Overview
This lecture explains why atoms form chemical bonds, describes the major types of bonds (covalent and ionic), introduces key concepts like electronegativity, polarity, and bond length, and highlights physical properties related to bonding.
Why Atoms Form Bonds
- Atoms bond to achieve lower energy by balancing attractive and repulsive forces.
- Electrostatic forces cause electrons of one atom to be attracted to protons of another (opposites attract, like charges repel).
- Minimum energy is achieved at a specific distance, called the bond length.
Types of Chemical Bonds
- Covalent bonds form when atoms share electrons, usually between non-metals or metalloids.
- Ionic bonds form when electrons are transferred from a metal to a non-metal, creating oppositely charged ions.
- Bond types are a continuum, not always strictly defined.
Covalent Bonds and Polarity
- In non-polar covalent bonds, electrons are shared equally (identical or similar electronegativities).
- In polar covalent bonds, electrons are shared unequally; higher electronegativity pulls electrons closer, creating polarity (separation of charge).
- Electronegativity measures how strongly an atom attracts shared electrons.
Ionic Bonds and Coulomb’s Law
- Ionic bonds result from electron transfer and the attraction between positive and negative ions.
- Coulomb’s law calculates ionic bond energy:
Energy = (Q₁ × Q₂) / r × 2.31 × 10⁻¹⁹ J·nm, where Q = ion charge, r = distance in nm.
- Ionic bonds are extremely polar because charges are fully separated.
Properties of Ionic vs Covalent Compounds
- Ionic compounds are usually crystalline, soluble in water, and conduct electricity when dissolved.
- Covalent compounds tend to be softer solids, liquids, or gases, are often not soluble in water, and do not conduct electricity in solution.
- Physical property differences stem from differences in bond polarity.
Key Terms & Definitions
- Bond length — Distance between two nuclei at minimum energy in a bond.
- Covalent bond — Bond formed by sharing electrons between atoms.
- Electronegativity — Atom’s ability to attract shared electrons.
- Polarity — Separation of charge due to uneven electron distribution in a bond.
- Ionic bond — Bond formed by the transfer of electrons and attraction between oppositely charged ions.
- Coulomb’s law — Formula for calculating energy between two charged ions.
Action Items / Next Steps
- Review electronegativity values for elements.
- Practice using Coulomb’s law to calculate ionic bond energy.
- Prepare for upcoming lessons on molecular polarity.