Chapter 19: Molecules and Compounds Section 19.2 Chemical Formulas

19.2 Chemical Formulas All compounds have an electrical charge of _____; that is, they are ______.

To be stable… the ____ electrical _____ of compounds must be ______.

Oxidation Numbers An oxidation number indicates how many electrons are ______ or ______ during the bonding process. Mg loses 2 electrons so it has an oxidation number of +2 Oxygen gains 2 electrons so it has an oxidation number of –2.

Common Oxidation Numbers 0 Copy on your Periodic Table Transition Metals

______ ______ have more than one oxidation #. _____ _____ show oxidation #.

Writing Chemical Formulas – monatomic ions 1. Symbol of (+) ion always written ____. 2. Symbol of (–) ion always written ____. 3. Add _______ so sum of oxidation #’s is zero.

Example: Write formula for ______ (2 element) compound made of iron(III) and oxygen.

1. Find oxidation #’s of elements: iron(III) = Fe 3+ oxygen =O 2- How do you make a cmpd electrically neutral?

Calculate Fe 3+ ions needed to combine with O 2- ions to make electrical charges equal zero. 2 (3+) added to 3 (2-) = 0 2 (Fe 3+ ) added to 3 (O 2- ) = 0

2- 2. To determine ratios, write chemical formulas…Use the ____________ _______ Fe 3+ O

Writing Chemical Formulas with polyatomic ions “_______” means many. See page 337: Oxidation #’s for polyatomic ions. Each polyatomic ion is treated like a ______ ion.

Common Polyatomic Ions

Using Polyatomics How would you write the chemical formula for Calcium with Hydroxide. Ca is located in Group 2 so it has a +2 oxidation number OH is a polyatomic ion with a charge of –1 In order to make the total charge 0 you need ( Ca +2 ) x (1) = +2 + (OH - ) x (2) = -2 0

Example: Write formula for aluminum sulfate. 1 st ion is always (+). Use PT to find oxidation #. Aluminum = Al 3+ 2 nd ion is always (-). Use Table Sulfate = SO 4 2-

LCM of 2 and 3? 0 (Al 3+ ) x 2 = 6+ 6 How many of each ion are needed? (SO 4 2- ) x 3 = 6-

Write chemical formula Al 2 (SO 4 ) 3 Don’t change subscripts in polyatomic ion!! Use ( ) ________ method Al 3+ SO

Naming binary ionic compounds Write name of 1 st element or polyatomic ion. Write _____ name of 2 nd element and add -ide. Exs: chlor-ine = chlor-ide phosph-orus = phosph-ide

Naming Formulas Write the name for the first element in the compound. Write the root name of the second element. Add the ending –ide to the root name Naming BaF 2 First element is Barium Second element is Fluorine (root is fluor) The name is Barium Fluoride

Naming Polyatomic Ions For polyatomic ions: (use chart) Write the name for the first positive ion. Write the name for the negative ion Naming MgCO 3 Positive ion is Magnesium Negative ion is Carbonate (see chart) The name is Magnesium Carbonate

Naming binary _______ compounds Add -ide to the neg. ion (anion) Add prefix (pg 340) If only one atom of 1 st element, don’t use mono-

Examples: CO carbon monoxide CO 2 carbon dioxide PCl 5 phosphorus pentachloride N2S6N2S6 dinitrogen hexasulfide Page 340 in Text

Examples SiF 4 Silicon tetrafluoride N2S3N2S3 Dinitrogen trisulfide P2O5P2O5 Diphosphorus pentoxide SF 6 Sulfur hexafluoride

Empirical vs Molecular formulas _________ formula – actual # of atoms of each element in a compound. _________ formula – simplest whole number ratio of elements in cmpd.

Example: Molecular formula -sugar C 6 H 12 O 6 Empirical formula -sugar CH 2 O