lecture 14

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Molecules and Compounds

From Elements to Molecules

When two or more nonmetallic elements combine, a molecule is formed. Molecules can be composed of the same element or of different elements, resulting in a wide variety of substances in nature.

Same element: Example: O2 (oxygen molecule)
Different elements: Example: H2O (water molecule)
The diversity of substances in nature is due to the ability of elements to form compounds.

Hydrogen, Oxygen, and Water: Properties and Differences

The properties of compounds are often dramatically different from the properties of the elements that compose them. For example, hydrogen and oxygen are both gases with distinct properties, but when combined to form water, the resulting compound has entirely new characteristics.

Selected Properties	Hydrogen	Oxygen	Water
Boiling Point	-253°C	-183°C	100°C
State at Room Temperature	Gas	Gas	Liquid
Flammability	Explosive	Necessary for combustion	Used to extinguish flame

Example: Water is used to extinguish flames, while hydrogen is explosive and oxygen is necessary for combustion.

The Law of Definite Proportion

Formation of Molecules

The Law of Definite Proportion states that a chemical compound always contains the same elements in the same proportion by mass. For example, a mixture of hydrogen and oxygen gases can have any ratio, but water (H2O) always has a fixed ratio of hydrogen to oxygen.

Each water molecule is composed of two hydrogen atoms and one oxygen atom.
Ratio: 2 hydrogen atoms : 1 oxygen atom

Types of Chemical Bonds

Overview of Chemical Bonds

Compounds are composed of atoms held together by chemical bonds. These bonds form because they lower the potential energy of the charged particles (electrons and protons) that make up atoms. Chemical bonds are classified into two main types:

Ionic bonds
Covalent bonds

Ionic Bonds

Ionic bonds occur between a metal atom and a nonmetal atom. They involve the transfer of electrons from one atom to another, resulting in the formation of ions:

The metal atom loses electrons to become a cation (positively charged ion).
The nonmetal atom gains electrons to become an anion (negatively charged ion).
Oppositely charged ions attract each other by electrostatic forces, forming an ionic compound.
In the solid phase, ionic compounds form a lattice—a regular three-dimensional arrangement of alternating cations and anions.

Covalent Bonds

Covalent bonds occur between two or more nonmetal atoms. They involve the sharing of electrons between atoms of high ionization energy. Instead of transferring electrons, the atoms share them, resulting in the formation of molecules (molecular compounds).

Molecular compounds are composed of atoms covalently bonded to each other.

Comparison of Ionic and Covalent Bonds

Bond Type	Participants	Electron Behavior	Resulting Species
Ionic	Metal + Nonmetal	Transfer of electrons	Ions (cation and anion)
Covalent	Nonmetal + Nonmetal	Sharing of electrons	Molecules

Representing Compounds: Chemical Formulas and Molecular Models

Chemical Formulas

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

Empirical formula: Gives the simplest whole-number ratio of atoms in a compound.
Molecular formula: Gives the actual number of atoms of each element in a molecule.
Structural formula: Shows how atoms are bonded to each other in a molecule, often using lines to represent covalent bonds.

Examples of Chemical Formulas

Empirical formula: For C2H6, the empirical formula is CH3.
Molecular formula: For C2H6, the molecular formula is C2H6.
Structural formula: For water, the structural formula is H–O–H.

Determining Empirical Formulas

To determine the empirical formula from a molecular formula, divide the subscripts by the greatest common factor.

Example: For C4H8, divide by 4 to get CH2.
Example: For B2H6, divide by 2 to get BH3.
Example: For CCl4, the empirical and molecular formulas are the same.

Summary Table: Types of Chemical Formulas

Type of Formula	Information Conveyed
Empirical	Simplest whole-number ratio of atoms
Molecular	Actual number of atoms of each element
Structural	Arrangement and bonding of atoms

Molecular Models: 3-D Representations

Ball-and-Stick and Space-Filling Models

Molecular models provide a three-dimensional representation of molecules. The two main types are:

Ball-and-stick model: Atoms are represented as balls and bonds as sticks. The way the balls connect reflects the molecule's shape.
Space-filling model: Atoms fill the space between each other, more closely representing the actual size and proximity of atoms in a molecule.

Example: In a space-filling model, each sphere represents the electron cloud of an atom. The nucleus would be too small to see at this scale.

Lewis Structure Model: Representing Valence Electrons

The Lewis Model

The Lewis structure (or Lewis electron-dot structure) is a way to represent the valence electrons of atoms within a molecule. Valence electrons are shown as dots around the chemical symbol of an element.

Lewis structures focus on valence electrons because chemical bonding involves the transfer or sharing of these electrons.
Example: Oxygen with 6 valence electrons is represented as O with 6 dots around it.

Application: Lewis structures are used to predict the arrangement of atoms and the distribution of electrons in molecules and ions.

Key Equations and Concepts

Law of Definite Proportion:
Empirical formula calculation: