Ionic Compound Formula Writing, Polyatomic Rules & Exam Reminders

Charge Patterns & the “Table” the Instructor Kept Referring To

• Positive ions (cations) increase their cumulative charge in multiples of their individual charge
  • Example shown: Tin(IV) $Sn^{4+}$ → $+4, +8, +12, +16$
  • Example shown: Aluminum $Al^{3+}$ → $+3, +6, +9, +12$
• Negative ions (anions) increase their cumulative charge in multiples of their individual charge
  • Phosphide $P^{3-}$ → $-3, -6, -9, -12$
  • Hydroxide $OH^-$ → $-1, -2, -3, -4$
• The table that “changes by nine” was illustrating how the two columns of running totals are compared until the algebraic sum reaches $0$ (electrical neutrality).

Step-by-Step Procedure for Writing an Ionic Formula

1. Locate each ion on your ion chart or periodic table.
   • Metals left of the staircase → form cations
   • Non-metals/right of staircase → form anions
2. Write the symbol and charge for each ion.
   E.g. Tin(IV) = $Sn^{4+}$, Phosphide = $P^{3-}$
3. Build two vertical columns of running totals of charge (multiples):
   • Cation column: $+q, +2q, +3q, \dots$
   • Anion column: $-p, -2p, -3p, \dots$
4. Stop when the magnitudes are equal (i.e., LCM of the two charges).
   • In instructor’s words: “Go down the list and find when the charges are equal.”
5. The row number in which you stop gives the subscripts.
   • Example: equality at $+12$ and $-12$ ⇒ 3 tin ions and 4 phosphide ions.
6. Drop the charges and write the subscripts:
   • $Sn<em>3P</em>4$ (Tin(IV) phosphide)
7. Never write a subscript of 1.
8. Parentheses rule for polyatomic ions:
   • If the polyatomic ion gets a subscript $\ge 2$, enclose it in parentheses: $Pb(OH)<em>4$ • If the subscript is 1, omit parentheses: $AlPO</em>4$

Short-Cut “Swap & Drop” (The Student Version)

• Cross-multiply the absolute values of the charges and place them as subscripts.
  • Works when the charges are already lowest‐terms.
  • The instructor cautioned: “They don’t always swap” because you might have to reduce.

Polyatomic Ion Essentials

• Polyatomic = ion containing more than one atom.
• Only polyatomic cation to memorize: Ammonium $NH_4^+$
• Polyatomic anions covered in class (must know name, formula, and charge):
  • Acetate $C<em>2H</em>3O<em>2^-$ • Bicarbonate / Hydrogen carbonate $HCO</em>3^-$
  • Bisulfate / Hydrogen sulfate $HSO<em>4^-$ • Carbonate $CO</em>3^{2-}$
  • Cyanide $CN^-$
  • Hydroxide $OH^-$
  • Nitrate $NO<em>3^-$ • Phosphate $PO</em>4^{3-}$
  • Sulfate $SO<em>4^{2-}$ • Thiosulfate $S</em>2O<em>3^{2-}$ • (Instructor also said “gophthalate” – usually written $C</em>8H<em>4O</em>4^{2-}$ in labs.)

Worked Examples from Class

1. Tin(IV) phosphide
   • $Sn^{4+}$ vs $P^{3-}$ ⇒ LCM $12$ ⇒ $Sn<em>3P</em>4$
2. Aluminum phosphate
   • $Al^{3+}$ vs $PO<em>4^{3-}$ ⇒ equal in first step ⇒ $AlPO</em>4$
3. Lead(IV) hydroxide
   • $Pb^{4+}$ vs $OH^-$ ⇒ need $4$ hydroxides ⇒ $Pb(OH)_4$
4. Iron(III) carbonate
   • $Fe^{3+}$ vs $CO<em>3^{2-}$ ⇒ LCM $6$ ⇒ $Fe</em>2(CO<em>3)</em>3$
5. Copper hydroxides
   • $Cu(OH)_2$ must contain $Cu^{2+}$ (charges cancel) ⇒ named Copper(II) hydroxide.
   • $CuOH$ would contain $Cu^+$ ⇒ Copper(I) hydroxide.
   • Naming relies on negative ion count to deduce which cation charge to use.
6. Tin iodide
   • Four iodides present ( $I^-$ each ) = $-4$ ⇒ tin must be $Sn^{4+}$ ⇒ Tin(IV) iodide $SnI_4$
7. Calcium chloride $CaCl_2$
   • $Ca^{2+}$ + 2 × $Cl^-$
8. Sodium phosphate $Na<em>3PO</em>4$ (derived verbally by students)

Naming Rules Recap

• Always name the cation first, anion second.
• For metals that have more than one possible charge (transition metals, Sn, Pb):
  • Indicate the charge with Roman numerals in parentheses, e.g., Iron(III), Tin(IV).
• Monoatomic anions → root + “-ide” (chloride, phosphide, sulfide, etc.).
• Polyatomic anions keep their standard names (carbonate, nitrate, sulfate…).

Parentheses Usage Cheat Sheet

Common Subscript Limits Mentioned

• Instructor: “You’re never gonna have a subscript beyond 4” for the exercises provided.

Quick Periodic-Table Cue

• Elements left of the staircase act as metals/cations in ionic compounds; elements right act as non-metals/anions.

Exam & Course Logistics (Verbatim Highlights)

• First Exam is tomorrow.
  • Coverage: up to Chapter 4 (Lewis structures & naming in Chapter 6 will appear on later test).
  • A posted practice exam on Canvas; hard copies passed out in class.
• Materials required
  • Pencil
  • NON-phone calculator (simple scientific fine)
  • Scantron form “50 questions / 5 choices” (sold at bookstore). Need four total for the term.
• Limited loaner calculators available – first-come, first-served.
• Ion memorization not tested on question 6 of practice exam; that problem will be omitted.
• Chapter 6 worksheet due Saturday (to be submitted online).

Study Advice Heard in Class

• Work through the handout from yesterday; be able to “go directly to the formula.”
• Practice deciding whether to use the swap-and-drop shortcut vs. the full LCM table.
• Make a personal flash-card set for the polyatomic ions (especially ammonium and the anions listed above).
• If you lack a chem tutor, rely on classmates and the posted practice materials.