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Chemical Practice

Chemical Practice. Using ion models to determine chemical formulas. How do the models work?. Designed to show electron movement. POSITIVE ions are trying to lose electrons so the model has an “arrow” showing electrons leaving.

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Chemical Practice

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  1. Chemical Practice Using ion models to determine chemical formulas

  2. How do the models work? • Designed to show electron movement. • POSITIVE ions are trying to lose electrons so the model has an “arrow” showing electrons leaving. • NEGATIVE ions are trying to gain electrons so the model can “accept” electrons. Arrow shows one electron leaving +1 Spaces show how many electrons the ion is trying to gain -2

  3. EXAMPLES: Potassium (K) and Chlorine (Cl) **Write the ions first. K+1 Cl-1 **Find the puzzle pieces representing each ion. **One molecule of the compound is whatever it takes to complete the puzzle. One of each element makes up the compound. **Write the compound formula: KCl +1 -1

  4. Find the ions: Find ion models: Put puzzle pieces together to find ONE molecule of compound: Write the formula: Mg+2 Cl-1 MgCl2 Magnesium (Mg) and Chlorine (Cl) -1 +2 -1 +2 Two NEGATIVE ions -1 One POSITIVE ion

  5. Another way to determine the formula … • Write the ions in order: POSITIVE ion before NEGATIVE ion. Mg+2 Cl-1 • Cross over just the numbers of the charge. Mg+2 Cl-1 Mg1Cl2 • And, since the “1” is understood: MgCl2

  6. PO4 polyatomic ion {These can be found on the back of your periodic table.} Find each ion: Put ion pieces together: NOTE: You can only use the ions that make up the compound. Write the formula: Polyatomic ion = ion made of more than one atom H+1 PO4-3 H3PO4 Hydrogen (H) and Phosphate (PO4) +1 -3 +1 +1

  7. Again, you could determine the formula using the cross-over formula … • Write the ions in positive  negative order. H+1 PO4-3 • Cross over just the numbers of the charges. H+1 PO4-3 H3PO41 • Again, the “1” is understood and should not be included in the formula: H3PO4

  8. NH4+1 PO4-3 Write the ions: Combine the puzzle pieces: How do you write more than one polyatomic ion? Write compound formula: Ammonium (NH4) and Phosphate (PO4) +1 -3 +1 +1 • Use parenthesis to show more than one polyatomic ion in a compound. • (NH4)3PO4

  9. You could also use the cross-over method for finding the formula: • Write the ions in positive  negative order. NH4+1 PO4-3 • Cross over just the numbers of the charges. NH4+1 PO4-3 (NH4)3PO41 • Again, the “1” is understood and should not be included in the formula: (NH4)3PO4

  10. Write the ions: Combine the puzzle pieces: Remember that you can only use the ions that are part of the compound. Write the formula: Al+3 SO4-2 Aluminum (Al) and Sulfate (SO4) -2 +3 -2 +3 -2 Al2(SO4)3

  11. Again, you could use the cross-over method for finding the formula: • Write the ions in positive  negative order: Al+3 SO4-2 • Cross over just the numbers of the charges. Al+3 SO4-2 Al2(SO4)3

  12. Write the ions: Combine the puzzle pieces: Notice that the molecule is complete with one of each ion. Write the formula: Ca+2 SO4-2 Calcium (Ca) and Sulfate (SO4) +2 -2 CaSO4

  13. What if you use the cross-over method? • Write the ions in positive  negative order: Ca+2 SO4-2 • If you cross over just the numbers, you would have this formula: Ca2(SO4)2, but remember Ca is losing 2 electrons to sulfate so this doesn’t make sense. • If the numbers on the charge are identical, you will only need one of each: CaSO4

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