Nomenclature of Ionic and Molecular Compounds

Naming Ionic Compounds

Basic Ionic Compound Naming

Cation (Metal) Naming: The name of the metal remains unchanged.
Anion (Nonmetal) Naming: The ending of the nonmetal's name is changed to "-ide." For example, chlorine (Cl) becomes chloride (Cl⁻).
Compound Name: Combine the name of the cation and the anion. For example, NaCl is sodium chloride.
Formula: Ionic compounds use the empirical formula, which represents the simplest whole-number ratio of ions, ensuring a zero net charge.
- Example: Sodium (Na⁺¹) and Chlorine (Cl⁻¹) combine in a 1:1 ratio to form NaCl. We do not write the subscript "1"

Ion Formation and Periodic Table Trends

Metals (Cations): Lose electrons to become isoelectronic with the nearest noble gas, forming positive ions (cations).
- Group 1 Elements (Alkali Metals): Lose one s electron, forming ions with a +1 charge (e.g., Na⁺).
- Group 2 Elements (Alkaline Earth Metals): Lose two s electrons, forming ions with a +2 charge (e.g., Ca²⁺).
Nonmetals (Anions): Gain electrons to become isoelectronic with the nearest noble gas, forming negative ions (anions).
- Halogens (Group 17): Gain one electron to form ions with a -1 charge (e.g., Cl⁻).
- Oxygen (Group 16): Needs two electrons, forming ions with a -2 charge (e.g., O²⁻).
- Nitrogen (Group 15): Needs three electrons, forming ions with a -3 charge (e.g., N³⁻), similar to Phosphorus (P³⁻).
Main Group Elements: These are elements in Groups 1A, 2A, and those on the outer side of the periodic table. Their ion charges can often be predicted by their group number or position.
- Aluminum (Al): Forms a +3 ion. Gallium (Ga) also forms a +3 ion.
Example: Calcium Chloride (CaCl₂)
- Calcium (Ca) is a metal from Group 2, so it forms a +2 ion (Ca²⁺).
- Chlorine (Cl) is a nonmetal (halogen), so it forms a -1 ion (Cl⁻), which is named chloride.
- To achieve a zero net charge (+2 from one Ca²⁺), two chloride ions (two times -1 charge = -2) are needed. Thus, CaCl₂.
Example: Aluminum Oxide (Al₂O₃)
- Aluminum (Al) forms a +3 ion (Al³⁺).
- Oxygen (O) forms a -2 ion (O²⁻), which is named oxide.
- To achieve a zero net charge:
  - Two aluminum ions (2 \times +3 = +6).
  - Three oxygen ions (3 \times -2 = -6).
  - Net charge = (+6) + (-6) = 0. Thus, Al₂O₃, named aluminum oxide.

Naming Ionic Compounds with Transition Metals

Variety of Charges: Transition metals (elements in the middle of the periodic table) can have variable charges (e.g., iron can form Fe²⁺ or Fe³⁺). This is often due to the loss of s electrons and sometimes the stability of half-filled or full d-subshells in electron configurations.
Naming Convention: To name ionic compounds involving transition metals, the charge of the metal ion must be indicated using Roman numerals enclosed in parentheses after the metal's name.
- The nonmetal's name still ends in "-ide."
- Example: Iron(II) for Fe²⁺, Iron(III) for Fe³⁺.

Exceptions to Transition Metal Naming Rules

Main Group Metals with Variable Charges: Some main group metals behave like transition metals by forming ions with variable charges, requiring Roman numerals in their names.
- Examples: Tin (Sn), Lead (Pb), Thallium (Tl).
- You are generally not required to memorize their specific charges but to be able to determine them from the compound formula.
- Common charges: Sn (+2, +4), Pb (+2, +4), Tl (+1, +3).
Transition Metals with Fixed Charges: Two transition metals consistently form only one type of ion, so Roman numerals are not needed.
- Silver (Ag): Always forms a +1 ion (Ag⁺).
- Zinc (Zn): Always forms a +2 ion (Zn²⁺).

