Periodic Table Development and Trends

Dobereiner's Triads: Early classification system grouping elements in sets of three based on similar properties.
Modern Periodic Law: Elements are arranged by increasing atomic number, with periodic trends in properties due to electron configuration.
Limitations of Early Classifications:
- Incomplete: Not all elements fit into triads.
- Arbitrary Grouping: Lacked systematic basis for placement.
Resolution of Limitations:
- Development of atomic theory and discovery of atomic structure provided a more consistent framework for classification.

Atomic Radius:
- Across a Period: Decreases due to increased nuclear charge pulling electrons closer.
- Down a Group: Increases due to added electron shells which outweigh nuclear charge effect.
First Ionization Energy Across Period 3:
- Increases due to greater nuclear attraction as atomic radius decreases.
- Anomalies:
- Occur (e.g., between Mg and Al) due to sublevel electron shielding.
- Justified by examining electron configurations (e.g., Al has a full 3s subshell).
Electronegativity Trends:
- Across a Period: Increases due to higher nuclear charge attracting bonding electrons more strongly.
- Down a Group: Decreases due to increased electron shielding and distance from nucleus.

Increased electronegativity across a period leads to stronger ionic and covalent bonds, whereas decreased electronegativity down a group results in weaker bonding characteristics.

Alkali Metals:
- Group 1; highly reactive, low ionization energy, soft solids.
- Tend to lose one electron, forming +1 cations.
Halogens:
- Group 17; highly reactive nonmetals, high ionization energy, exist as diatomic molecules.
- Tend to gain one electron, forming -1 anions.
Relation to Position in Periodic Table:
- Alkali metals exhibit metallic properties, while halogens exhibit nonmetallic properties, reflecting their placement in contrasting groups.