Polyatomic Ions

Let's touch upon polyatomic ions again. They are chemicals in which atoms fill their octet with electrons by using both covalent and ionic bonding. A simple example of this is the polyatomic ion, hydroxide. (The way that it can be formed is also shown in example 24 in your workbook.)

Oxygen can fill up its "octet" using three different options. The ionic option (left side) involves the atom gaining two electrons to become an ion with a -2 charge. The covalent option (right side) involves the atom sharing two electrons from other atoms (hydrogen in this case). The polyatomic ion option (middle) requires the oxygen atom to obtain one electron by sharing with another atom (hydrogen in this case) and gain the other electron by taking it away from some willing electron-losing atom. In hydroxide ions, the shared electrons create a covalent bond which holds the oxygen and hydrogen atoms together. The gained electron makes this combination of atoms an ion. The attraction between the negative charge on this ion and the positive charge on other ions results in ionic bonding between this ion and others.

8Ogeopt.JPG (3525 bytes)

This situation is typical of polyatomic ions--covalent bonding holds the atoms together within the ion and ionic bonding joins the entire ion to other oppositely charged ions.

Practice Determining Charge from "Extra" Electrons

It is possible, given the right information, to figure out the charge on polyatomic ions. I'd like you to try to figure out what charge there should be on each of the polyatomic ions shown here (and in exercise 25 in your workbook). However, you should already know what charge each has. So what you actually have to do is match or verify the charge for each with the Lewis diagram.

ЗЗ : O : ЗЗ ЗЗ ЗЗ : O : S : O : ЗЗ ЗЗ ЗЗ : O : ЗЗ	ЗЗ : O : ЗЗ ЗЗ ЗЗ : O : P : O : ЗЗ ЗЗ ЗЗ : O : ЗЗ	H ЗЗ H : N : H ЗЗ H
ЗЗ ЗЗ : O : N : O : ЗЗ :: ЗЗ : O :	ЗЗ ЗЗ : O : C : O : ЗЗ :: ЗЗ : O :	ЗЗ : O : ЗЗ H

To do this, start with what you know about how many valence electrons each atom has, and compare that to the number of electron dots shown in each of the diagrams. If there are more dots shown than can be accounted for by the number of valence electrons that those atoms should have, then the polyatomic ion as a whole will have a negative charge equal to the number of additional electrons. If it's short some electrons, then it will have a positive charge equal to how many electrons that it's short. After you have figured those out, compare your answers to the known charges. If you don’t want to look them up, they are listed below.

Answers

The charge on SO₄ is -2. The charge on PO₄ is -3. The charge on NH₄ is +1. The charge on NO₃ is -1. The charge on CO₃ is -2. The charge on OH is -1.

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E-mail instructor: Eden Francis

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