Compounds, Bonding, and Nomenclature
Core concepts
Ionic bonds form between metals and nonmetals through electron transfer, creating charged ions held by electrostatic attraction. Covalent bonds form between nonmetals through electron sharing, producing molecules. Metallic bonds involve delocalized electrons. Ionic compounds have high melting points and conduct when molten or dissolved; covalent compounds generally have lower melting points. Use oxidation numbers to write formulas and the rules of nomenclature to name them: metal name plus nonmetal ending in -ide, with Roman numerals for variable-charge metals, and prefixes (mono-, di-, tri-) for molecular compounds. Know common polyatomic ions such as nitrate, sulfate, carbonate, and hydroxide.
Traps to watch
- Using prefixes with ionic compounds ("sodium dichloride" is wrong).
- Omitting Roman numerals for metals like iron or copper.
- Forgetting that the formula must be electrically neutral.
- Confusing a compound with a mixture.
- Assuming all covalent compounds are nonconductors in every state.
Worked example
Name FeCl3. Iron can be +2 or +3; three chloride ions give -3 total, so iron is +3. The name is iron(III) chloride. For CO2, a molecular compound, use prefixes: carbon dioxide.
Rule of thumb
Charges must sum to zero in an ionic formula. Metal first, nonmetal with -ide, Roman numeral if the metal varies, prefixes only for molecules.