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Ionic Formulas Writing Exercise

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Learn to write formulas from ionic compound names. Understand cations and anions, rules for naming, and how to calculate charges to create balanced formulas.

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Ionic Formulas Writing Exercise

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  1. How to Write Ionic Formulas In this exercise you will learn to write formulas from the written name of the ionic compound

  2. Ionic compounds consist of: a cation and an anion • Cation = an atom which has lost one or more electrons resulting in a positive charge on the resulting ion • Ex: Na0 → Na1+ + e • Anion = an atom which has gained one or more electrons resulting in a negative charge on the resulting ion • Ex: Cl0 + e → Cl1-

  3. CATIONS • SODIUM chloride , NaCl • The cation is listed first • The cation is named for the element without any change in the name • Ex: Na0 is sodium, the atom • Ex: Na1+ is sodium, the ion • The cation is a metal (exceptions: H1+ and NH41+)

  4. ANIONS (monatomic) • sodium CHLORIDE, NaCl • The anion is listed second • Monatomic anions represent only one element • The anion is named for the element, but the name is changed to end in “-ide” • Ex: Cl0 is chlorine, the atom • Ex: Cl1- is chloride, the ion • The anion is a nonmetal and found directly on the front of the Blue Sheet

  5. ANIONS (polyatomic) • sodium CHLORATE, NaClO3 • The anion is listed second • Polyatomic anions represent more than one element, usually an element and oxygen • The anion is named for the unique element, but ends in “ate” (Some polyatomics have an “ite” form) • Ex: ClO31- is named chlorate after the chlorine; note that the oxygen is not mentioned specifically • NOTE: the name chlorate does not suggest the amount of oxygen present nor the charge on the ion • All the polyatomic ions you will need are listed on the back of the Blue Sheet

  6. How to write a formula • Sodium chloride • #1 Find sodium on the Blue Sheet • #2 Note its oxidation number (i.e. charge) listed above the symbol (1, which means 1+) • #3 Find chlorine on the Blue sheet • #4 Note its oxidation number (i.e. charge) listed above the symbol (±1, 5, 7) Use only the negative value for the anion. The other values are not used in ionic compounds. • #5 The charges on an ionic compound must add up to zero. SO……………………. Na1+ and Cl1- add up to NaCl • SEE THE NEXT SLIDE FOR EXAMPLES AND SUGGESTIONS

  7. If the two ions involved have different oxidation numbers find a common factor and write them in the correct proportion • Aluminum oxide • Aluminum, Al3+ • Oxide (comes from oxygen), O2- • REMEMBER: the charges must add up to zero! • SO………. two Al3+ gives a six postive charge while three O2- gives a six minus charge • TOGETHER, they are written Al2O3 • Note each subscript represents the number of the element written before it

  8. Other things to consider : #1 The same system of proportions works for polyatomic ions; the total positive charge and total negative charge must add up to ZERO #2 Do NOT change any of the subscripts on the polyatomic ions #3 See the “Rules for Writing Formulas” sheet for rules about using parentheses with polyatomic ions (See Rule 6) #4 See the “Rules for Writing Formulas” sheet for rules about which cations will require Roman numerals (See Rule 7) #5 See the “Rules for Writing Formulas” sheet for rules about the various exceptions to “ide” and “ate” endings and the polyatomic ions that end in “ite” (See Rule 4)

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