Atoms, Molecules, and Ions: Chemical Formulas and Compound Nomenclature

Chemical Formulas

Types of Chemical Formulas

Chemical formulas are symbolic representations that convey the composition of chemical substances. There are several types of formulas, each providing different levels of information about the compound.

• Empirical Formula: Shows the simplest whole-number ratio of atoms of each element in a compound. For example, the empirical formula for glucose (C6H12O6) is CH2O.

• Molecular Formula: Indicates the exact number of atoms of each element in a molecule. For example, water has the molecular formula H2O.

• Structural Formula: Illustrates how atoms are bonded to one another, showing the connectivity but not the three-dimensional shape.

• Perspective Drawings, Ball-and-Stick Models, Space-Filling Models: These models provide three-dimensional representations of molecules, showing spatial arrangement of atoms.

Key Point: The molecular formula can be used to determine the empirical formula, but the reverse is not always possible without additional information.

Empirical vs. Molecular Formulas

The empirical formula gives the lowest whole-number ratio of elements, while the molecular formula gives the actual number of atoms in a molecule.

Molecular Formula	Empirical Formula
H2O	H2O
C6H12O6	CH2O
O3	O
N2H4	NH2

Example: The molecular formula of hydrogen peroxide is H2O2, and its empirical formula is HO.

Standard Types of Formulas and Models

Different compounds can be represented in various ways to highlight their composition and structure.

Compound	Molecular Formula	Structural Formula	Ball-and-Stick Model	Space-Filling Model
Hydrogen	H2	H—H	●—●	●●
Water	H2O	H—O—H	●—●—●	●●●
Ammonia	NH3	H—N—H	●—●—●	●●●
Methane	CH4	H—C—H	●—●—●	●●●

Additional info: Ball-and-stick and space-filling models are used to visualize molecular geometry and atomic connectivity.

Formulas of Ionic Compounds

Characteristics of Ionic Compounds

Ionic compounds are formed from the electrostatic attraction between positive ions (cations) and negative ions (anions). They do not exist as discrete molecules but as extended lattices.

• Formula Unit: The simplest ratio of ions that results in electrical neutrality.

• Empirical Formula: For ionic compounds, the formula unit is usually the empirical formula.

• Charge Balance: The sum of the charges in the formula unit must be zero.

Example: For ZnBr2, Zn2+ and 2Br- combine so that $(2+) + 2(-1) = 0$.

Examples of Ionic Compound Formulas

Compound	Cation	Anion	Charge Calculation
Aluminum oxide	Al3+	O2-	2 × (+3) + 3 × (-2) = 0
Calcium bromide	Ca2+	Br-	1 × (+2) + 2 × (-1) = 0
Sodium carbonate	Na+	CO32-	2 × (+1) + 1 × (-2) = 0

Key Point: The subscripts in the formula reflect the ratio needed to balance the charges.

Writing Formulas for Ionic Compounds

To write the formula for an ionic compound:

1. Write the symbol and charge for each ion.

2. Balance the charges by adjusting subscripts so the total positive and negative charges are equal.

3. If necessary, reduce subscripts to the lowest whole-number ratio.

Example: Magnesium nitride: Mg2+ and N3- combine to form Mg3N2.

Naming Compounds (Chemical Nomenclature)

Classification of Compounds

Chemical compounds are classified as organic or inorganic:

• Organic Compounds: Contain carbon, usually with hydrogen, oxygen, nitrogen, or sulfur.

• Inorganic Compounds: Include ionic compounds, molecular compounds, acids, bases, and hydrates.

Naming Ionic Compounds

Ionic compounds are named by identifying the cation and anion.

• Binary Ionic Compounds: Name the metal cation first, then the nonmetal anion with the suffix "-ide".

• Polyatomic Ions: Name the cation, then the polyatomic anion (e.g., Mg(OH)2 is magnesium hydroxide).

• Transition Metals: Indicate the charge of the metal with Roman numerals (e.g., FeCl2 is iron(II) chloride).

Example: BaCl2 is barium chloride; KNO3 is potassium nitrate.

