Molecules and Compounds

Elements to Molecules

When two or more 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 collections of molecules.

• Molecule: A group of atoms bonded together, representing the smallest fundamental unit of a chemical compound.

• Compound: A substance formed when two or more elements are chemically bonded.

Hydrogen, Oxygen, and Water: Properties Comparison

The properties of compounds are often dramatically different from those of their constituent elements. For example, hydrogen and oxygen are both gases with distinct properties, but when combined to form water, the resulting compound is a liquid with unique characteristics.

Law of Definite Proportion

The Law of Definite Proportion states that a chemical compound always contains exactly the same proportion of elements by mass. For example, water (H2O) always has two hydrogen atoms for every one oxygen atom, regardless of the sample size.

Types of Chemical Bonds

Chemical Bonds Overview

Chemical bonds are the forces that hold atoms together in compounds. They result from the attractions between charged particles (electrons and protons) in atoms. There are three main types of chemical bonds:

• Ionic Bonds

• Covalent Bonds

• Metallic Bonds

Ionic vs. Covalent Bonds

Ionic bonds occur between metals and nonmetals and involve the transfer of electrons, resulting in the formation of cations and anions. Covalent bonds occur between nonmetals and involve the sharing of electrons.

Formation of Ionic Compounds

In ionic bonding, a metal atom loses electrons to become a cation, while a nonmetal atom gains electrons to become an anion. The resulting oppositely charged ions attract each other, forming an ionic compound, typically arranged in a crystal lattice.

Representing Compounds

Chemical Formulas and Molecular Models

Chemical compounds are represented by chemical formulas, which indicate the types and numbers of atoms present. There are several ways to represent compounds:

• Empirical Formula: Simplest whole-number ratio of elements (e.g., CH2 for C4H8).

• Molecular Formula: Actual number of atoms of each element (e.g., C4H8).

• Structural Formula: Shows how atoms are bonded (e.g., H–O–O–H for H2O2).

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 the relative sizes and proximity of atoms.

Summary Table: Ways of Representing Compounds

Lewis Structure Model

Valence Electrons and Lewis Structures

The Lewis structure represents valence electrons as dots around atomic symbols. These structures help visualize the transfer or sharing of electrons in chemical bonding, especially for covalent and simple ionic compounds.

Octet Rule

Atoms tend to gain, lose, or share electrons to achieve a noble gas configuration (eight valence electrons, or an octet). There are exceptions, such as hydrogen (duet), and elements in period 3 or below, which can have expanded octets.

Lewis Structures for Ionic Compounds

Lewis symbols can be used to predict the formulas of ionic compounds by showing the transfer of electrons from metals to nonmetals, resulting in stable ions.

Ionic Compounds: Properties and Conductivity

Crystal Lattice and Conductivity

Ionic compounds form a crystal lattice, maximizing attractions between cations and anions. In the solid state, ions are fixed and do not conduct electricity. When melted or dissolved in water, ions are free to move, allowing the compound to conduct electricity.

Writing and Naming Ionic Compounds

The formula of an ionic compound reflects the smallest whole-number ratio of ions. The sum of the charges of the cations and anions must be zero. Naming conventions depend on whether the metal forms only one type of ion (Type I) or more than one (Type II).

Polyatomic Ions and Hydrates

Common Polyatomic Ions

Polyatomic ions are ions composed of two or more atoms covalently bonded, carrying a charge. Many are oxyanions, containing oxygen and another element. Their names depend on the number of oxygen atoms present (e.g., nitrate, nitrite, sulfate, sulfite).

Hydrated Ionic Compounds

Hydrates are ionic compounds with a specific number of water molecules associated with each formula unit. Prefixes (mono-, di-, tri-, etc.) indicate the number of water molecules (e.g., MgSO4·7H2O is magnesium sulfate heptahydrate).

Covalent Bonding

Bonding and Lone Pair Electrons

In covalent bonds, electrons are shared between atoms. Shared electrons are called bonding pairs, while non-shared electrons are lone pairs (or nonbonding pairs).

Single, Double, and Triple Bonds

Single bond: One pair of shared electrons (e.g., H–H). Double bond: Two pairs of shared electrons (e.g., O=O). Triple bond: Three pairs of shared electrons (e.g., N≡N).

Naming Molecular Compounds

Rules for Naming 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-, etc.). The prefix mono- is usually omitted for the first element.

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 chemical formula. It is used to convert between mass and number of particles using Avogadro's number.

Percent Composition and Empirical Formulas

Mass Percent Composition

The mass percent of an element in a compound is calculated as:

$\text{\% mass of element Z} = \frac{\text{molecular mass of element Z}}{\text{mass of 1 mol of compound}} \times 100\%$

Empirical and Molecular Formulas

The empirical formula gives the simplest whole-number ratio of atoms in a compound. The molecular formula is a whole-number multiple of the empirical formula, determined by the compound's molar mass.

Combustion Analysis

Determining Empirical Formulas by Combustion

Combustion analysis is a technique used to determine the empirical formula of organic compounds by burning a known mass of the compound and measuring the amounts of products formed (CO2 and H2O).

Introduction to Organic Compounds

Organic vs. Inorganic Compounds

Organic compounds are primarily composed of carbon and hydrogen, sometimes with oxygen, nitrogen, phosphorus, sulfur, and other elements. Carbon's ability to form four covalent bonds and create chains and rings is central to organic chemistry.