Chapter Two: Naming Simple Compounds

Introduction to Compound Naming

When chemistry was a young science, the number of known compounds was small enough for chemists to memorize their names. Many of these names derived from compounds' physical appearance, properties, origins, or applications. Examples of such names include:

• Sugar of lead, represented by $ext{Pb(CH}_3 ext{COO)}_2$.
• Blue vitriol, which is $ext{CuSO}_4 \bullet ext{H}_2 ext{O}$.
• Quicklime, known as $ext{CaO}$.
• Epsom salt, which corresponds to $ext{MgSO}_4 \bullet ext{H}_2 ext{O}$.
• Milk of magnesia.
• Laughing gas, commonly referred to as $ext{N}_2 ext{O}$.
  These names are classified as "common names." As the field of chemistry expanded and the number of compounds increased significantly, it became impractical to memorize all the names. Consequently, a systematic approach to naming compounds was developed, wherein the name conveys information regarding the composition of the compound.

Types of Simple Compounds

Binary Ionic Compounds (Type I)

Binary ionic compounds consist of two elements. Characteristics include:

1. Cation and Anion Formation:
      - The positive metal ion (cation) is always written first in the formula.
      - The negative nonmetal ion (anion) follows.
2. Naming Rules:
      1. Name the cation first, followed by the anion.
      2. A monoatomic cation is named after the element's name (e.g., Na+ is sodium ion, Li+ is lithium ion, and Ca2+ is calcium ion).
      3. A monoatomic anion's name derives from the element's root with the suffix -ide added (e.g., Cl- is chloride, N3- is nitride).
   

Examples of Binary Ionic Compounds (Type I)

• $ext{NaCl}$: Sodium chloride (Na+, Cl-)
• $ext{CaS}$: Calcium sulfide (Ca2+, S2-)
• $ext{MgO}$: Magnesium oxide (Mg2+, O2-)
• $ext{Li}_3 ext{N}$: Lithium nitride (Li+, N3-)

Binary Ionic Compounds (Type II)

Type II binary ionic compounds are characterized by the presence of metals that can form more than one cation. Notably, transition metals are included in this category. Unlike Type I compounds, the charge of the cation must be specified:

• Example:
     - $ext{FeCl}_2$ contains Fe2+.
     - $ext{FeCl}_3$ contains Fe3+.

Naming Rules for Type II Binary Ionic Compounds

• The charge of the metal ion is denoted in parentheses using Roman numerals following the cation's name (e.g.,
  $ext{FeCl}_2$ is iron (II) chloride and $ext{FeCl}_3$ is iron (III) chloride).

Older Naming System

For metals that form only two ions, the following naming conventions were utilized:

• The ion with a higher charge utilizes an -ic ending.
• The ion with a lower charge ends in -ous.
  For instance:
• $ext{FeCl}_2$: Ferrous chloride
• $ext{FeCl}_3$: Ferric chloride

Common Type II Cations

Some common systematic names for type II cations include:

• Fe2+: Iron(II)
• Fe3+: Iron(III)
• Cu2+: Copper(II)
• Cu+: Copper(I)
• Pb2+: Lead(II)
• Pb4+: Lead(IV)
• Co2+: Cobalt(II)
• Co3+: Cobalt(III)

Naming Ionic Compounds with Polyatomic Ions

Polyatomic ions consist of multiple atoms, such as $ext{NO}_3^-$, $ext{NH}_4^+$, and $ext{SO}_4^{2-}$.

• Polyatomic ions possess designated names that must be memorized for compound naming. A selection of important polyatomic ions and their systematic names is provided as follows:

Important Polyatomic Ions

• $ext{NH}_4^+$: Ammonium
• $ext{SO}_4^{2-}$: Sulfate
• $ext{CO}_3^{2-}$: Carbonate
• $ext{C}_2 ext{H}_3 ext{O}_2^-$: Acetate
• $ext{OH}^-$: Hydroxide

Oxyanions

Oxyanions are a series of anions consisting of the same element but differing in the number of oxygen atoms. Naming conventions based on the number of oxygen atoms include:

• The name of the anion with fewer oxygen atoms ends in -ite.
• The name of the anion with more oxygen atoms ends in -ate.
• When three oxyanions are present, hypo- and per- are used as prefixes for low and high amounts of oxygen, respectively.

