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Ionic
compounds can be considered as a regular arrangement of oppositely
charged particles. Their chemical formulas represent the lowest,
whole number ratio ‑ empirical
formula ‑ of the component ions.
For this reason, ionic compounds do not exist as molecules. A crystal of sodium chloride is composed of many Na+ and Cl‑ ions. Under normal conditions we cannot isolate a single NaCl unit, so a “molecule” of NaCl does not exist.
Let
us look at the ionic compound, KCl, potassium chloride. When learning the nomenclature
of ionic compounds, you learned that the charges in the compound must add to
give zero. That is, potassium chloride has the formula KCl
because potassium forms +1 cations and chlorine forms ‑1 anions.
+1
+ (-1) = 0
If
we look at this fact in light of Lewis's theory, we also see the octet rule being followed. The ground
state electron configuration of K is 1s22s22p63s23p64s1. It can also be depicted by a valence
shell or planetary model.
In this element, K, the valence shell is the 4th energy level and the first three energy levels represent the core of electrons.
The core electrons are very tightly held, and therefore do not participate in bonding. Recognizing that it is only the outermost electron shell that participates in electron transfer, Lewis developed a means to show the electron configuration. Let us look at the ground state of potassium depicted by a Lewis dot diagram. The single valence electron is represented by the dot.
Compare
K with this representation for the ground state of Cl, 1s2s2 2p63s23p5 .
Notice the 7 dots which represent the 7 valence electrons for Cl. The valence electrons consist of the two 3s
electrons and the five 3p electrons.
