IonicCompounds

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7-4-7-6 Atoms are made of protons, with positive charges, and electrons, with negative charges. When an atom, usually of a metal, loses an electron, it becomes an ion with a positive charge. When an atom of a non-metal gains electrons, it becomes a negative ion. The charge of the ion is determined for metals by subtracting the number of electrons from the number protons and vice versa for non-metals.

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 givin in the question, so you don't need to look them up.

Rule 3: The number