Example Sodium (Na) ion with 11 protons and 10 electrons. The net charge is thus:
11 protons....11+
-10 electrons..10-
Net Charge= +1
Therefore the ion is labled Na+1
Ionic Compounds are formed when ions combine. They are represented by empirical formulas not molecular formulas. For example: Sodium Chloride, made up of Na+ ions and Cl- ions, is represented by the empirical formula NaCl . The formula unit shows the lowest whole number ratio of the ions in the compound. The symbol for an ion consists of the symbol and a superscript showing the charge.
Positive ions formed by metals are called cations. Negative ions formed by nonmetals are called anions. All ionic compounds contain cations and anions in a fixed ratio, in which the charges balance eachother off, making the compound neutral.
A binary compound is a compound formed by two elements. One of these elements is a metal while the other is a nonmetal.
Rules for writing formulas of binary ionic compounds. Rule 1 Write the symbols for the two elements in the compound. Write the symbol of the cation first, followed by the symbol of the anion.
Rule 2 Determine the charges of the atoms of each element by looking them up or using the periodic table. (See figure 7-12, page 155 for explaination of determining charges using the periodic table.)
Rule 3 Using the known charges on the ions, select the subscripts that make the total positive charge equal to the total negative charge, making the compound electrically neutral.
Example Write the formula for Silver oxide, an ionic compound made up of Ag+1 and O-2 ions.
Solution Rule 1: The periodic table shows that Silver is a metal and Oxygen is a nonmetal, so the formula will be written Ag?O?.
Rule 2: The charges are given in the question, so you don't need to look them up.
Rule 3: The number of ions must ben such that the total positive charge equals the total negative charge.
Number of "+" charges in 1 formula unit: .................................. Number of "-" charges in 1 formula unit:
(Number of Ag+1 ions) X (Charge on Ag+1 ion) ..................(Number of O-2 ions) X (Charge on a O-2 ion)
? X 1+ ..................................................................................... ? X 2-
2 X 1+ ..................................................................................... 1 X 2-
Therefore the formula unit will be written: Ag2O
Polyatomic ions Some ions contain more than one element. Groups of atoms of more than one element are called polyatomic ions. These ions function as a unit and generally do not break up, and they function as one ion. So, a fourth rule must be added for dealing with polyatomics.
Rule 4 When using subscripts with polyatomics, the formula of the ion is placed in parentheses and the subscript is placed on the outside of the parentheses. For example: Magnesium nitrate is written Mg (N O3)2
Naming ionic compounds: THe names of ionic compounds are formed by the cation name first, followed by the name of the anion, with the suffix -ide added at the end.
For example, an ionic compound made up of silver and chlorine is called Silver chloride.
For elements with more than one charge, the Stock System is used. It concists of the cation followd by the charge written in Roman Numerals in parentheses, and then the anion with the suffix -ide. For example, the compound formed with Fe+2 and O-2 is named Iron (II) oxide. While the compound formed with Fe+3 and O-2 is called Iron (III) oxide.