Overview
This lecture introduces covalent bonds and structural formulas, explaining how atoms share electrons to achieve stability and describes how to represent molecules using structural formulas.
Review of Ionic Bonding and the Octet Rule
- Atoms are stable when their outer orbitals are full (octet rule: 2 electrons in the first shell, 8 in the second and third).
- Ionic bonding involves transfer of electrons between atoms, creating oppositely charged ions that attract each other.
Covalent Bonds
- Covalent bonds form when atoms share one or more pairs of electrons.
- Hydrogen gas (H₂) is formed when two hydrogen atoms share electrons, resulting in each having a full outer shell.
- Covalent bonds hold together the molecules that make up living things.
Examples of Covalently Bonded Molecules
- Methane (CH₄): One carbon shares electrons with four hydrogens, achieving full outer shells for all.
- Ethane (C₂H₆): Each carbon bonds with three hydrogens and the other carbon; all atoms reach stable configurations.
- Ethylene (C₂H₄): Carbonds connect via a double bond (two shared pairs of electrons), with each also bonding to hydrogens.
- Oxygen gas (O₂): Two oxygens share two pairs of electrons (double bond), resulting in stable, full outer shells.
- Acetylene (C₂H₂): Two carbons form a triple bond (three shared pairs), each also bonded to one hydrogen.
Structural Formulas
- Structural formulas represent atoms by their chemical symbols.
- Each shared pair of electrons (covalent bond) is shown with a dash (–); double or triple bonds use double or triple dashes.
- These formulas simplify and clarify molecular structures for biological study.
Key Terms & Definitions
- Octet Rule — Atoms are most stable when their outer electron shells are full.
- Ionic Bond — Chemical bond formed through the transfer of electrons resulting in attraction between oppositely charged ions.
- Covalent Bond — Chemical bond formed when two atoms share one or more pairs of electrons.
- Structural Formula — Diagram showing the arrangement of atoms and the bonds between them using symbols and dashes.
Action Items / Next Steps
- Practice drawing structural formulas for molecules such as methane, ethane, and ethylene.
- Complete flashcards, quizzes, and activities on the instructor's website.
- Consider and comment on whether a poem is more like a covalent or ionic bond.