Determining Ion Charge from Compound Formula

The zero net charge rule can be used to determine the charge of a variable-charge metal ion if the anion's charge is known.
Example 1: CuBr
- Bromine (Br) is a halogen, so it forms a bromide ion with a -1 charge (Br⁻).
- Since there is one Br⁻ ion, the total negative charge is -1. For a zero net charge, the copper (Cu) ion must have a +1 charge.
- One copper ion has a charge of +1. Therefore, it is Cu⁺.
- Name: Copper(I) bromide.
Example 2: CuBr₂
- Bromine (Br) forms a -1 charge (Br⁻). There are two bromide ions, so the total negative charge is 2 \times -1 = -2.
- For a zero net charge, the copper (Cu) ion must have a +2 charge.
- One copper ion has a charge of +2. Therefore, it is Cu²⁺.
- Name: Copper(II) bromide.

Polyatomic Ions

Definition and Characteristics

Definition: Ions composed of more than one atom (usually nonmetals) held together by covalent bonds, possessing an overall net charge.
Naming: Polyatomic ions have specific names (e.g., carbonate, ammonium) that are retained when they are part of an ionic compound; their endings are not changed to "-ide" (unless they are a simple monoatomic anion).
Examples:
- Carbonate: CO₃²⁻ (one carbon, three oxygens, net charge -2).
- Ammonium: NH₄⁺ (one nitrogen, four hydrogens, net charge +1). This is a common polyatomic cation.

Patterns in Polyatomic Ion Naming

Compounds of Nonmetals with Oxygen (-ite vs. -ate):
- Many nonmetals combine with oxygen at different ratios to form polyatomic ions.
- The ion with fewer oxygen atoms ends in "-ite." (e.g., Nitrite: NO₂⁻, Sulfite: SO₃²⁻)
- The ion with more oxygen atoms ends in "-ate." (e.g., Nitrate: NO₃⁻, Sulfate: SO₄²⁻)
Halogen-Oxygen Polyatomic Ions (Hypo-, Per- Prefixes): Halogens (e.g., chlorine, bromine) can combine with oxygen in an even wider range of ratios, leading to additional prefixes.
- Chlorite: ClO₂⁻
- Chlorate: ClO₃⁻
- Perchlorate: ClO₄⁻ (more oxygen than chlorate, uses "per-" prefix)
- Hypochlorite: ClO⁻ (fewer oxygen than chlorite, uses "hypo-" prefix)
- These patterns apply to other halogens as well (e.g., bromite, bromate, perbromate, hypobromite).
Distinction between Monoatomic and Polyatomic Ions: Spelling is crucial as "bromide" (Br⁻, monoatomic) is different from "hypobromite" (BrO⁻, polyatomic).

Polyatomic Ions with Hydrogen

When polyatomic anions combine with hydrogen ions (H⁺), the name typically adds "hydrogen" as a prefix.
Example 1: Phosphate (PO₄³⁻)
- With one H⁺: HPO₄²⁻ (charge -3 + 1 = -2) is Hydrogen phosphate.
- With two H⁺: H₂PO₄⁻ (charge -3 + 2 = -1) is Dihydrogen phosphate.
Example 2: Carbonate (CO₃²⁻)
- With one H⁺: HCO₃⁻ (charge -2 + 1 = -1) is Hydrogen carbonate.
- Historically, this is also known as Bicarbonate (e.g., Sodium bicarbonate - baking soda).
Example 3: Sulfate (SO₄²⁻)
- With one H⁺: HSO₄⁻ (charge -2 + 1 = -1) is Hydrogen sulfate.
- Historically, this is also known as Bisulfate.

Naming Ionic Compounds with Polyatomic Ions

Polyatomic ions are incorporated into ionic compound names based on whether they are the cation or anion.

