2.7: Applications of Electron Configurations: Valence Electrons and Electron Dot Structures (2024)

Valence Electrons

As was mentioned in a previous section of this chapter, electrons are highly important, because a specific subset of electrons, calledvalence electrons, are solely-responsible for determininghow elements bondwith one another.The number of valence electrons that are present in an atom can be determined from that atom's electron configuration. Valence electrons are found in the orbitals associated with an atom's highest occupied energy level. The remaining electrons, which are called inner shell electrons, do not participate in bonding and are, therefore, not important to study.

Consider sulfur's electron configuration, which was determined in the previous section and is replicated below.

1s²2s²2p⁶3s²3p⁴

Recall that the energy levelsin an electron configuration are theleading red numbers that denotethe start of a new energy level/orbital combination. Sulfur has electrons in the first, second, and third energy levels, as indicated by the leading red 1, 2's, and 3's, respectively.Valence electrons are those found in thehighest occupiedenergy level. Therefore, in this case, only those electrons associated with anenergy level/orbital combination beginning with a3need to be considered. Since two energy level/orbital combinations beginwith a3, both orbitals are selected for further consideration:

3s²3p⁴

The superscripts associated with these orbitals total to 6. Therefore, sulfur has 6 valence electrons.

While an electron configuration representsallof the electrons present in an atom of an element, chemists are only truly interested in an atom'svalenceelectrons, since, as indicated above, those are the electrons that are solely-responsible for determininghow elements bondwith one another. Therefore, finding a "shortcut" for determining how many valence electrons are present in an atom would be highly convenient. Such a "shortcut" does, indeed, exist. In a previous section of this chapter, three systemsfor labeling the groups, or columns, on the periodic table were presented. The second system, which is called the "A/B System," was indicated to provide insight into the electronic character of elements found within that group.

Again, consider sulfur, S, which,based on its electron configuration, has6 valence electrons.

Sulfur is located in the 16^thcolumnof the periodic table. However, the "A/B System"is used to labelthemain group elements. Group 16 is the 6^thcolumn in the main group, or "A-Block," columnsof the periodic tableandsois labeled asGroup 6A. Note that sulfur's valence electron count matches its group number in the "A/B System." This connection applies tonearlyallelements found in themain groupcolumns of the periodic table. Helium is the only exception to this rule, as itis found in Group 8A, but only contains two total electrons. This inconsistency invalidates the "A/B shortcut" method, and the electron configuration method must be employed to determine that both of helium's electrons are valence electrons.

Since the "A/B System" group number corresponds to the number of valence electrons that are present in an atom, all elements found within the same columnhave the same number of valence electrons. Since an atom's valence electrons are solely-responsible for determininghow elementsbondwith one another, this commonality in electronic character explains why all of the elements within the same group share similar properties.

Electron Dot Structures

Electron dot structuressurround the elemental symbol of an element with one dot for every valence electron that the element contains.When drawing an electron dot structure, three rules must be followed:

1. The first dot can be placed on any "side" of the elemental symbol (top, bottom, left, or right).
2. The first four dots must each be placed on their own "side" of the elemental symbol. In other words, if the first dot is placed on the topof the elemental symbol, the second dot can be placed on the bottom, left, or right of the symbol, butcannotbe placed at the top, alongside the first dot.
3. The final four dots can again be placed on any "side" of the elemental symbol, but must be arranged such that no more than two dots exist on any "side" of the elemental symbol.

Again, consider sulfur, whichhas 6 valence electrons.

The elemental symbol for sulfur is S. Since an electron dot structure surrounds an elemental symbol with one dot for everyvalence electron that the element contains, sulfur's elemental symbol must be surrounded by 6 dots. Based on the rules given above, the dot representing sulfur's first valence electron can be placed on any "side" of the symbol, as shown below inFigure \(\PageIndex{1}\).

If the first structure in Figure \(\PageIndex{1}\) is chosen as the basis of sulfur's electron dot structure,the dot representing sulfur's second valence electron can be placed on the bottom, left, or right of the elemental symbol, butcannotbe placed at the top, alongside the first dot.Figure \(\PageIndex{2}\) shows three structures with acceptable placements for sulfur's first two valence electrons, as well as a structure with an incorrect electron arrangement.

If the final structure in Figure \(\PageIndex{2}\) is chosen as the basis of sulfur's electron dot structure,the dots representing sulfur's third and fourth valence electrons must be placedon the bottom and to the leftof the elemental symbol, butcannotbe placed at the top or to the right of the elemental symbol.Figure \(\PageIndex{3}\) shows the onlystructurewith an acceptable placementfor sulfur's first four valence electrons.

The dots representing sulfur's fifth and sixthvalence electrons can again be placed on any "side" of the elemental symbol, but cannot both be placed on the same "side," so thatno more than two dots exist on any "side" of the elemental symbol. Figure \(\PageIndex{4}\) shows all of the structures with acceptable placements for sulfur's six valence electrons. Therefore, any of the structures inFigure \(\PageIndex{4}\) is a valid electron dot structurefor sulfur.