Overview
This lecture explains the organization, history, and major periodic trends of the periodic table, highlighting the reasons for elemental behavior and key chemical properties.
Structure and History of the Periodic Table
- The periodic table organizes elements by increasing atomic number into rows (periods) and columns (groups).
- Dmitri Mendeleev created the first widely-accepted table by grouping elements with similar properties and predicting undiscovered elements.
- Elements in the same group have similar chemical behavior due to having the same number of valence (outer shell) electrons.
Valence Electrons and Element Groups
- Group 1 elements have one electron in their outermost shell (one valence electron).
- Group 2 elements have two valence electrons.
- The number of valence electrons determines many of an element's chemical properties.
Periodic Trends
- Atomic Radius: Increases down a group (more shells), decreases across a period (greater nuclear charge pulls electrons closer).
- Ionic Radius: Increases when gaining electrons (more electron-electron repulsion), decreases when losing electrons; ions with the same electron configuration get smaller as atomic number increases.
- Ionization Energy: Energy needed to remove an outer electron; decreases down a group, increases across a period.
- Successive Ionization Energies: Each successive electron removed requires more energy; a large jump occurs after removing all valence electrons.
- Exceptions in Ionization Energy: Explained by orbital symmetry; elements with half-filled or fully-filled subshells are exceptionally stable.
- Electron Affinity: Energy change when an atom gains an electron; highest for elements needing one more for a full shell (e.g., fluorine), lowest for those inclined to lose electrons.
- Electronegativity: The ability of an atom to attract shared electrons; increases up and to the right across the table, highest for fluorine (excluding noble gases).
Key Terms & Definitions
- Group — A column of elements with similar chemical properties and the same number of valence electrons.
- Period — A row of elements arranged by increasing atomic number.
- Valence Electrons — Electrons in the outermost shell of an atom.
- Atomic Radius — The size of an atom's electron cloud.
- Ionization Energy — Energy required to remove an electron from an atom.
- Electron Affinity — Energy change when an atom gains an electron.
- Electronegativity — An atom's ability to attract electrons in a bond.
Action Items / Next Steps
- Review trends for atomic radius, ionization energy, electron affinity, and electronegativity on the periodic table.
- Prepare for the next lesson on chemical bonds.