Long Form of Periodic Table — Core Principles
Core Principles
The long form of the periodic table arranges elements based on their increasing atomic number, which is the number of protons in an atom. This arrangement follows the Modern Periodic Law, stating that properties are periodic functions of atomic number.
Elements are organized into 7 horizontal rows called periods, corresponding to the principal quantum number of the outermost electron shell, and 18 vertical columns called groups, where elements share similar chemical properties due to identical valence electron configurations.
The table is further divided into four blocks: s-block (Groups 1-2, alkali and alkaline earth metals), p-block (Groups 13-18, non-metals, metalloids, noble gases), d-block (Groups 3-12, transition metals), and f-block (lanthanides and actinides, inner transition metals, placed separately).
For elements with atomic numbers greater than 100, a systematic IUPAC nomenclature is used, combining numerical roots for digits with the suffix '-ium'. This structure is fundamental for predicting chemical behavior and understanding periodic trends.
Often confused with
Side-by-side differences the NEET paper likes to test.
| Aspect | Long Form of Periodic Table | Mendeleev's Periodic Table |
|---|---|---|
| Basis of Classification | Atomic Number (Z) | Atomic Mass |
| Periodic Law | Properties are periodic functions of atomic numbers. | Properties are periodic functions of atomic masses. |
| Position of Isotopes | Same position (same Z). | Different positions (different atomic masses), creating ambiguity. |
| Anomalous Pairs | No anomalous pairs (e.g., Ar-K, Te-I are correctly placed). | Anomalous pairs existed (e.g., Ar (39.9) before K (39.1), Te (127.6) before I (126.9)). |
| Position of Hydrogen | Still debated, but often placed in Group 1. | No fixed position, placed with alkali metals or halogens. |
| Position of Lanthanides/Actinides | Separate f-block below the main table. | No specific place for these elements. |
| Number of Groups | 18 groups (IUPAC system). | 8 groups (including subgroups). |
| Predictive Power | Predicts properties based on electronic configuration and position. | Predicted properties of undiscovered elements based on gaps. |
The shift from Mendeleev's atomic mass-based classification to the modern atomic number-based long form resolved several inconsistencies and provided a more fundamental understanding of elemental properties.
The modern table accurately places isotopes, eliminates anomalous pairs, and systematically incorporates f-block elements. It reflects the electronic configuration of elements, which is the true determinant of their chemical behavior, making it a more robust and predictive tool for chemists and students alike, especially for understanding periodic trends.
Why it is tested: NEET relevance: Understanding this distinction is crucial for conceptual clarity in periodic classification. Questions often test the fundamental basis of the modern periodic table and its advantages over earlier models, particularly regarding the role of atomic number and electronic configuration in determining properties.