Chapter 3 Molecules and Compounds: Polyatomic Ions, Ionic and Molecular Compound Naming

Ch.3 - Molecules and Compounds

Polyatomic Ions

Polyatomic ions are groups of atoms covalently bonded together that carry an overall charge. These ions are fundamental in general chemistry, as they frequently appear in compounds and reactions.

• Definition: A polyatomic ion is a tightly bound group of two or more atoms that acts as a single unit and possesses an overall positive or negative charge.

• Examples: Nitrate (NO3-), Sulfate (SO42-), Ammonium (NH4+).

• Polyatomic Oxyanions: These are negatively charged polyatomic ions containing oxygen. Their names and formulas depend on the number of oxygens present.

• Trioxides: When the name ends with -ate, the ion possesses three oxygens (e.g., nitrate).

• Tetraoxides: When the name ends with -ate, the ion possesses four oxygens (e.g., sulfate).

• Deriving Oxyanions: Decreasing the number of oxygens by one changes the ending to -ite, while keeping the overall charge the same (e.g., nitrite NO2-).

Example: The systematic name for NO2- is nitrite.

Example: The systematic name for PO33- is phosphite.

Example: The systematic name for CO22- is carbonate.

Example: The systematic name for AsO43- is arsenate.

Example: The formula for thiosulfate ion, where one oxygen in sulfate (SO42-) is replaced by sulfur, is S2O32-.

Example: The systematic name for FO- is fluorite.

Example: The systematic name for IO3- is iodate.

Example: The silicate ion is the silicon version of the carbonate ion, with the structure SiO32- or SiO44- depending on context.

Additional info: The thiocyanate ion (SCN-) consists of sulfur, carbon, and nitrogen atoms, with the negative charge typically residing on the nitrogen.

Halogen Oxyanions

Halogen oxyanions are polyatomic ions containing halogens (F, Cl, Br, I) and oxygen. Their names are based on the halogen's base name and the number of oxygens present, which determines the prefix and suffix.

• Base Name: The root of the halogen's name (e.g., chlor- for chlorine).

• Charge: All halogen oxyanions possess a charge of -1.

• Naming: The number of oxygens affects the prefix/suffix:

  • per-...ate: Most oxygens (e.g., perchlorate ClO4-)

  • -ate: Second most oxygens (e.g., chlorate ClO3-)

  • -ite: Fewer oxygens (e.g., chlorite ClO2-)

  • hypo-...ite: Least oxygens (e.g., hypochlorite ClO-)

Other Polyatomic Ions

Some polyatomic ions do not fit predictable patterns and must be memorized. These include ions like cyanide, hydroxide, peroxide, dichromate, and acetate.

• Cyanide: CN-

• Hydroxide: OH-

• Peroxide: O22-

• Dichromate: Cr2O72-

• Acetate: CH3COO- or C2H3O2-

Polyatomic Cations

Most polyatomic ions are anions, but a few are cations:

• Ammonium: NH4+ (the only major polyatomic ion with a +1 charge)

• Mercury(I): Hg22+ (composed of two mercury ions bonded together)

Naming Ionic Compounds

Concept and Rules

Ionic compounds consist of a positive ion (cation) and a negative ion (anion). The cation is usually a metal or a polyatomic ion, while the anion is a nonmetal or a polyatomic ion.

• STEP 0: Write the cation first, then the anion.

• STEP 1: The metal keeps its name and is written first. If the metal has more than one possible charge, use a Roman numeral to indicate its charge (e.g., iron(III) chloride).

• STEP 2: The anion keeps its base name but changes its ending to -ide (e.g., chloride, oxide).

• STEP 3: If a polyatomic ion is present, it keeps its name (e.g., nitrate, sulfate).

Example: CaCl2 is named calcium chloride.

Example: Al(BrO4)3 is named aluminum perbromate.

Example: CoCO3 is named cobalt(II) carbonate.

