 What is a chemical formula?  It indicates the relative number of atoms of each kind in an ionic compound.  Ex Al 2 O 3 has 2 atoms of Al and 3 atoms of O.  It indicates the number of atoms of each element contained in a single molecule of a molecular compound. Ex. C 8 H 18 has 8 atoms of carbon and 18 atoms of H in a molecule of octane.

 There are different rules for naming and writing formulas for ionic and covalent compounds.  The compound is ionic if it is composed of a cation and an anion, or a metal and a nonmetal. The elements will be located on opposite sides of the periodic table. They have a high electronegativity difference.  The compound is covalent (or molecular) if both elements are nonmetals.

 Binary because composed of only 2 elements.  Ionic because composed of a metal and a nonmetal.  Example aluminum oxide.

1). Write the symbol and charge for the cation. 2). Write the symbol and charge for the anion. 3). Criss cross the oxidation numbers and drop the charges. The oxidation number will become the subscript for the other element. 4). Reduce the subscripts if possible. If they are the same, they cancel completely. 5). If the subscript is 1, do not write anything.

Write the formula for calcium iodide.  Write symbols and charges.  Ca +2 I -1  Criss-cross oxidation numbers, drop charges.  Ca 1 I 2  Cancel and take away ones.  Answer: CaI 2

 Question: Write the formula for Magnesium Oxide.  1) Write the symbol and charge.  Mg +2 O -2  2) Criss cross oxidation number and drop charges.  Mg 2 O 2  3). Cancel and don’t write 1’s.  Answer: MgO  Complete page 4 and the bottom of page 5 of formula writing packet.

 If a compound consists of a metal and a nonmetal, we use the name of the first element (always the cation) followed by the second element (always the anion). For the anion only, drop the last syllable and add –ide. Subscripts are not included in the name. Examples: CaO: calcium oxide LiCl: lithium chloride K 2 S: potassium sulfide

 P. 223 sample problem A  P. 223 practice #1 a-e, and #2 a-f  Complete packet page 1 and 8.

 Some elements, such as iron, form 2 or more cations with different charges. To distinguish the ions formed by such elements, scientists use the Stock system of nomenclature. This system uses a Roman numeral to indicate the ion’s charge. The numeral is enclosed in parentheses and placed immediately after the metal name.  Fe +2 is iron (II) and Fe +3 is iron (III)  Names of metals that commonly form only one cation, do not include a Roman numeral.

 CuCl 2  Determine the charges of each element, by writing ions side by side.  Backwards criss-cross to determine charge of cation.  Name by writing name of cation, roman numeral, name of anion with ide instead of last syllable.  CuCl -1  Cu +2 Cl -1  Copper(II) chloride

 Example: Write the formula of lead(IV) oxide.  Write symbols with charge.  Pb +4 and O -2  Criss cross and drop signs.  Pb 2 O 4  Reduce and remove 1’s.  PbO 2

 Page 225 #1(a-f) and #2 (a-d)  Complete packet page 3 and 5(top)

A covalently bonded group of atoms that has a positive or negative charge and act as a unit is a polyatomic ion. The prefix poly means “many”. Most simple polyatomic ions are anions. Ammonium (NH 4 ) is the only polyatomic cation. Sometimes there are parentheses in the formula, so you can tell its polyatomic.

 If it contains a polyatomic ion, it is ionic, so it follows the rules for naming ionic compounds.  If anion is polyatomic, do not change the ending to ide. Keep it the same as is listed on the chart. Name cation then name anion.  If the cation is polyatomic and the anion is binary, name is just like before. The ide ending means the anion is not polyatomic.

 Al 2 (SO 4 ) 3  Al +3 and SO 4 -2  Aluminum sulfate  Fe(OH) 2  Fe +2 and OH -1  Iron(II)hydroxide

 1. Write the symbols for the monatomic and polyatomic ions in the compound.  2. Look up the oxidation numbers of the ions involved.  If a single atom, use the periodic table.  If roman numeral (variable oxidation number) or polyatomic, use the chart.  3. Criss cross the oxidation numbers, and drop the charges.  4. Put polyatomic ions in parentheses if they have a subscript.  5. Reduce (simplify) and erase ones.  6. DO NOT ADD< SUBTRACT, MULTIPLY, OR DIVIDE. Just criss cross and reduce if necessary!

 Write the formula for Lead (II) hydroxide.  1. Write the symbol and oxidation number.  Pb +2 OH -1  2. Criss cross charges and drop signs.  Pb 1 OH 2  3. Use parentheses if needed.  Pb 1 (OH) 2  4. Reduce and drop 1’s.  Pb(OH) 2

 Write the formula for ammonium sulfate.  1. Write the symbol and oxidation number.  NH 4 +1 SO 4 -2  2. Criss cross charges and drop signs. Use parentheses if needed.  (NH 4 ) 2 (SO 4 ) 1  3. Reduce and drop 1’s.  (NH 4 ) 2 (SO 4 )

 Molecular/covalent compounds are made of 2 nonmetals.  Generally, the most metallic element is written first. These elements are farther to the left in the periodic table. If both elements are in the same group, the more metallic element is closer to the bottom of the group.  The name of the second element is changed to -ide.  Because molecular formulas can combine in many combinations, prefixes are used to indicate the subscript. Do not criss cross and do not use oxidation numbers.  Don’t use mono on first element.

 1- mono  2- di  3- tri  4- tetra  5- penta  6- hexa  7- hepta  8- octa  9- nona  10- deca

 P 2 O 5  Prefix of 1 st element, name of 1 st element  Prefix of 2 nd element, name of 2 nd element, change ending to ide. Diphosphorus pentaoxide  CO  Carbon monoxide  (Don’t write mono on first element.)

 Write the symbols for the elements in the order that they appear in the name.  The prefixes indicate the number of atoms of each element in the molecule.  The prefixes appear as subscripts in the formulas.  If there is no prefix, there is only one atom. (1 st element only)  Remember, do not criss cross.  Example tetraphosphorus dichloride  P 4 Cl 2

***1 ST PUT I (ionic) OR C (covalent) BESIDE THE QUESTION FRONT LEFT COLUMN: 1—20, 25, 26 FRONT RIGHT COLUMN: 1—18, 23, 25, 26

 HCl  This is binary, H and Cl. Follow rule #1.  Hydrochloric acid  H 2 SO 4 - This is polyatomic. Hydrogen and sulfate (SO 4 ) -2. Must find polyatomic anion and name. Ending is -ate so change to -ic.  Sulfuric acid (notice no hydro, hydro is only for binary acids).  HNO 2 - Polyatomic, H and nitrite. -ite becomes –ous. Nitrous acid.

 Cation is always H +1. From the name of the acid, you can figure out the anion.  Write symbols and charges for cation and anion.  Criss cross oxidation numbers and drop signs.  Reduce and drop 1’s.  Example  Phosphoric acid  H +1 and phosphoric so phosphate, so  H +1 and (PO 4 ) -3  H 3 PO 4