Writing Formulas and Naming Compounds Section 19.3

Writing Formulas Binary compound- contains 2 elements –Examples: KCl, NaCl, KI, H 2 O Oxidation number- tells you how many electrons an atom gains, loses or shares to become stable –Rules: For ionic compounds- the oxidation number is the same as the charge on the ion The oxidation number of hydrogen in a compound is +1 unless it is combined with a metal

Rules for Oxidation numbers When hydrogen combines with a metal, like NaH, hydrogen has an oxidation number of -1 Oxidation number of oxygen in a compound is -2 unless it is part of a peroxide. If it is part of a peroxide, O 2 -2, each oxygen has an oxidation number of -1

Rules for Oxidation Numbers The oxidation numbers of atoms that are in their elemental form is 0. For any neutral compound, the sum of the oxidation numbers must be zero For a polyatomic ion, the sum of oxidation numbers must equal the charge of the ion There are some elements that can form more than one ion. –Copper (I), Copper (II), Iron (II), Iron (III), Chromium (II), Chromium (III), Lead (II), Lead (IV). Also Mn (II), Mn (IV), Mn (VII), V(II), V(IV), V(V), Cr(III), Cr(VI), Co (II), Co (III)

Formulas Polyatomic ion- an ion with more than one atom Steps for writing IONIC formulas: –Write the symbol of the element or polyatomic ion that has the positive oxidation number or charge: Hydrogen, ammonium, all metals –Write the symbol of the element or polyatomic ion with the negative oxidation number: Nonmetals, except H, Polyatomic ions, except ammonium –Use subscripts next to each ion so the sum of the charges is zero –OR USE THE CRISSCROSS METHOD

Naming Binary Ionic Compounds Write the name of the positive ion. (same as the name of the element) Check to see if the ion is capable of forming more than one oxidation number. –If so, determine the oxidation number of the ion from the formula of the compound. –Write the charge of the positive ion using roman numerals in parentheses after the ion’s name. –If not, move to step 3. Write the root name of the negative ion. Add –ide to the end of the root.

Naming & Formulas with polyatomic ions First write the name of the positive ion. Then write the name of the negative ion – check the back of your PT to find names Formulas: 1- write the symbol of the positive ion 2- identify the negative ion 3- balance the charges If there is more than 1 polyatomic ion, use parentheses to indicate this

Hydrates Hydrate is a compound that has water chemically attached to its ions –When heated, steam is released- becomes anhydrous –Rules for writing the formula of a hydrate –Write the formula of the compound –Place a dot between the formula for the compound and the next portion –Write the number of water molecules and the formula of water

Covalent Naming Use Greek prefixes to indicate how many atoms of each element are in the compound. –Mono 1, di- 2, tri – 3, tetra- 4, penta- 5, hexa- 6, hepta- 7, octa – 8, nona- 9, deca - 10 prefix mono- is omitted for the 1 st element last vowel of the prefix is dropped when the second element begins with a vowel (Pentoxide) The same prefixes are used to indicate the number of water molecules in hydrates

Covalent vs. Ionic Why name them differently? –2 elements may form many different covalent compounds –2 elements may form only one ionic compound