Chapter 3. The Periodic Table and Periodicity
Elements are arranged by increasing atomic number. Columns are groups (similar valence electron configurations); rows are periods [1][8].
Major regions: alkali metals (Group 1), alkaline earth metals (Group 2), transition metals (Groups 3–12), halogens (Group 17), noble gases (Group 18), plus the lanthanides and actinides.
Periodic trends
| Property | Across a period | Down a group |
| Atomic radius | Decreases | Increases |
| Ionization energy | Increases | Decreases |
| Electronegativity | Increases | Decreases |
| Metallic character | Decreases | Increases |
The trends arise from effective nuclear charge increasing across a period and from added electron shells down a group [2]. Noble gases are the usual exception to electronegativity trends since they rarely form compounds.
Review: Which is larger, Na or Mg? Cl or Cl⁻? Explain.