notes-04_Lecture-2025spring

Chapter 4: Basic Chemical Bonding

Elements and Compounds

• Elements combine to form numerous compounds similar to how letters form words.

• Key Point: The properties of compounds differ from those of the constituent elements.

Chemical Bonds

• Compounds are made up of atoms connected through chemical bonds.

• Definition: Chemical bonds represent the attraction forces between atoms, originating from the interactions between protons and electrons.

Why Do Atoms Bond?

• Atoms bond to lower their energy state, forming stable interactions:

  • Nucleus–to–nucleus repulsion

  • Electron–to–electron repulsion

  • Nucleus–to–electron attraction

• A chemical bond forms when the potential energy of bonded atoms is lower than that of separate atoms.

• Energy decreases when bonds form, foundational concept in quantum mechanics.

Bond Types

• Ionic Bonds: Result from the transfer of electrons, usually between metals and nonmetals.

• Covalent Bonds: Formed when two atoms share electrons, typically between nonmetals.

Page 2: Representing Compounds with Chemical Formulas

Chemical Formulas

• Chemical formulas convey information about the elements in a compound using symbols.

• Not all formulas accurately represent the structure of the compound.

Types of Formula

Empirical Formula

• Represents the relative number of each atom in a compound, does not provide info on the structure or order.

  • Example: Calcium Fluoride (CaF2).

Molecular Formula

• Gives the actual number of atoms in a compound.

  • Example: Oxalic Acid (C2H2O4).

Structural Formula

• Illustrates how atoms are connected using lines for bonds.

  • Single bond: 2 electrons, double bond: 4 electrons, triple bond: 6 electrons.

Molecular Models

• Ball-and-stick models: Visual representation of atoms and their bonds.

• Space-filling models: Show electron clouds of atoms.

Classifying Elements & Compounds

• Atomic Elements: Single atoms.

• Molecular Elements: Multi-atom molecules (e.g., diatomics)

• Molecular Compounds: Made of nonmetals.

• Ionic Compounds: Composed of cations and anions.

Page 3: Molecular View of Elements and Compounds

Elements

• Atomic Elements: Most consist of single atoms not chemically bound together.

• Molecular Elements: Made of multi-atom molecules bound by covalent bonds (e.g., diatomics: H2, O2).

Compounds

• Comprised of ions in a 3D arrangement (ionic compounds) or molecules with covalent bonds (molecular compounds).

Differences Between Ionic and Molecular Compounds

• Example of propane as a molecular compound versus sodium chloride as an ionic compound.

Classification Exercise

• Classify elements and compounds as atomic, molecular, ionic, or molecular compounds.

  • Examples: Aluminum (Al) = Atomic Element, Aluminum Chloride (AlCl3) = Ionic Compound.

Page 4: Ionic Compounds

Formation of Ionic Compounds

• Ionic compounds consist of metals and nonmetals exhibiting ionic bonds; metals form cations and nonmetals form anions.

• Compounds must overall balance charge.

Examples of Ionic Compounds

1. Sodium and Sulfur: Na2S is needed to balance charges.

2. Aluminum and Oxide: Al2O3 results from balancing Al3+ and O2− ions.

Naming Ionic Compounds

• Involves determining cation and anion names, includes special rules for metals with invariant or variable charges.

Page 5: Naming Ionic Compounds

Rules for Naming

• Systematic naming involves naming the cation first followed by the anion, noting charges for variable cations.

• Binary Ionic Compounds: Metal cation + nonmetal anion.

  • Cation is named first; anion's name modified to -ide.

Metals with Variable Charges

• Named using Roman numerals to indicate charge based on the oxidation state.

Page 6: Polyatomic Ions

Characteristics of Polyatomic Ions

• Composed of multiple atoms that act as one ion.

• Identified through their names and charges without changing them.

Naming Ionic Compounds with Polyatomic Ions

• Example: Na2SO4 involves identifying sodium (Na+) and sulfate (SO4^2−).

Page 7: Writing Names of Binary Molecular Compounds

Rules for Molecular Compounds

1. Name the first nonmetal using the full element name.

2. Name the second nonmetal with an -ide suffix and use prefixes to indicate quantities.

Page 8: Acids

General Features of Acids

• Produce H+ in solution and are characterized by sharp taste.

• Binary Acids: Contain H+ and a nonmetal anion.

• Oxyacids: Contain H+ and polyatomic oxyanions.

Naming Acids

• Binary Acids: Prefix “hydro-” + anion name with -ic + “acid”.

• Oxyacids: Names change based on the ending of the polyatomic ion (-ate to -ic, -ite to -ous).

Page 9: Organic Compounds

Alkanes

• Only consist of carbon (C) and hydrogen (H).

• Formula: CnH(2n+2).

• Naming follows the format of prefix + -ane.

Bonding Theories

• Fundamental to understanding chemical bonding, determining stability, shapes, and properties of compounds.

Page 10: Lewis Bonding Theory

Valence Electrons

• Key in chemical bonding, primarily concerned with the transfer or sharing of electrons.

Drawing Lewis Structures

• Represent valence electrons and predict molecular properties like stability and shape.

Page 11: Energetics of Ionic Bond Formation

Formation Processes

• Involve ionization energy and electron affinity; generally an exothermic process.

• Crystal Lattice: Arrangement of ions providing extra stability and energy release upon formation.

Page 12: Trends in Lattice Energy

Factors Influencing Lattice Energy

• Ion size and charge play crucial roles in the strength of ionic bonds; larger charge indicates stronger attraction and lattice energy.

Page 13-20: Covalent Bonding

Characteristics of Covalent Bonds

• Form through sharing of electrons and demonstrate directional properties, resulting in distinct molecular shapes.

Bond Types and Stability

• Different bonds (single, double, triple) have varying strengths and lengths, with trends observed across the periodic table and between similar atoms.

• Molecular compounds typically exhibit lower melting points compared to ionic compounds.

Resonance and Formal Charge

• Understanding resonance in structures allows predictions of molecular behavior and stability, guiding the conception of Lewis structures and their modifications (e.g., radicals, incomplete octets).

Bond Energies and Lengths

• Bond energies correlate with bond types and strength. Bond lengths decrease with greater shared electrons.

Properties of Metals

• Metal bonding characterized by electron delocalization provides conductive, malleable properties.

Summary of Key Concepts

• Chemical Bonds: Atoms bond via ionic or covalent means.

• Naming Conventions: Systematic approaches to naming ionic and molecular compounds.

• Bonding Theories: Lewis theory and resonance concepts are crucial for predicting molecular behavior and stability.