Chemistry
Periodic Law and the Periodic Table
The Periodic Law
Discovered by D. I. Mendeleev in 1869.
Modern formulation: the properties of elements and their compounds depend periodically on the charge of the atomic nucleus (Z).
The periodic table has 7 periods and 8 groups (A and B subgroups).
Changes in Properties
| Property | Across a period → | Down a group ↓ |
|---|---|---|
| Radius | decreases | Radius increases down a group |
| Metallic character | Metallic character decreases across a period | increases |
| Electronegativity | increases | decreases |
| Oxidizing ability | increases | decreases |
- Across a period from left to right, oxides change from basic to acidic.
- Down a group from top to bottom (group A): radius, metallic character and reducing ability increase, while electronegativity decreases.
- Ionization energy increases across a period and decreases down a group from top to bottom.
- Usually the highest oxidation state = the group number (for group A elements).
Important Elements
- The most electronegative is F, the most metallic is Cs (Fr).
- Amphoteric: Be, Al, Zn, Cr (III). Their oxides and hydroxides show both acidic and basic properties.
- IA: alkali metals. VIIA: halogens (F, Cl, Br, I). VIIIA: noble gases (He, Ne, Ar, Kr, Xe, Rn).
- Families: s-, p-, d- and f-elements. Lanthanides and actinides are f-elements.