Chemical Bonding, Formulas, and Nomenclature
Chemical Bonding, Formulas, and Nomenclature
This domain tests how atoms combine, how to write formulas from ion charges, and how to name ionic compounds, covalent compounds, and common acids. Expect electronegativity differences, polyatomic ions, and oxidation numbers.
Core rules
- Ionic bonds form between metals and nonmetals by electron transfer; covalent bonds form between nonmetals by electron sharing. A rough guide: electronegativity difference >1.7 is ionic, <0.4 is nonpolar covalent, and between is polar covalent.
- In ionic formulas, total positive charge equals total negative charge. Write the cation first, then the anion. Use parentheses around a polyatomic ion when more than one is needed: Ca(NO3)2.
- Name ionic compounds: cation name + anion name. Monatomic anions end in -ide (Cl− = chloride). Transition metals with variable charge need a Roman numeral: FeCl3 = iron(III) chloride.
- Name covalent compounds with prefixes: mono-, di-, tri-, tetra-, penta-, hexa-. The first prefix "mono-" is usually omitted. N2O4 = dinitrogen tetroxide.
- Acids: binary acids use hydro-...-ic acid (HCl = hydrochloric acid); oxyacids with -ate become -ic acid and -ite become -ous acid.
- Common polyatomic ions: nitrate NO3−, sulfate SO4²−, carbonate CO3²−, phosphate PO4³−, ammonium NH4+, hydroxide OH−.
Worked example
Write the formula for calcium nitrate. Calcium is Group 2, so Ca²⁺. Nitrate is NO3−. To balance charge, use one Ca²⁺ and two NO3−: Ca(NO3)2. Do not write CaNO3 or CaN2O6; the polyatomic ion stays intact, and parentheses show two nitrate groups. Name check: calcium nitrate.
Traps that cost marks
Do not crisscross subscripts into a polyatomic ion and split it (Mg(NO3)2 is correct, MgN2O6 is not). Do not use prefixes for ionic compounds. Do not omit Roman numerals for metals such as iron, copper, or tin when the charge varies. Do not forget that "ide" is used for monatomic anions, while polyatomic anions keep their names (sulfate, not sulfide). And in covalent names, "mono-" is not used on the first element.