Periodic Table

The periodic table is an orderly chart of elements arranged in order of their increasing atomic numbers.  

Its classification and arrangement are among the most important achievements in chemistry, providing a systematic way to understand the properties of elements.  

Early Attempts at Classification  

- In the early 19th century, scientists such as De Chancourtois and Lothar Meyer attempted to classify elements.  

- Their work laid the foundation for later developments.  

Mendeleev’s Periodic Table (1869)  

- Russian scientist Dmitri Mendeleev constructed the first widely accepted periodic table.  

- He arranged elements in order of increasing atomic mass, leading to the Old Periodic Law:  

  “If elements are arranged in order of their increasing atomic weight, a periodic repetition of their chemical properties is observed.”  

- Mendeleev left gaps in his table, predicting the existence of undiscovered elements.  

- Later discoveries of scandium, gallium, and germanium confirmed his predictions.  

Features of Mendeleev’s Table:  

- Published the first organised table of elements.  

- Grouped elements with similar chemical properties.  

- Arranged elements by increasing mass.  

Mendeleev’s Anomalies  

Despite its success, Mendeleev’s table had limitations:  

- Lanthanides and actinides had no place.  

- Some elements of higher atomic weight were placed before those of lower weight.  

- Hydrogen could not be properly placed.  

- Noble gases were not predicted.  

Modern Periodic Law (1913)  

- In 1913, Henry Moseley showed that the atomic number is more fundamental than atomic weight.  

- Modern Periodic Law:  

  “The properties of elements are a periodic function of their atomic numbers.”  

- This corrected Mendeleev’s anomalies and gave rise to the modern periodic table.  

Structure of the Modern Periodic Table  

- Groups: Eight vertical columns numbered 0–7.  

- Elements in the same group have the same number of valence electrons and similar chemical properties.  

- Group Names:  

  - Group 1 → Alkali Metals  

  - Group 2 → Alkaline Earth Metals  

  - Group 3 → Boron Family  

  - Group 4 → Carbon Family  

  - Group 5 → Nitrogen Family  

  - Group 6 → Oxygen Family  

  - Group 7 → Halogens  

  - Group 8 → Noble Gases (Inert Gases)  

- Other Classifications:  

  - Central elements → Transition Metals  

  - Separate rows → Lanthanides and Actinides