Overview
This lecture discusses how electrons are arranged in atoms, why full outer shells make atoms stable, and how ions form when atoms gain or lose electrons.
Electron Arrangement in Atoms
- Atoms have electrons arranged in shells or energy levels around the nucleus.
- The first shell holds up to 2 electrons; the second and third shells each hold up to 8 electrons.
- Atoms are unstable if their outermost shell (valence shell) is not full.
Examples of Electron Configurations
- Sodium (atomic number 11): electron arrangement is 2,8,1; unstable as the outer shell has 1 electron.
- Argon (atomic number 18): electron arrangement is 2,8,8; full outer shell, making it stable.
- Calcium (atomic number 20): electron arrangement is 2,8,8,2; not stable since the outer shell has only 2 electrons.
- Fluorine (atomic number 9): electron arrangement is 2,7; needs 1 more electron for a full outer shell.
Stability and Reactivity
- Atoms with incomplete outer shells tend to react to achieve a full shell.
- Most atoms form molecules or compounds to fill their outer shells.
- Noble gases (Group 0) already have full outer shells and are unreactive.
Formation of Ions
- Calcium can lose 2 electrons to form a Ca²⁺ ion with a stable electron arrangement of 2,8,8.
- Fluorine gains 1 electron to form an F⁻ ion with a stable configuration of 2,8.
- The overall charge of ions reflects the electrons lost or gained.
Notation and Drawing Electron Structures
- Electron arrangements can be written as numbers separated by commas (e.g., 2,8,8).
- Electrons can be represented by dots or crosses in diagrams.
- Ions are drawn with square brackets and the charge in the top right corner.
Key Terms & Definitions
- Electron Shell — Energy level where electrons are found around an atom's nucleus.
- Valence Shell — The outermost electron shell of an atom.
- Ion — Atom that has gained or lost electrons, acquiring a charge.
- Noble Gases — Group 0 elements with full outer electron shells and low reactivity.
Action Items / Next Steps
- Practice drawing electron arrangements for the first 20 elements.
- Learn how to represent ions in diagrams.
- Review how electron configuration relates to atomic stability and reactivity.