivity or in the formation
of compounds. Lithium has a single electron in the second principal energy level, so we say that lithium has one
valence electron. Beryllium has two valence electrons. How many valence electrons does boron have? Because the