Naming and Formulas of Ionic and Molecular Compounds

Binary Ionic Compounds with Metals Forming Only One Type of Cation

Binary ionic compounds consist of a metal and a nonmetal. When the metal forms only one type of cation (typically Group 1A and 2A elements), naming is straightforward.

• Name of Metal + Base name of Nonmetal + -ide suffix

• Do not indicate the charge of the metal in the name.

• Examples:

  • CaO: calcium oxide

  • MgBr2: magnesium bromide

Binary Ionic Compounds with Metals Forming More Than One Type of Cation (Transition Metals)

Many transition metals can form cations with different charges. Their names must specify the charge using Roman numerals.

• Name of Metal (Roman numeral for charge) + Base name of Nonmetal + -ide

• Examples:

  • FeCl2: iron(II) chloride

  • FeCl3: iron(III) chloride

  • Fe2S3: iron(III) sulfide

• Some transition metals have only one common charge (e.g., Zn2+, Ag+, Ca2+), so Roman numerals are not used for them.

Polyatomic Ions and Compounds Containing Polyatomic Ions

Polyatomic ions are ions composed of two or more atoms covalently bonded, carrying a net charge. Compounds can contain both ionic and covalent bonds.

• Common Polyatomic Ions: NO3- (nitrate), SO42- (sulfate), NH4+ (ammonium), PO43- (phosphate)

• Naming: Name the cation (metal or polyatomic ion) first, then the anion (polyatomic ion or nonmetal with -ide suffix).

• Examples:

  • NH4NO3: ammonium nitrate

  • Be3(PO4)2: beryllium phosphate

  • CoSO4: cobalt(II) sulfate (ionic bonding between Co2+ and SO42-; covalent bonds within SO42-)

• Some ionic compounds do not contain metals (e.g., NH4NO3).

Writing Chemical Formulas for Ionic Compounds

To write the formula for an ionic compound:

1. Write the symbol and charge for the cation and anion.

2. Use arrows to cross the magnitude of each ion's charge (ignore the sign) to become the subscript for the other ion.

3. Write the formula with these subscripts, reducing to the lowest whole-number ratio if necessary.

• Example: Lithium sulfide

  • Li+ and S2-

  • Cross charges: Li2S

• Example: Calcium nitride

  • Ca2+ and N3-

  • Cross charges: Ca3N2

• Example: Sodium carbonate

  • Na+ and CO32-

  • Cross charges: Na2CO3

Naming Binary Molecular (Covalent) Compounds

Binary molecular compounds are formed between two nonmetals. Prefixes indicate the number of each atom present.

• Format: Prefix + Name of First Element + Prefix + Base Name of Second Element + -ide

• Prefixes:

  • mono- (1) [usually omitted for the first element]

  • di- (2)

  • tri- (3)

  • tetra- (4)

  • penta- (5)

  • hexa- (6)

  • hepta- (7)

  • octa- (8)

  • nona- (9)

  • deca- (10)

• Examples:

  • P4S10: tetraphosphorus decasulfide

  • N2O: dinitrogen monoxide

  • CO2: carbon dioxide

  • SO2: sulfur dioxide

• If there is only one atom of the first element, the prefix mono- is omitted.

The Mole Concept

Definition of the Mole

The mole (mol) is the SI unit for the amount of substance. One mole contains as many elementary entities (atoms, molecules, ions, or formula units) as there are atoms in exactly 12 grams of carbon-12.

• Avogadro's number: entities per mole

• Elementary entities can be atoms, molecules, ions, or formula units, depending on the substance.

• The smallest unit of an element is an atom.

Conversions Using the Mole

• For elements:

• For molecules:

• For ions:

Example: Calculating Number of Atoms

• How many atoms are in 5.00 mol of Na?

• Calculation:

Atomic Mass and the Mole

The atomic mass unit (amu) is defined as .

• To convert between moles and grams, use the molar mass (g/mol) of the element or compound.

Example: Calculating Mass from Moles

• Given the atomic mass of Na, calculate the mass of 2.4 mol Na.

• Calculation:

Summary Table: Prefixes for Molecular Compounds

Additional info: Some content was inferred and clarified for completeness, such as the stepwise process for writing ionic formulas and the explicit definition of the mole and Avogadro's number.