Writing Formulas and Naming Compounds Chapter 20 Section 3 Writing Formulas and Naming Compounds

Compound Names and Formulas Compounds have specific names and formulas so you can tell how many and what atoms are in the compound/molecule. For example, the formula for Carbon Dioxide is CO2. This tells us that Carbon Dioxide has one atom of carbon and two atoms of oxygen.

Important Vocabulary Binary compound: a compound composed of 2 elements. They can be covalent or ionic. Oxidation number: tells you how many electrons an atom has gained, lost, or shared to become stable. Polyatomic ion: a positively or negatively charged, covalently bonded group of atoms. Cation: a positively charged ion. Anion: a negatively charged ion.

Naming Ionic Compounds Cations: Use the element’s name. Anion: Add “ide” to the end of the element’s name. Sodium Chloride Cation is ALWAYS FIRST Anion is ALWAYS LAST Ionic compounds MUST have a total charge of zero, meaning that all negative and positive charges cancel each other out!! Na+1 + Cl-1 = NaCl = sodium chloride = all charges are cancelled out! Ca+2 + F-1 + F-1 = CaF2 = calcium fluoride = all charges are cancelled out!

Would a transition metal forming a compound with a nonmetal still be considered an ionic compound? YES! You are still dealing with a metal and a nonmetal forming a compound. The rules for naming these and writing the formulas are a little different though….

Oxidation numbers are important Oxidation numbers are important! You can kind of think of think of them like charges.

Aluminum will always have an oxidation number of +3. Al+3 Silver will always have an oxidation number of +1. Ag+1 Zinc will always have an oxidation number of +2. Zn+2

Transition Metal Naming Transition metals show their charges as Roman Numerals because their charges don’t follow a set pattern and they often change. FeO = Fe+2 + O-2 Iron (II) Oxide Fe2O3 = Fe+3 + O-2 Iron (III) Oxide

Practice Makes Perfect! What is the formula for Lithium Oxide? Li2O What is the name of this compound: CaBr2 ? Calcium Bromide What is the formula for Iron (III) Sulfide? Fe2S3 What is the name of this compound: CuCl2 ? Copper (II) Chloride

ALL CHARGES MUST CANCEL! Complex compounds Compounds with polyatomic ions are named ALMOST EXACTLY the same way that ionic compounds are. Cations: Use the element’s name. Anions: Use the polyatomic ion’s name. ***The exception to the rule would be if your cation happens to be a polyatomic ion. There are only two that we’ll use: ammonium and dimercury. If the cation bonds to an element and not another polyatomic ion, drop the ending of the element and add “ide.” ALL CHARGES MUST CANCEL! USE PARENTHESES WHEN YOU NEED MORE THAN ONE OF A PARTICULAR POLYATOMIC ION!

More Practice… What is the formula for Potassium Carbonate? K2CO3 What is the name of this compound: NH4CH3COO ? Ammonium acetate What is the formula for Magnesium Nitrate? Mg(NO3)2

Covalent Naming To name covalent compounds, you must put a prefix in front of each element’s name. Add “ide” to the last element’s name. mono- 6. hexa- di- 7. hepta- tri- 8. octa- tetra- 9. nona- penta- 10. deca- ***Often, the prefix “mono” is omitted, but is sometimes used for emphasis.

Practice, Practice, Practice… What is the formula for Dihydrogen Monoxide? H2O What is the name of this compound: H2S ? Dihydrogen Sulfide (or monosulfide)