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Ionic Charges of the Periodic Table
An ionic charge is a positive or negative charge on an atom. The charge depends on the atom's electron configuration and the number of valence electrons. On the periodic table, elements in the same group have the same number of valence electrons. So, their ionic charge is usually the same. In general, atoms that form positive ions (cations) are found on the left of the periodic table. Atoms that form negative charges (anions) are found on the right. The lower charge values are on the outside of the periodic table and increase as you move inward.
Charges Among Periodic Groups
Let's look at each group on the periodic table.
Ionic charges by Group:
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Group 1, the alkali metals: charge of +1
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Group 2, the alkaline earth metals: charge of +2
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Groups 3 - 12, the transition metals: charge can vary from negative to positive values. Some transition metals have been known to access up to a +7 charge.
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Group 15 nonmetals (N, P): charge of -3
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Group 16 nonmetals (O, S, Se): charge of -2
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Group 17 halogens (F, Cl, Br, I): charge of -1
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Group 18, the noble gases (He, Ne, Ar, Kr, Xn, Rn): charge of 0.
- The noble gases already have a full octet. So, they don't have an ionic charge. There are a few Xenon compounds that are octet rule exceptions.
Determining Ionic Charges from the Electron Configuration
The trends of ionic charge on the periodic table are generally true and reliable. But have you ever wondered how the charge is determined? Atoms have the same number of electrons as their atomic number. The electrons fill the first shell or energy level of an atom with two electrons. The remaining electrons fill the next sets of shells with eight electrons. The valence electron shell is the outermost shell with electrons in it. What If there aren't eight electrons? The atom is unstable. So, electrons will fill or leave the valence shell. For example, sodium () has one electron in its valence shell. It is more favorable to lose that electron than add seven more. When that electron is lost, the sodium atom has one more proton than it does electrons. So, there is a +1 charge associated with . This same charge and ion electron configuration are also true for the other element in group 1. Let's consider another example. Chlorine has seven electrons and needs one more for a complete octet or full valence shell. So chlorine will gain an electron and have a -1 charge. It has a large electron affinity. This is true for the other halogens.
Why Do We Care about Ionic Charge?
So, atoms can gain or lose valence electrons. Why do we care? The ionic charge is the basis of ionic bonding. For example, consider table salt. Do you know its chemical formula? It's , or sodium chloride. The sodium atom gave its one electron to chlorine. So, we have a cation and an anion. But, opposite charges experience electrostatic attraction. So, they bond together.
But, how does salt become a crystal? The atoms' charges actually radiate in every direction. So, each sodium is surrounded by six chlorine atoms. Two are above and below. The other four are in front and behind, and on the right and left. The same is true for each chlorine atom! Ionic bonds form a crystal lattice. They have repeating units of sodium surrounded by chlorine and vice versa. All of this happens because of the ionic charge!
