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, resulting in new substances with properties distinct from their constituent elements.

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 consists of 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. The three main types of chemical bonds are:

• Ionic bonds

• Covalent bonds

• Metallic bonds

Ionic Bonds

Ionic bonds occur between metals and nonmetals and involve the transfer of electrons from the metal to the nonmetal. The metal becomes a positively charged cation, and the nonmetal becomes a negatively charged anion. These oppositely charged ions attract each other, forming an ionic compound, which in the solid phase is a lattice of alternating cations and anions.

Covalent Bonds

Covalent bonds occur between two or more nonmetals and involve the sharing of electrons between atoms. The atoms are held together as a molecule, and these compounds are called molecular compounds.

Representing Compounds

Chemical Formulas

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

• Empirical formula: Simplest whole-number ratio of atoms in a compound.

• Molecular formula: Actual number of atoms of each element in a molecule.

• Structural formula: Shows how atoms are bonded together.

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 of atoms and how they fill space in a molecule.

Lewis Structures and the Octet Rule

Lewis Structure Model

The Lewis structure represents valence electrons as dots around the chemical symbol. It is used to illustrate the arrangement of electrons in molecules and ions, focusing on the transfer or sharing of valence electrons.

Octet Rule

The octet rule states that atoms tend to gain, lose, or share electrons to achieve a noble gas configuration (eight valence electrons). Some elements, such as H, Li, Be, and B, are exceptions and achieve stability with fewer electrons.

Ionic Bonding: Model versus Reality

Properties of Ionic Compounds

Ionic compounds are characterized by:

• High melting and boiling points

• Hardness and brittleness

• Electrical conductivity in the liquid state or when dissolved in water

Lattice Energy

Lattice energy is the energy released when gaseous ions form an ionic solid. It is always exothermic and is a measure of the stability of the ionic lattice.

Conductivity of Ionic Compounds

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

Naming Ionic Compounds

Type I and Type II Ionic Compounds

Ionic compounds are named based on the type of metal present:

• Type I: Metal forms only one type of cation (e.g., Na+, Ca2+).

• Type II: Metal forms more than one type of cation (e.g., Fe2+, Fe3+).

Naming Compounds with Polyatomic Ions

When a compound contains a polyatomic ion, the name of the polyatomic ion is used directly in the compound's name.

Covalent Bonding and Molecular Compounds

Bonding and Lone Pair Electrons

Electrons shared between atoms are called bonding pairs, while electrons not shared are called lone pairs or nonbonding pairs.

Single, Double, and Triple Bonds

• Single bond: Two atoms share one pair of electrons.

• Double bond: Two atoms share two pairs of electrons.

• Triple bond: Two atoms share three pairs of electrons.

Naming Molecular Compounds

Binary Molecular Compounds

Binary molecular compounds are named using prefixes to indicate the number of each type of atom present. The first element is named first, followed by the second element with the suffix -ide.

Formula Mass and Mole Calculations

Calculating Formula Mass

The formula mass (or molecular mass) is the sum of the atomic masses of all atoms in a chemical formula. For example, the formula mass of glucose (C6H12O6) is calculated as:

Using Molar Mass to Count Molecules

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

Percent Composition and Empirical Formulas

Mass Percent Composition

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

Empirical Formula from Experimental Data

The empirical formula is the simplest whole-number ratio of atoms in a compound. It can be determined from percent composition or combustion analysis by converting masses to moles and finding the simplest ratio.

Combustion Analysis

Combustion analysis is a technique used to determine the empirical formula of compounds containing C, H, and O by burning a known mass and measuring the products.

Organic Compounds

Introduction to Organic 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.