Metal Cation and Polyatomic Anion:
- Name the metal (cation) first, followed by the name of the polyatomic anion.
- Example: LiNO₃ is Lithium nitrate (Lilithium⁺ and Nitrate NO₃⁻).
Transition Metal Cation and Polyatomic Anion:
- Name the transition metal, indicating its charge with Roman numerals in parentheses, followed by the name of the polyatomic anion.
- Example: FeSO₄ is Iron(II) sulfate (Fe²⁺ and Sulfate SO₄²⁻).
Polyatomic Cation (Ammonium) and Nonmetal Anion:
- The only common polyatomic cation to memorize is ammonium (NH₄⁺).
- Name the ammonium ion, followed by the nonmetal anion with its name changed to "-ide."
- Example: NH₄Cl is Ammonium chloride.
Polyatomic Cation and Polyatomic Anion:
- Name the polyatomic cation first, followed by the polyatomic anion's name.
- Example: (NH₄)₃PO₄ is Ammonium phosphate (Ammonium NH₄⁺ and Phosphate PO₄³⁻).

Naming Hydrates

Definition: Hydrates are ionic compounds that have a specific number of water molecules associated with each formula unit.
- Anhydrous: The ionic compound without water.
- Hydrate: The ionic compound with the water of hydration.
Formula Representation: The formula of a hydrate is written by listing the ionic compound's formula, followed by a slightly elevated dot, and then the number of water molecules (e.g., Ba(OH)₂ \cdot 8H₂O). The dot means "associated with" or "attached to."
Naming Convention:
1. Name the ionic compound (cation then anion, using Roman numerals for transition metals if necessary).
2. Add a Greek prefix corresponding to the number of water molecules.
3. Add the word "hydrate."
Greek Prefixes for Hydrates:
- 1: mono-
- 2: di-
- 3: tri-
- 4: tetra-
- 5: penta-
- 6: hexa-
- 7: hepta-
- 8: octa-
Example 1: Ba(OH)₂ \cdot 8H₂O
- Ionic compound: Barium hydroxide (Ba²⁺ and Hydroxide OH⁻).
- Water molecules: 8 (octa-).
- Name: Barium hydroxide octahydrate.
Example 2: CuSO₄ \cdot 5H₂O
- Ionic compound: Copper(II) sulfate (Cu²⁺ and Sulfate SO₄²⁻).
- Water molecules: 5 (penta-).
- Name: Copper(II) sulfate pentahydrate.
Significance: Hydrates often exhibit distinct properties, such as color, compared to their anhydrous forms (e.g., anhydrous copper sulfate is white, while copper(II) sulfate pentahydrate is vivid blue).

Naming Molecular (Covalent) Compounds

Definition: Compounds formed between two nonmetals (or a nonmetal and a metalloid behaving as a nonmetal in combination), existing as individual discrete units (molecules).
Variety of Ratios: Nonmetals can combine in various ratios (e.g., H₂O and H₂O₂), requiring specific naming to differentiate them.
Naming Convention (Binary Covalent Compounds):
1. First Element: Use the full name of the first element. Add a Greek prefix to indicate the number of atoms, unless there is only one atom of the first element (the prefix "mono-" is omitted for the first element).
2. Second Element: Change the ending of the second element's name to "-ide." Always use a Greek prefix to indicate the number of atoms, even for one atom.
**Greek Prefixes (same as for hydrates): **
- 1: mono- (used for second element only)
- 2: di-
- 3: tri-
- 4: tetra-
- 5: penta-
- 6: hexa-
- 7: hepta-
- 8: octa-
- 9: nona-
- 10: deca-
Examples:
- Si₂Br₆: Disilicon hexabromide (two silicon atoms, six bromine atoms).
- PCl₃: Phosphorus trichloride (one phosphorus, three chlorine atoms, "mono-" omitted for phosphorus).
- CO₂: Carbon dioxide (one carbon, two oxygen atoms).
Prefixes with Oxygen (Omitting Vowels):
- If the prefix ends in "o" or "a" and the second element is "oxide" (starting with "o"), the final vowel of the prefix is usually omitted for ease of pronunciation.
- Mono-oxide becomes monoxide (e.g., CO is Carbon monoxide).
- Tetra-oxide becomes tetroxide (e.g., N₂O₄ is Dinitrogen tetroxide).
- It sounds better this way and is the accepted convention.