Writing Formulas and Naming Compounds

Binary Ionic Compounds A binary compound is one that is composed of two elements. Example: Potassium iodide. But before you can write the chemical formula, you must have all the information you will need.

Are Electrons Gained or Lost? You need to know the elements involved and what number of electrons they lose, gain, or share to become stable. The number of electrons that an elements loses or gains to become stable is called the oxidation number. For ionic compounds, the oxidation number is the same as the charge on the ion. For example: A sodium ion has a charge of 1+ and an oxidation number of 1+. A chlorine ion has a charge of 1- and an oxidation number of 1-.

Oxidation Numbers The numbers with positive and negative signs on your periodic table in the back of your journal are the oxidation numbers for these elements. Notice how they fit with the periodic table grouping. Some elements need a different way of expressing their oxidation number. This is done by using a roman numeral with the element’s name. Example: The oxidation number of iron in iron (III) oxide is 3+.

Special Ions NameOxidation Number Copper (I)1+ Copper (II)2+ Iron (II)2+ Iron (III)3+ Chromium (II)2+ Chromium (III)3+ Lead (II)2+ Lead (IV)4+

Compounds are Neutral Even though each of the elements in a compound carries its own charge, the compound as a whole will have a zero (neutral) charge. A formula must have the right number of positive ions and negative ions so the charges balance. Examples: Sodium chloride is made up of one sodium ion with a 1+ charge and one chlorine ion with a 1- charge. However, calcium fluoride is made up of one calcium ion with a 2+ charge and two fluorine ions each with a 1- charge. Some compounds need more figuring like Aluminum oxide. Aluminum has a 3+ charge and oxygen has a 2- charge. This requires you to find the least common multiple of 2 and 3. So you will need two aluminum ions and three oxygen ions in order to have a 6+ and 6- charge making the compound Al 2 O 3.

Rules for Writing Formulas for Ionic Compounds: 1) Write the symbols of the elements or polyatomic ions (ions with more than one atom). Li 1+ O 2- 2) Cross the numbers not the signs. Li 1+ 2 O ) Reduce numbers or drop ones and put the symbols together. Li 2 O

How to Name a Binary Ionic Compound Use the name of the first element, the root name of the second element, and the suffix -ide to write the name of a binary ionic compound. Example: NaCl sodium chlor-ide = sodium chloride EXAMPLES: Write the chemical formula for the following compounds. 1) Lithium nitride 2) Potassium iodide 3) Lead (IV) phosophide

How to Name Binary Ionic Compounds EXAMPLES: What would a chemist name the following compounds? 1) CuCl 2) CuO 3) AlCl 3 4) KCl

Compounds with Complex Ions Some compounds are composed of more than two elements. They contain polyatomic ions (positively or negatively charged, covalently bonded groups of atoms).

Polyatomic Ions ChargeNameFormula 1+AmmoniaNH Acetate Chlorate Hydroxide Nitrate C 2 H 3 O 2 - ClO 3 - OH - NO Carbonate Sulfate CO 3 2- SO PhosphatePO 4 3-

Writing the Name and Formulas of Polyatomic Ions To write names, write the name of the positive ion first. Then write the name of the negative ion. To write formulas, use the oxidation numbers, their least common multiple, and put parentheses around the polyatomic ion before adding a subscript.

Practice EXAMPLES: Write the name for the following compounds. 1) NH 4 Cl 2) Mg(OH) 2 3) Al 2 (SO 4 ) 3 EXAMPLES: What is the formula for the following compounds? 1) Calcium phosphate 2) Aluminum nitrate 3) Potassium hydroxide 4) Magnesium sulfate

Compounds with Added Water Some ionic compounds have water molecules as part of their structure. These are called hydrates.

Binary Covalent Bonds Covalent bonds form between nonmetals. Some pairs of nonmetals can form more than one compound with each other. Example: nitrogen and oxygen can form N 2 O, NO,NO 2, and N 2 O 5. In the system of naming you have learned so far, these would all be called nitrogen oxide, but we do not know the composition of the compound.

Naming Binary Covalent Compounds Therefore, we use prefixes to indicate how many atoms of each element are in the binary covalent compound. Number of AtomsPrefix 1Mono- 2Di- 3Tri- 4Tetra- 5Penta- 6Hexa- 7Hepta- 8Octa-

Rules and Practice 1. Do NOT use mono- with the first element. 2. Sometimes drop the last vowel of the prefix when the second element begins with a vowel. Only do this when it sounds or looks awkward. EXAMPLES: Write the formula for the following molecules. 1) Chlorine heptoxide 2) Dihydrogen monoxide 3) Carbon dioxide 4) Carbon tetrachloride EXAMPLES: Write the name for the following compounds. 1) CO 2) SF 6 3) NO 2 4) N 2 O 3