Writing Ionic Compounds

Rules for Writing Formulas

To write the formula for an ionic compound:

• STEP 1: Identify the ions involved from the compound's name.

• STEP 2: Combine the ions so that the total positive and negative charges balance to zero.

• Same charges: If the charges are numerically equal, simply combine the elements (e.g., Al3+ and N3- form AlN).

• Different charges: If the charges are different, use subscripts to balance the charges (e.g., Ba2+ and PO43- form Ba3(PO4)2).

Example: Magnesium sulfate is MgSO4.

Example: Copper(I) nitrate is CuNO3.

Example: Sodium dichromate is Na2Cr2O7.

Naming Ionic Hydrates

Concept and Rules

Ionic hydrates are ionic compounds linked to at least one molecule of water (H2O). The water portion is indicated by a numerical prefix and the term "hydrate."

• STEP 4: The water portion is called "hydrate," and the number of water molecules is described by a prefix (mono-, di-, tri-, etc.).

Example: PbO2 · 5H2O is named lead(IV) oxide pentahydrate.

Example: K2Cr2O7 · 3H2O is named potassium dichromate trihydrate.

Example: Calcium selenide hexahydrate is CaSe · 6H2O.

Naming Acids

Binary Acids

Binary acids are covalent compounds containing hydrogen and a nonmetal anion (not oxygen). The naming uses the prefix "hydro-" and the suffix "-ic acid."

• STEP 1: Prefix "hydro-" for the H+ ion.

• STEP 2: Use the base name of the nonmetal.

• STEP 3: Suffix "-ic acid."

Example: H2S is named hydrosulfuric acid.

Example: HCN is named hydrocyanic acid.

Example: Hydroiodic acid is HI.

Example: Hydroselenic acid is H2Se.

Example: Hydrofluoric acid is HF.

Oxyacids

Oxyacids are covalent compounds containing hydrogen and a polyatomic ion with oxygen. The naming depends on the ending of the polyatomic ion:

• If the ion ends with -ate, change the ending to -ic acid (e.g., nitrate → nitric acid).

• If the ion ends with -ite, change the ending to -ous acid (e.g., nitrite → nitrous acid).

Example: HNO3 is nitric acid.

Example: HNO2 is nitrous acid.

Example: H2CO3 is carbonic acid.

Example: H3PO3 is phosphorous acid.

Example: H2SO4 is sulfuric acid.

Naming Molecular (Covalent) Compounds

Concept and Rules

Molecular compounds contain only nonmetals bonded together. Numerical prefixes are used to indicate the number of each atom present.

• STEP 1: The first nonmetal is named normally and uses all numerical prefixes except "mono-" for one.

• STEP 2: The second nonmetal uses any numerical prefix and changes its ending to -ide.

• If the letter "a" of the prefix is next to "o" in the element name, drop the "a" (e.g., tetraoxide becomes tetroxide).

Example: SeF6 is named selenium hexafluoride.

Example: IO5 is named iodine pentoxide.

Example: N2S4 is named dinitrogen tetrasulfide.

Example: Disulfur monochloride is S2Cl.

Example: Tetraphosphorus pentaselenide is P4Se5.

Example: Dibromine heptoxide is Br2O7.

Numerical Prefixes Table

Prefix	Number
Mono-	1
Di-	2
Tri-	3
Tetra-	4
Penta-	5
Hexa-	6
Hepta-	7
Octa-	8
Nona-	9
Deca-	10

Summary Table: Common Polyatomic Ions

Ion Name	Formula
Nitrate	NO3-
Carbonate	CO32-
Sulfate	SO42-
Phosphate	PO43-
Ammonium	NH4+
Hydroxide	OH-
Acetate	CH3COO-
Thiosulfate	S2O32-
Chromate	CrO42-
Dichromate	Cr2O72-

Key Equations

• Charge Balance in Ionic Compounds: $\text{Total positive charge} + \text{Total negative charge} = 0$

• General formula for hydrate: $\text{Compound} \cdot n\,\text{H}_2\text{O}$