Molecules and Compounds

Elements to Molecules

When two or more nonmetallic elements combine, they form a molecule. Molecules can consist of the same element (e.g., O2) or different elements (e.g., H2O). The diversity of substances in nature arises from the ability of elements to form compounds, which are new substances with distinct properties from their constituent elements.

Hydrogen, Oxygen, and Water

The properties of compounds differ dramatically from those of their constituent elements. For example, water (H2O) is a compound formed from hydrogen and oxygen, both of which are gases, but water is a liquid with unique properties.

Law of Definite Proportion

The Law of Definite Proportion states that a compound always contains the same elements in the same proportion by mass. For water, the ratio is always 2 hydrogen atoms to 1 oxygen atom.

• Mixtures can have any proportion of elements.

• Compounds have a fixed, definite proportion.

Types of Chemical Bonds

Chemical Bonds: Ionic vs. Covalent

Chemical bonds are forces that hold atoms together in compounds, lowering their potential energy. There are two main types:

• Ionic Bonds: Formed between metals and nonmetals; involve transfer of electrons.

• Covalent Bonds: Formed between nonmetals; involve sharing of electrons.

Ionic Bonds: Formation and Structure

In ionic bonding, a metal atom transfers electrons to a nonmetal atom, forming a cation (positive ion) and an anion (negative ion). These ions attract each other by electrostatic forces, forming an ionic compound with a lattice structure.

Covalent Bonds: Molecular Compounds

Covalent bonds occur between nonmetal atoms, which share electrons to achieve stable electron configurations. The resulting compounds are called molecular compounds.

Representing Compounds: Chemical Formulas and Models

Chemical Formulas

A chemical formula indicates the type and number of each element in a compound. There are three main types:

• Empirical Formula: Simplest whole-number ratio of atoms.

• Molecular Formula: Actual number of atoms in a molecule.

• Structural Formula: Shows how atoms are bonded.

Structural Formula Example

Structural formulas use lines to represent covalent bonds and show the arrangement of atoms.

Molecular Models

Molecular models provide three-dimensional representations of molecules. The ball-and-stick model shows atoms as balls and bonds as sticks, while the space-filling model shows atoms filling the space between them.

Ways of Representing a Compound

Compounds can be represented by molecular formulas, structural formulas, ball-and-stick models, and space-filling models.

Lewis Structure Model

Lewis Electron-Dot Structures

The Lewis model represents valence electrons as dots around the element symbol. Lewis structures are used to illustrate bonding and predict molecular structure.

Octet Rule

Atoms tend to gain, lose, or share electrons to achieve a noble gas-like configuration, usually eight valence electrons (octet). Some elements, like hydrogen and helium, follow the duet rule (two electrons).

Ionic Bonding and Crystal Lattice

Ionic Compounds and Lattice Structure

Ionic compounds are composed of cations and anions arranged in a crystal lattice. The lattice maximizes attractions between ions, resulting in a stable structure.

Lattice Energy

Lattice energy is the energy released when ions form a solid crystal from the gas phase. It is always exothermic and depends on the charge and distance between ions.

Conductivity of Ionic Compounds

Solid vs. Aqueous Ionic Compounds

In the solid state, ions are fixed and do not conduct electricity. When dissolved in water or melted, ions are free to move and conduct electricity.

Naming Ionic Compounds

Type One and Type Two Cations

Ionic compounds are named based on the type of metal:

• Type One: Metals with invariant charge (e.g., alkali metals, alkaline earth metals).

• Type Two: Metals with variable charge (e.g., transition metals).

Naming Binary Ionic Compounds

Binary compounds contain two elements. The name consists of the cation name followed by the base name of the anion with the suffix -ide.

Naming Type Two Ionic Compounds

For metals with variable charge, the cation name is followed by a Roman numeral indicating its charge.

Polyatomic Ions and Hydrates

Polyatomic Ions

Polyatomic ions are groups of atoms with a charge, often containing oxygen (oxyanions). Their names depend on the number of oxygen atoms (e.g., -ate, -ite, hypo-, per-).

Hydrated Ionic Compounds

Hydrates are ionic compounds with a specific number of water molecules associated with each formula unit. Prefixes indicate the number of water molecules (e.g., mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-).

Covalent Bonding

Bonding and Lone Pair Electrons

Electrons shared between atoms are called bonding pairs, while those not shared are lone pairs (nonbonding pairs).

Single, Double, and Triple Covalent Bonds

• Single Bond: One pair of shared electrons.

• Double Bond: Two pairs of shared electrons.

• Triple Bond: Three pairs of shared electrons.

Naming Molecular Compounds

Binary Molecular Compounds

Molecular compounds are composed of two or more nonmetals. The element with the smallest group number is named first, and prefixes indicate the number of atoms (mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-).

Formula Mass and Molar Mass

Calculating Formula Mass

The formula mass (or molecular mass) is the sum of the atomic masses of all atoms in a compound's formula.

Using Molar Mass to Count Molecules

Molar mass and Avogadro's number are used to convert between mass, moles, and number of molecules.

Composition of Compounds

Mass Percent Composition

The mass percent of an element in a compound is calculated using the formula:

Empirical and Molecular Formulas

Determining Empirical Formula

The empirical formula is the simplest whole-number ratio of atoms in a compound. It can be determined from percent composition or combustion analysis.

From Empirical to Molecular Formula

The molecular formula is a whole-number multiple of the empirical formula. To find it:

Combustion Analysis

Combustion Analysis Set-up

Combustion analysis is used to determine the empirical formula of organic compounds by burning a known mass and measuring the products.

Organic Compounds

Modern Organic Compounds

Organic compounds are mainly composed of carbon and hydrogen, sometimes with oxygen, nitrogen, phosphorus, sulfur, and other elements. Carbon forms four covalent bonds and can create chains, branches, and rings.

Common Hydrocarbons

Hydrocarbons are organic compounds consisting only of carbon and hydrogen. They can be straight-chain (normal) or branched.

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