Accepted answer
Answer included
Compound formulas from names
“Provide the formula for each compound.” Chemistry-formula writing relies on charge neutrality for ionic compounds (including salts with polyatomic ions) and atom-count prefixes for molecular compounds.
Charge balance for ionic compounds
Ionic formulas satisfy electrical neutrality: \[ z_{+}n_{+} + z_{-}n_{-} = 0, \] where \(z\) denotes ionic charge and \(n\) denotes the number of ions in the formula unit.
- Total positive charge equals total negative charge.
- Polyatomic ions remain intact as a unit.
- Parentheses appear when a polyatomic ion occurs more than once.
Atom counts for molecular compounds
- Greek prefixes encode subscripts (mono-, di-, tri-, tetra-, penta-, …).
- Element symbols reflect the named elements in order.
- Prefix “mono-” is commonly omitted for the first element in many naming conventions.
Formulas for a representative compound set
The list below reflects common general-chemistry nomenclature practice spanning binary ionic compounds, salts with polyatomic ions, and one molecular compound example.
| Compound name | Formula | Structural note |
|---|---|---|
| Sodium chloride | NaCl | Binary ionic; \(+1\) and \(−1\) charges balance \(1:1\). |
| Magnesium bromide | MgBr2 | Binary ionic; Mg2+ pairs with two Br−. |
| Aluminum oxide | Al2O3 | Binary ionic; Al3+ and O2− balance to a \(2:3\) ratio. |
| Iron(III) chloride | FeCl3 | Roman numeral indicates Fe3+; three chlorides balance the charge. |
| Calcium nitrate | Ca(NO3)2 | Polyatomic anion repeated; parentheses preserve the nitrate unit. |
| Copper(II) sulfate | CuSO4 | Cu2+ with SO42−; \(1:1\) balance. |
| Sodium carbonate | Na2CO3 | CO32− requires two Na+. |
| Potassium permanganate | KMnO4 | MnO4− paired with K+. |
| Ammonium phosphate | (NH4)3PO4 | PO43− requires three NH4+; parentheses required. |
| Aluminum sulfate | Al2(SO4)3 | Al3+ and SO42− balance in a \(2:3\) ratio; parentheses required. |
| Ammonium chloride | NH4Cl | Polyatomic cation; no parentheses needed because only one ammonium occurs. |
| Dinitrogen tetroxide | N2O4 | Molecular compound; prefixes “di-” and “tetra-” set subscripts. |
Visualization of charge balancing with polyatomic ions
The diagram links ionic charges to the final formula unit, highlighting parentheses usage for repeated polyatomic ions in salts.
Common pitfalls
- Parentheses omitted for repeated polyatomic ions (for example, Ca(NO3)2 rather than CaNO32).
- Roman numerals ignored for variable-charge metals (for example, iron(III) as Fe3+).
- Subscripts applied inside a polyatomic ion incorrectly (for example, nitrate remaining NO3−, not altered to N2O6− when doubled).
- Formula mass logic substituted for charge logic in ionic compounds (charge neutrality governs subscripts, not relative atomic masses).
Vote on the accepted answer
Upvotes: 0
Downvotes: 0
Score: 0