Nomenclature of Covalent and Ionic Compounds

Nomenclature of Covalent Compounds

Covalent molecules are formed through bonds between two nonmetals. These elements are typically found on the right side of the periodic table and include atoms such as $H$, $C$, $N$, $O$, $S$, $P$, and the halogens.

General Formula and Naming Structure

The general molecular formula for a covalent compound is represented as $A_x B_y$.

• Atom A (The first element): The name consists of the prefix indicating the number of atoms followed by the name of the atom.

• Atom B (The second element): The name consists of a prefix indicating the number of atoms, the name of the atom, and the suffix "-ide".

Covalent Prefixes

Prefixes are used to denote the number of atoms present in the molecule:

• 1: mono-

• 2: di-

• 3: tri-

• 4: tetra-

• 5: penta-

• 6: hexa-

• 7: hepta- (also referred to as septa-)

• 8: octa-

• 9: nona-

• 10: deca-

Specific Naming Rules and Exceptions

• The First Atom Exception: The prefix "mono-" is never used for the first element ($A$) in a compound. It is only used for the second element ($B$).

• Vowel Drop Rule: If the element name begins with a vowel, the "o" or "a" at the end of the prefix is often dropped to ensure better phonetic flow.

  • Examples include:

    • penta- + oxide becomes pentoxide

    • tetra- + oxide becomes tetroxide

    • hexa- + oxide becomes hexoxide

    • Note that "di-" remains "dioxide" without dropping a vowel.

Examples of Covalent Nomenclature

• Carbon disulfide: $CS_2$

• Sulfur hexafluoride: $SF_6$

• Dinitrogen pentoxide ($N_2O_5$): Named as "dinitrogen" (2 nitrogens) and "pentoxide" (5 oxygens).

• Carbon monoxide ($CO$): Named as "carbon" (prefix mono- omitted for the first atom) and "monoxide" (prefix mono- included for the second atom).

• Silicon tetrafluoride ($SF_4$): Named as "silicon" and "tetrafluoride".

Nomenclature of Ionic Compounds

Ionic compounds, also known as simple salts, are formed through bonds between a metal and a nonmetal.

Distinction Between Elements and Polyatomic Ions

• Elements: Found directly on the periodic table. They appear as one capital letter (e.g., $H, K, O$) or one capital letter followed by one lowercase letter (e.g., $Cu, Ag$).

• Polyatomic Ions: Groups of two or more elements that stick together and function as a single unit (e.g., $ClO_3, NH_4, OH$).

Identification of Polyatomic Ions to Memorize

A critical rule for polyatomic ions is that the "-ate" ending indicates the presence of one more oxygen atom than the similar anion ending in "-ite".

• Nitrate: $NO_3^-$

• Nitrite: $NO_2^-$

• Sulfate: $SO_4^{2-}$

• Sulfite: $SO_3^{2-}$

• Phosphate: $PO_4^{3-}$

• Phosphite: $PO_3^{3-}$

• Carbonate: $CO_3^{2-}$

• Ammonium: $NH_4^+$

Naming Procedures for Ionic Compounds

• Binary Ionic Compounds: These involve two elements without transition metals. The name of the metal comes first, followed by the nonmetal with the "-ide" suffix.

  • $NaCl$: Sodium chloride

  • $K_2O$: Potassium oxide

• Compounds with Transition Metals: Because transition metals can have multiple ionic states, a Roman numeral is used to indicate the charge of the metal ion.

  • $FeCl_2$: Iron (II) chloride

  • $Ag_2S$: Silver (I) sulfide

• Compounds with Polyatomic Ions: When naming these, the name of the polyatomic ion is used directly without changing its ending.

  • $(NH_4)_2S$: Ammonium sulfide

  • $Ca(ClO_3)_2$: Calcium chlorate

Determining Structure and Formula Balancing

To determine the molecular formula of an ionic salt, the positive and negative charges of the ions must be balanced to equal zero.

General Structural Forms

Common ratios for ionic compound structures include:

• $AB$: $NaCl, CaO, AlN$

• $AB_2$ or $A_2B$: $CaCl_2, Na_2S$

• $AB_3$ or $A_3B$: $AlCl_3, Na_3N$

• $A_2B_3$ or $A_3B_2$: $Ca_3N_2, Al_3O_2$

Step-by-Step Example: Barium Iodide

1. Identify the ionic states: Barium is $Ba^{2+}$ and Iodine is $I^-$.

2. Balance the charges: One barium ion has a $+2$ charge. Since each iodide ion has a $-1$ charge, two iodide ions are required to reach neutrality.

3. Mathematical verification: $(+2) + (-1) + (-1) = 0$.

4. Resulting structure: $BaI_2$.