The purpose of this week's assignment is to predict ion charges and to practice writing chemical formulas for ionic compounds.

The electron configurations of the noble gases are extremely stable. When an atom undergoes a chemical change to become an ion, the atom will gain or lose electrons until it is isoelectronic (has the same electron configuration) with the nearest noble gas. Metals tend to lose electrons and form positively charged ions called cations. Non-metals tend to gain electrons and form negatively charged ions called anions.

For example, sodium metal, Na, [Ne]3s¹, loses one electron to form the Na⁺ ion which is isoelectronic with neon. (All the alkali metals have one valence electron and form 1+ cations.) Chlorine, [Ne]3s¹3p⁵, gains one electron to form the Cl^– ion which is isoelectronic with argon. (All of the halogens have 7 valence electrons and form 1- anions.)

Ionic compounds are composed of metal cation(s) and non-metal anion(s) and have a net charge of zero. Sodium chloride, NaCl, is composed of Na⁺ and Cl^– in a one to one ratio which gives the formula zero charge. When writing the formula for an ionic compound, the symbol for the cation is written first.

Note that forming ions from neutral atoms is a chemical reaction. Click here to view sodium metal and chlorine gas reacting to form sodium chloride. This video clip is from the McMurry textbook companion web site (http://cwx.prenhall.com/bookbind/pubbooks/mcmurrygob/). If you can't view the reaction, there is a picture of it on page 74 in your text.

The following questions are designed to help you become proficient at predicting ions and writing formulas of ionic compounds. You may wish to look at chapter 4 section 7 (page 79-81) and section 10 (bottom of page 84-87) in your textbook.

When writing the formula for ionic compounds, the total positive charge from the cations plus the total negative charge from the anions must add up to zero for the formula to have a zero net charge. When writing formulas for an alkali metal reacting with a halogen, the ratio of ions is one to one. Remember our example of NaCl above? NaCl is composed of Na⁺ and Cl^– ions. So 1(1+) + 1(1-) = 0

What about the compound that forms when calcium reacts with bromine? Calcium forms the 2+ ion, Ca²⁺, because it needs to lose 2 electrons to be isoelectronic with argon. Bromine forms the 1- ion, Br^–. The formula for calcium bromide is CaBr₂. You need two bromide ions (-1 each) to balance the 2+ charge of the calcium ion.