Examples of Oxyanions

• ext{ClO}^-$: Hypochlorite
• ext{ClO}_2^-$: Chlorite
• ext{ClO}_3^-$: Chlorate
• ext{ClO}_4^-$: Perchlorate

Naming Binary Covalent Compounds (Type III)

Binary covalent compounds are formed between two nonmetals and do not contain ions. The following naming conventions apply:

1. The first element is named in full.
2. The second element is treated as if it were an anion, with prefixes indicating the number of atoms present.

Prefixes for Covalent Compounds

• mono-: 1
• di-: 2
• tri-: 3
• tetra-: 4
• penta-: 5
• hexa-: 6
• hepta-: 7
  Note: The prefix mono- is not used for the first element (e.g., carbon monoxide, $ext{CO}$, not monocarbon monoxide).

Examples of Binary Covalent Compounds

1. $N_2F_4$: Dinitrogen tetrafluoride
2. $CO_2$: Carbon dioxide
3. $P_2O_5$: Diphosphorous pentoxide
4. $PCl_3$: Phosphorous trichloride

Formulas from Names

The reverse of naming is deriving the chemical formula from the systematic name, which can be applied to the following examples:

• Calcium hydroxide's formula is $ext{Ca(OH)}_2$ because calcium forms only $ext{Ca}^{2+}$ ions, and hydroxide is $ext{OH}^-$, necessitating two hydroxides to obtain a neutral compound.

Formulas for Naming Practice

1. Sulfur difluoride: $ext{SF}_2$ (Type III)
2. Sodium oxide: $ext{Na}_2 ext{O}$ (Type I)
3. Lead(IV) sulfide: $ext{PbS}_2$ (Type II)
4. Chlorine monoxide: $ext{ClO}$ (Type III)
5. Lead(II) sulfate: $ext{PbSO}_4$ (Type II)
6. Potassium chlorate: $ext{KClO}_3$ (Type I)

Naming Acids

Acids are defined as molecules with one or more $ext{H}^+$ ions attached to an anion. The associated anions may include:

1. Anions without Oxygen:
      - Named with the prefix hydro- and the suffix -ic acid.
      - Examples:
        - $ext{HCl}$: Hydrochloric acid
        - $ext{HBr}$: Hydrobromic acid
        - $ext{HCN}$: Hydrocyanic acid
        - $ext{H}_2 ext{S}$: Hydrosulfuric acid

2. Anions with Oxygen:
      - Acidic names are derived using the root name of the anion:
      - If the anion ends in -ate, add -ic to the root name.
        - $ext{H}_2 ext{SO}_4$ is sulfuric acid due to the sulfate anion ($ext{SO}_4^{2-}$).
        - $ext{H}_3 ext{PO}_4$ is phosphoric acid due to the phosphate anion ($ext{PO}_4^{3-}$).
      - If the anion ends in -ite, add -ous to the root name.
        - $ext{H}_2 ext{SO}_3$ contains sulfite ($ext{SO}_3^{2-}$) and is called sulfurous acid.
        - $ext{HNO}_2$ contains nitrite ($ext{NO}_2^{-}$) and is called nitrous acid.
      - For the oxyanions of chlorine:
        - $ext{HClO}_4$: Perchlorate → Perchloric acid
        - $ext{HClO}_3$: Chlorate → Chloric acid
        - $ext{HClO}_2$: Chlorite → Chlorous acid
        - $ext{HClO}$: Hypochlorite → Hypochlorous acid

Hydrates

Hydrates are compounds that include specific numbers of water molecules attached to them. For example, copper (II) sulfate contains five water molecules associated with it, leading to the systematic name copper (II) sulfate pentahydrate, represented as $ext{CuSO}_4 \bullet 5 ext{H}_2 ext{O}$. The water can be removed by heating the compound, resulting in the anhydrous form $ext{CuSO}_4$, meaning it no longer contains attached water molecules. Other examples of hydrates include:

• $ext{BaCl}_2 \bullet 2 ext{H}_2 ext{O}$: Barium chloride dihydrate.
• $ext{LiCl} \bullet ext{H}_2 ext{O}$: Lithium chloride monohydrate.
• $ext{MgSO}_4 \bullet 7 ext{H}_2 ext{O}$: Magnesium sulfate heptahydrate.

Familiar Inorganic Compounds

Certain compounds are widely recognized by their common names rather than their systematic chemical names. A table of familiar examples is as follows: