The periodic table is a chart that arranges all chemical elements in a systematic way. It helps predict the properties of elements and is a frequent source of questions in competitive exams.
Mendeleev's Periodic Table
Dmitri Mendeleev (1869) arranged elements by increasing atomic mass. He left gaps for undiscovered elements like eka-silicon (germanium). His law: properties of elements are a periodic function of their atomic masses.
Modern Periodic Law
Henry Moseley changed the basis to atomic number. The modern law states that properties of elements are a periodic function of their atomic numbers.
Structure of the Table
- Periods: 7 horizontal rows.
- Groups: 18 vertical columns.
- The modern table has 118 known elements.
- Elements in the same group have similar chemical properties.
Important Groups
- Group 1: Alkali metals (Na, K).
- Group 2: Alkaline earth metals (Ca, Mg).
- Group 17: Halogens (F, Cl).
- Group 18: Noble gases (He, Ne, Ar).
Trends in the Table
Across a period, atomic size decreases and metallic character decreases. Down a group, atomic size increases and metallic character increases. Hydrogen is the lightest element; fluorine is the most reactive non-metal.
Quick Revision Points
- Mendeleev arranged elements by atomic mass.
- Modern periodic table is based on atomic number (Moseley).
- Table has 7 periods and 18 groups.
- 118 elements are known.
- Group 18 elements are noble gases.
- Fluorine is the most reactive non-metal.
- Down a group, metallic character increases.