Overview
This lecture covers the origins, structure, and organization of the periodic table, key groups, periodic properties, and neutron-proton ratio, preparing students for exam-focused understanding.
Origins & Evolution of the Periodic Table
- Early attempts by Dobereiner, Newlands, and Mendeleev arranged elements by increasing atomic mass.
- Mosley improved the table by arranging elements by increasing atomic number (Z), not mass (A).
- Modern periodic table has 7 horizontal periods (rows) and 18 vertical groups (columns).
Structure & Features of the Modern Periodic Table
- Elements in the same period (row) have the same number of electron shells.
- Elements in the same group (column) have the same valence shell configuration and show similar chemical properties.
- Group 1: Alkali metals (very reactive, 1 valence electron).
- Group 2: Alkaline earth metals (reactive metals).
- Groups 3–12: Transition metals (incomplete d-subshell).
- Groups 13–16: Post-transition elements (mix of metals, metalloids, non-metals).
- Group 17: Halogens (highly electronegative, 1 less than complete octet).
- Group 18: Noble gases/inert gases (stable, complete octet/duplet).
- Two bottom rows (lanthanides and actinides) are called inner transition elements.
Periodic Properties & Their Trends
Atomic Radius
- Distance from nucleus to outermost electron shell.
- Decreases across a period (left to right) due to increasing nuclear charge.
- Increases down a group due to adding electron shells.
Ionization Potential (IP)
- Energy required to remove an electron from an isolated gaseous atom.
- Increases across a period (nuclear charge increases, harder to remove electrons).
- Decreases down a group (atomic radius increases, electrons are farther from nucleus).
Electron Affinity (EA)
- Energy released when an electron is added to an atom to form an anion.
- Increases across a period (greater tendency to gain electrons).
- Decreases down a group (atomic radius increases, weaker pull on added electron).
- Exception: Chlorine has higher electron affinity than fluorine due to less repulsion.
Electronegativity
- Tendency of an atom to attract electrons in a bond.
- Increases across a period.
- Decreases down a group.
- Fluorine is the most electronegative, cesium is the least.
Metallic & Non-Metallic Character
- Metallic character: tendency to lose electrons (increases down a group, decreases across a period).
- Non-metallic character: tendency to gain electrons (decreases down a group, increases across a period).
- Cesium is most metallic, fluorine is most non-metallic.
Special Relationships & Classifications
- Bridge/Diagonal Relationship: Similar properties between diagonally placed elements (e.g., Li-Mg, Be-Al).
- Inner transition elements: Lanthanides (period 6) are rare earths; actinides (period 7) are mostly radioactive.
Neutron-Proton (N/P) Ratio
- Light elements: N/P ratio ≈ 1 (e.g., sodium).
- Heavy elements: N/P ratio ≈ 1.5 (e.g., uranium).
Key Terms & Definitions
- Atomic Number (Z) — Number of protons in the nucleus.
- Atomic Mass (A) — Number of protons plus neutrons.
- Period — Horizontal row in the periodic table.
- Group — Vertical column in the periodic table.
- Valence Shell — Outermost electron shell of an atom.
- Ionization Potential — Energy needed to remove an electron from an atom.
- Electron Affinity — Energy change when an atom gains an electron.
- Electronegativity — Tendency to attract bonded electrons.
- Metallic Character — Tendency to lose electrons.
- Non-Metallic Character — Tendency to gain electrons.
Action Items / Next Steps
- Memorize the first 20 elements and key group members (halogens, alkali metals, noble gases).
- Practice drawing and labeling the periodic table.
- Make summary tables for trends of periodic properties.
- Review definitions and exceptions, especially for electron affinity.