Common Monatomic Anions

Group	Anion Name
Group 7A (Halogens)	Fluoride (F-), Chloride (Cl-), Bromide (Br-), Iodide (I-)
Group 6A	Oxide (O2-), Sulfide (S2-), Selenide (Se2-), Telluride (Te2-)
Group 5A	Nitride (N3-), Phosphide (P3-)

Naming Molecular Compounds

Molecular (covalent) compounds are typically formed between nonmetals. Prefixes are used to indicate the number of atoms of each element.

• First element: Use the element name.

• Second element: Use the root plus "-ide" and a prefix to indicate the number.

• Prefixes: mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-.

Examples:

• CO2: Carbon dioxide

• SO2: Sulfur dioxide

• N2O4: Dinitrogen tetroxide

• NO: Nitrogen monoxide

Naming Acids and Bases

Naming Acids

Acids are substances that yield hydrogen ions (H+) when dissolved in water.

• Binary Acids: Composed of hydrogen and one other element. Named as "hydro-" + root of nonmetal + "-ic acid" (e.g., HCl in water is hydrochloric acid).

• Oxoacids: Contain hydrogen, oxygen, and another element. The name depends on the polyatomic ion present:

Oxoacid	Formula	Oxoanion	Formula
Carbonic acid	H2CO3	Carbonate	CO32-
Nitric acid	HNO3	Nitrate	NO3-
Nitrous acid	HNO2	Nitrite	NO2-
Sulfuric acid	H2SO4	Sulfate	SO42-
Sulfurous acid	H2SO3	Sulfite	SO32-

Example: H3PO4 (phosphoric acid) can form three different ions: dihydrogen phosphate, hydrogen phosphate, and phosphate.

Naming Bases

Bases are substances that yield hydroxide ions (OH-) when dissolved in water.

• Named as "metal name + hydroxide" (e.g., NaOH is sodium hydroxide).

Hydrates

Definition and Naming

Hydrates are compounds that contain a specific number of water molecules associated with each formula unit. The water is called water of hydration or water of crystallization.

• Named by writing the formula of the anhydrous compound, followed by a dot and the number of water molecules (e.g., BaCl2·2H2O is barium chloride dihydrate).

• Water can be removed by heating.

Example: MgSO4·7H2O is magnesium sulfate heptahydrate.

Organic Compounds and Nomenclature

Alkanes

Alkanes are hydrocarbons containing only single bonds between carbon atoms. The names are based on the number of carbon atoms:

• meth- (1), eth- (2), prop- (3), but- (4), pent- (5), hex- (6), hept- (7), oct- (8), non- (9), dec- (10)

• Suffix "-ane" indicates an alkane.

Examples: Methane (CH4), Ethane (C2H6), Propane (C3H8).

Alcohols

Alcohols are organic compounds where a hydrogen atom in an alkane is replaced by a hydroxyl group (–OH). The name ends in "-ol".

• Methanol (CH3OH)

• Ethanol (C2H5OH)

• 1-Propanol (C3H7OH)

Isomers: Compounds with the same molecular formula but different structures (e.g., 1-propanol and 2-propanol).

Common Names and Systematic Names

Formula	Common Name	Systematic Name
H2O	Water	Dihydrogen monoxide
NH3	Ammonia	Trihydrogen nitride
CO2	Dry ice	Solid carbon dioxide
NaCl	Table salt	Sodium chloride
N2O	Laughing gas	Dinitrogen monoxide
CaCO3	Marble, chalk, limestone	Calcium carbonate
CaO	Quicklime	Calcium oxide
Ca(OH)2	Slaked lime	Calcium hydroxide
NaHCO3	Baking soda	Sodium hydrogen carbonate
Na2CO3·10H2O	Washing soda	Sodium carbonate decahydrate
MgSO4·7H2O	Epsom salt	Magnesium sulfate heptahydrate
Mg(OH)2	Milk of magnesia	Magnesium hydroxide
CaSO4·2H2O	Gypsum	Calcium sulfate dihydrate

Summary Table: Prefixes for Molecular Compounds

Number	Prefix
1	mono-
2	di-
3	tri-
4	tetra-
5	penta-
6	hexa-
7	hepta-
8	octa-
9	nona-
10	deca-

Key Equations

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

• Empirical Formula Calculation: $\text{Empirical formula} = \text{Simplest whole-number ratio of atoms}$