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.

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 example, water (H2O) always has a ratio of 2 hydrogen atoms to 1 oxygen atom.

• Mixtures can have any proportion of components.

• Compounds have fixed ratios of elements.

Types of Chemical Bonds

Ionic and Covalent Bonds

Atoms in compounds are held together by chemical bonds, which lower the potential energy of the system. 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

Ionic bonds occur when a metal atom transfers electrons to a nonmetal atom, resulting in a cation (positive ion) and an anion (negative ion). These ions attract each other by electrostatic forces, forming an ionic compound with a crystal 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

Types of Chemical Formulas

• 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: Shows how atoms are bonded and arranged in a molecule.

Example: The structural formula for hydrogen peroxide (H2O2):

Molecular Models

Molecular models provide three-dimensional representations of molecules:

• Ball-and-stick model: Atoms as balls, bonds as sticks; color-coded by element.

• Space-filling model: Atoms fill the space between each other, representing the molecule's actual shape.

Ways of Representing a Compound

Compounds can be represented by molecular formulas, structural formulas, ball-and-stick models, and space-filling models. For example, methane (CH4) and benzene (C6H6) can be depicted in various ways:

Lewis Structure Model

Valence Electrons and Lewis Structures

The Lewis Model represents valence electrons as dots around the element symbol. Lewis structures are used to illustrate the transfer or sharing of electrons in chemical bonding.

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 valence electrons).

Ionic Bonding and Crystal Lattice

Properties of Ionic Compounds

• Hardness: Ionic solids are hard due to strong electrostatic forces.

• Brittleness: Ionic solids are brittle; they shatter when struck.

• Conductivity: Ionic solids do not conduct electricity, but ionic compounds conduct electricity when melted or dissolved in water.

Crystal Lattice and Lattice Energy

The crystal lattice is a three-dimensional arrangement of ions in an ionic compound. Lattice energy is the energy released when the crystal forms from separate ions in the gas state. It is always exothermic and depends on the charge and size of the ions.

Naming Ionic Compounds

Type One and Type Two Cations

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

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

Binary ionic compounds are named by stating the cation first, followed by the base name of the anion with the suffix -ide.

Polyatomic Ions and Hydrates

Polyatomic Ions

Polyatomic ions are composed of two or more atoms with a specific charge. Most are oxyanions, and 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 such as mono-, di-, tri-, etc., indicate the number of water molecules.

Covalent Bonding

Bonding and Lone Pair Electrons

Electrons shared between atoms are called bonding pairs, while electrons 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 present (mono-, di-, tri-, etc.). The prefix mono- is usually omitted for the first element.

Formula Mass and Molar Mass

Calculating Formula Mass

The molecular mass (or formula mass) is the sum of the atomic masses of all atoms in a molecule or formula unit. For example, the molecular mass of water is 18.02 grams per mole.

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 as:

Empirical and Molecular Formulas

Determining Empirical Formula from Experimental Data

Steps to determine empirical formula:

1. Convert mass percent to grams (assume 100 g sample).

2. Convert grams to moles using molar mass.

3. Divide all mole values by the smallest to get ratios.

4. Multiply ratios to obtain whole numbers if necessary.

From Empirical to Molecular Formula

The molecular formula is a whole-number multiple of the empirical formula:

Combustion Analysis

Determining Empirical Formula from Combustion Data

Combustion analysis is used to determine the empirical formula of compounds containing C, H, and O by measuring the masses of CO2 and H2O produced.

Organic Compounds

Modern Organic Chemistry

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.

Summary Table: Ways of Representing Compounds

Key Equations