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Chemical Formula

Chemical Formula. Tells the relative number of atoms of each element in a compound Molecular: ( covalent bonding) Formula represents the number of atoms of each element in a single molecule Ionic: ( ionic bonding)

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Chemical Formula

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  1. Chemical Formula • Tells the relative number of atoms of each element in a compound • Molecular: (covalent bonding) • Formula represents the number of atoms of each element in a single molecule • Ionic: (ionic bonding) • Formula represents the simplest ratio of positive and negative ions in one formula unit Episode 601

  2. Ionic Bond • The attraction that binds opposite charged ions together. • Metals • Give away electrons • Ions are positively charged • Cations • Non-metals • Take electrons • Ions are negatively charged • Anions Al +3 Cl -1 AlCl3 Episode 601

  3. Writing Ionic Formulas • Write the symbols for the elements • The cation is written first (metal) • The anion is written second (non-metal) • Determine the charge on each ion • Select subscript that will make the total positive charge equal to the total negative charge. sodium chloride magnesium chloride +1 -1 +2 -1 Na Cl Mg Cl NaCl MgCl2 Episode 601

  4. Criss-Cross Method for Formula Writing • Write the symbol of cation followed by symbol of anion along with their ionic charges. • Use the absolute value of the charge of each ion as the subscript for the other. • If the subscript is one, omit it. • If the subscripts are the same, omit them. • Subscripts must be simplified in an ionic formula. barium iodide calcium sulfide Ba+2 I-1 Ca+2 S-2 Ba1 I2 Ca2 S2 BaI2 CaS Episode 601

  5. Polyatomic Ions in formula writing • Use the same steps to write formula • Treat the polyatomic ion as a single ion • If the subscript for the polyatomic is more than 1, put polyatomic ion in ( ) with subscript on the outside. sodium nitrate calcium nitrate Na+1 NO3-1 Ca+2 NO3-1 Na1 (NO3)1 Ca1 (NO3)2 NaNO3 Ca(NO3)2 Episode 601

  6. Examples: Ca+2 O-2 Calcium Oxide Potassium chlorate Aluminum sulfate CaO Ca2 O2 K+1 ClO4-1 KClO4 K1 (ClO4)1 Al+3 SO4-2 Al2(SO4)3 Al2 (SO4)3 Episode 601

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