# Writing Formulas for Ionic Compounds

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Writing Formulas for Ionic Compounds

Oxidation Numbers for the Group A Elements 1+ 2+ 3+ 3- 2- 1- NA

When you write ions, use the element symbol and the oxidation number written as a superscript.
bromine sulfur cesium called bromide in a compound Br-1 called sulfide in a compound S-2 Anions change end of name to –ide. Cs+1

If you need the oxidation number for a transition or inner transition metal, it will be given to you because these elements typically have more than one oxidation number. Ox. # for transition/inner transition metals will be in the form of a Roman Numeral! Exceptions: Pb and Sn use Roman numerals even though they are NOT transition metals Ag and Zn do NOT use Roman Numerals even though they ARE transition metals

example: nickel (II) chloride (NiCl2) The (II) belongs to nickel which means nickel has a +2 charge while chlorine has a -1 charge!

What is the oxidation number for the metal in each compound?
Pb+4 Lead (IV) oxide Scandium (II) nitride Sc+2

Monatomic Ions A monatomic ion is an ion made up of one element.
examples: H O F Ba+2 Monatomic ions bond to make a binary compound. Binary compounds are made of a monatomic cation and a monatomic anion.

To Write Formulas with Monatomic Ions, use the criss-cross method!
bromine: name changed to bromide in a compound Write the formula for barium bromide. First: Write each element symbol and oxidation number. ***Always write the positive ion first!!! Next: Criss-cross the oxidation numbers. Swap the ox. #’s and write them as SUBSCRIPTS (lose the + and – signs) Ba+2 Br-1 Ba Br-1 Ba1Br 1 = BaBr2 2

Write the formulas for:
Iron (III) chloride Sodium oxide Magnesium nitride

Polyatomic Ions A polyatomic ion is an ion made up of more than one element. These are on the back of your periodic table! The entire polyatomic ion has an oxidation number (NOT the individual elements).

Polyatomic Ions Example: PO4-3 is the polyatomic ion phosphate with an oxidation number of -3. Example: SO3-2 is the polyatomic ion sulfite with an oxidation number of -2. Most polyatomic ions end in the suffix –ate or –ite.

To write formulas with polyatomic ions:
First: Write each element and polyatomic ion symbol and oxidation number. ***Always write the positive ion first!!! Next: Place polyatomic ion in PARANTHESES Finally: criss cross the oxidation numbers NEVER, NEVER, NEVER, EVER CHANGE THE SUBSCRIPT WITHIN THE POLYATOMIC ION ITSELF!

Write formulas for: Calcium Phosphate Magnesium Nitrate:

ONE LAST NOTE: Chemical formulas must be in the lowest, reduced form!!!
Write formulas for: lead (II) oxide calcium sulfide

Let’s Try More!

Nomenclature of Ionic Compounds
Nomenclature means naming. Naming Ionic Compounds with Monatomic Ions First: Name the cation (positive ion) – DO NOT CHANGE THE CATION’S NAME IN ANY WAY and write it down. Note: If the cation is a transition or inner transition metal, you must indicate its charge (the oxidation number) using Roman numerals in parentheses after the metal’s name. Ex: iron (II) chloride; its chemical formula is: FeCl2

Oxygen ______________ Sulfur ______________ Nitrogen ______________
Ex: What would these anions be named in a compound? Oxygen ______________ Sulfur ______________ Nitrogen ______________ Iodine ______________ Phosphorus ______________ Chlorine ______________

Name These: Ex: NaCl ____________________ Pb3P2 ___________________

Naming Ionic Compounds with Polyatomic Ions
First: Name the cation ion first (NEVER CHANGE THE CATION’S NAME). Note: If the cation is a transition or inner transition metal, you must indicate its charge using Roman Numerals in parentheses after the metal’s name. Ex: iron (II) hydroxide (its chemical formula is Fe(OH)2) Next: Name the polyatomic ion – ON THE BACK OF THE PT – (NEVER CHANGE THE POLYATOMIC’S NAME). HINT: Polyatomic ions usually end in –ate or –ite!

Try These: Li2SO4 (NH4)2O

I will do the rest with you on the board!