Molecules and Compounds - Definitions, Properties, and Nomenclature

Section 3.1 - Molecules and Compounds

Definition and Properties

Compounds are a fundamental concept in chemistry, representing substances composed of two or more elements chemically combined in fixed proportions. Understanding the distinction between elements and compounds is essential for grasping chemical behavior and reactions.

• Compound: A pure substance made of two or more elements in a definite ratio. Compounds can be broken down into simpler substances by chemical means.

• Element: A substance that cannot be broken down into simpler substances by chemical reactions.

• Example: Water (H2O) always contains two hydrogen atoms for every one oxygen atom. Sodium chloride (NaCl) always contains one sodium atom for every one chlorine atom.

Properties of Elements vs. Compounds

The properties of a compound are often very different from those of the elements that compose it. This is due to the chemical bonds formed between the elements.

• Hydrogen (H2): Boiling point: -253°C; explosive gas.

• Oxygen (O2): Boiling point: -183°C; essential for respiration.

• Water (H2O): Boiling point: 100°C; used to extinguish fires.

• Sodium (Na): Highly reactive metal; explodes on contact with water.

• Chlorine (Cl): Corrosive, toxic gas.

• Sodium chloride (NaCl): Safe, edible compound used as table salt.

Key Point: When elements combine to form compounds, the resulting substance has new properties distinct from its constituent elements.

Chemical Bonds

Types of Chemical Bonds

Chemical bonds are the forces that hold atoms together in compounds. They are broadly classified into two main types:

• Ionic Bonds: Formed by the transfer of electrons from a metal atom to a nonmetal atom, resulting in the formation of cations and anions.

• Covalent Bonds: Formed by the sharing of electrons between two nonmetal atoms, resulting in the formation of molecules.

Example: Sodium and chlorine form an ionic bond in NaCl; hydrogen and oxygen form covalent bonds in H2O.

Representing Compounds

Chemical Formulas and Models

Chemical formulas provide a concise way to represent the composition of compounds. They indicate the elements present and the relative number of atoms of each.

• Empirical Formula: Shows the simplest whole-number ratio of atoms in a compound.

• Molecular Formula: Shows the actual number of atoms of each element in a molecule.

• Structural Formula: Uses lines to represent covalent bonds and shows how atoms are connected.

Examples:

• Water: $\mathrm{H_2O}$ (2:1 ratio of H:O)

• Sodium chloride: $\mathrm{NaCl}$ (1:1 ratio of Na:Cl)

• Carbon tetrachloride: $\mathrm{CCl_4}$ (1:4 ratio of C:Cl)

Classification of Substances

Substances can be categorized as elements or compounds, and further subcategorized based on their composition:

• Elements: Atomic (single atoms) or molecular (diatomic/polyatomic molecules, e.g., O2, P4).

• Compounds: Molecular (covalently bonded nonmetals) or ionic (ions held together by electrostatic forces).

Naming Ionic Compounds

Type I Ionic Compounds

Type I ionic compounds contain metals that form only one type of cation (usually Group 1A and 2A metals). The name consists of the metal followed by the nonmetal with an -ide ending.

• Example: KCl is potassium chloride; CaO is calcium oxide.

Type II Ionic Compounds

Type II ionic compounds contain metals (often transition metals) that can form more than one type of cation. The charge of the metal is specified using Roman numerals in parentheses.

• Example: FeCl2 is iron(II) chloride; FeCl3 is iron(III) chloride.

Polyatomic Ions

Polyatomic ions are groups of covalently bonded atoms with an overall charge. Ionic compounds containing polyatomic ions are named using the name of the cation and the polyatomic ion.

• Example: NaNO2 is sodium nitrite; CaCO3 is calcium carbonate.

Oxyanions

Oxyanions are polyatomic ions containing oxygen. Their names depend on the number of oxygen atoms:

• -ate: More oxygen atoms (e.g., NO3- is nitrate)

• -ite: Fewer oxygen atoms (e.g., NO2- is nitrite)

• Prefixes: hypo- (least), per- (most), used when more than two oxyanions exist (e.g., ClO- hypochlorite, ClO4- perchlorate)

Hydrates

Hydrates are ionic compounds containing a specific number of water molecules associated with each formula unit. The number of water molecules is indicated by a prefix.

• Example: MgSO4·7H2O is magnesium sulfate heptahydrate; CuSO4·6H2O is copper(II) sulfate hexahydrate.

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

Naming Molecular Compounds

Formulas and Names

Molecular compounds are composed of two or more nonmetals. Their formulas cannot always be predicted from the elements alone, as multiple compounds may form from the same elements.

• Example: Nitrogen and oxygen form NO, NO2, N2O, N2O3, N2O4, N2O5.

Names are constructed using prefixes to indicate the number of atoms of each element, with the second element ending in -ide.

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

• Example: NO2 is nitrogen dioxide; N2F4 is dinitrogen tetrafluoride.

Key Point: Prefixes are used for molecular compounds but not for ionic compounds.

Summary Table: Types of Compounds and Naming Rules

Type	Constituents	Naming Rule	Example
Type I Ionic	Metal (fixed charge) + Nonmetal	Metal + Nonmetal-ide	NaCl: sodium chloride
Type II Ionic	Metal (variable charge) + Nonmetal	Metal (Roman numeral) + Nonmetal-ide	FeCl2: iron(II) chloride
Ionic w/ Polyatomic Ion	Metal + Polyatomic Ion	Metal + Polyatomic Ion name	NaNO2: sodium nitrite
Molecular	Nonmetal + Nonmetal	Prefix + 1st element + Prefix + 2nd element-ide	CO2: carbon dioxide
Hydrate	Ionic compound + water	Compound name + prefix-hydrate	CuSO4·5H2O: copper(II) sulfate pentahydrate

Additional info: Some context and examples have been expanded for clarity and completeness, including systematic naming conventions and classification tables.