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WRITING IONIC FORMULAS

This guide provides a comprehensive overview of writing ionic formulas, including the use of cation and anion symbols, balancing charges with the crisscross method, and the proper handling of polyatomic ions. It covers the determination of charges for monatomic and polyatomic ions using the periodic table, as well as recognizing multiple oxidation states for cations through the Stock and Classical systems. Practical examples and exercises are included for reinforcing knowledge, ensuring you can accurately distinguish between ionic, covalent, and acid compounds.

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WRITING IONIC FORMULAS

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  1. WRITING IONIC FORMULAS • Write the symbol for the cation, then the symbol for the anion. • Balance the charges - use the crisscross method - by placing subscripts and “( )” for polyatomic ions if needed. • Monatomic ions don’t need “( )”. • Net charges must be zero. Use ion sheet to get charges.

  2. DETERMINING THE CHARGE Monotomic Ions • An ion with only one atom; Mg+2, Na+1, O-2 • Most of the charges can be determined by the atom’s position on the periodic table. • It is the atom’s oxidation number. • Most transition elements have more than one oxidation number (see back of PT or cover of textbook)

  3. DETERMINING THE CHARGE Polyatomic Ions • An ion made up of more than one atom NH4+1, SO4-2, C2H3O2-1; • The charges given to polyatomic ions apply to the whole group of atoms. • NEVER change the subscripts of a polyatomic ion. • If more than one ion is needed, a parenthesis is placed around the ion and the subscript is written outside; Example: Al(OH)3

  4. Criss Cross for writing formulae • Determine the charges on the atoms in the formula • +ve Cation listed first and –ve Anion listed second • Criss Cross the charge each and use the charge as the subscript of the other.

  5. Ex. Aluminum chloride. Al and Cl. • What charge does Al get? +3 Cl ? -1 • Let’s write them down Al3+ Cl1- Al3 Cl1 Don’t need to record “1”s • Step 2 to get the neutral formula AlCl3 • Check to see if formula is neutral +3 -1 -1 -1 = 0 • Remember that if you have a polyatomic ion there will be a few atoms grouped together that have a charge together. So use parenthesis.

  6. MULTIPLE OXIDATION NUMBERS Cations can have more than one oxidation number. Example: Cu+1 and Cu+2. It is important to distinguish which ion is in the compound. There are two ways used in chemistry.

  7. MULTIPLE OXIDATION NUMBERS Stock SystemClassical (Latin) Cu+1 copper (I) cuprous Cu+2 copper (II) cupric Sn+2 tin(II) stannous Sn+4 tin (IV) stannic Hg2+2 mercury (I) mercurous Hg+2 mercury (II) mercuric The lower charge ends in –ous and the higher charge ends in –ic.

  8. MULTIPLE OXIDATION NUMBERS You are only responsible for only the Stock system but I just wanted you to see the other.

  9. MULTIPLE OXIDATION NUMBERS You are only responsible in class for the Stock system but I wanted to make you familiar with the classic.

  10. Examples cupric sulfate/copper (II) sulfate CuSO4 ferrous oxide/iron (II) oxide FeO cobaltic chloride/cobalt (III) chloride CoCl3 mercurous nitrate/mercury (I) nitrate Hg2(NO)2

  11. PRACTICE • lithium fluoride • magnesium chloride • calcium bromide • lithium iodide • lithium oxide • calcium sulfate • barium cyanide • Iron (II) oxide • barium sulfate • aluminum oxide

  12. PRACTICE • sodium nitrate • iron(III) sulfate • copper (II) sulfite • tin (IV) fluoride

  13. PRACTICE • lithium fluoride LiF • magnesium chloride MgCl2 • calcium bromide CaBr2 • lithium iodide LiI • lithium oxide Li2O • calcium sulfate CaSO4 • barium cyanide BaCN • Iron (II) oxide FeO • barium sulfate BaSO4 • aluminum oxide Al2O3

  14. PRACTICE • sodium nitrate NaNO3 • iron(III) sulfate Fe2(SO4)3 • copper (II) sulfite CuSO3 • tin (IV) fluoride SnF4

  15. WRITING FORMULAS • Potassium fluoride • Potassium iodide • Potassium iodate 11. Iron (II) hydroxide • Beryllium chloride 12. Iron (III) oxalate • Beryllium oxide. • Beryllium nitride • Beryllium hydroxide • Sodium acetate • Sodium sulfate • Copper (II) sulfate

  16. Putting It Altogether You need to make sure that you can distinguish between ionic, covalent, and acid compounds.

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