The chapter “Classification of Elements and Periodicity in Properties” is one of the foundational topics in Class 11 Chemistry and holds great importance for NEET 2025 aspirants. Understanding how elements are classified and how their properties change periodically helps students predict the behavior of atoms, ions, and compounds — a skill that’s essential for mastering inorganic chemistry.
Introduction
Chemistry is often described as the study of matter and its transformations. To study the vast number of elements systematically, scientists developed methods to classify elements based on their atomic structure and chemical behavior.
The idea of periodic classification evolved over time — from early attempts like Dobereiner’s Triads and Newlands’ Law of Octaves, to the modern Periodic Law proposed by Dmitri Mendeleev and refined by Henry Moseley.
Historical Development of the Periodic Table
1. Dobereiner’s Triads (1829)
Johann Dobereiner grouped elements into triads based on similar chemical properties. The atomic mass of the middle element was approximately the average of the other two.
Example:
- Li (7), Na (23), K (39) → (7 + 39)/2 = 23
Although simple, this law failed when more elements were discovered.
2. Newlands’ Law of Octaves (1866)
John Newlands arranged elements in increasing atomic mass and found that every eighth element had similar properties, just like musical octaves.
However, this rule only worked up to calcium, failing for heavier elements.
3. Mendeleev’s Periodic Law (1869)
Mendeleev’s Periodic Law stated that “the physical and chemical properties of elements are a periodic function of their atomic masses.”
He arranged 63 known elements in a table and left gaps for undiscovered ones — successfully predicting elements like gallium and germanium.
Yet, placing some elements in order of increasing atomic mass caused irregularities (e.g., cobalt and nickel).
4. Modern Periodic Law (Moseley, 1913)
Henry Moseley discovered that atomic number (not mass) is the true basis of periodicity. The Modern Periodic Law states:
“The physical and chemical properties of elements are periodic functions of their atomic numbers.”
This led to the Modern Periodic Table, which we use today.
Structure of the Modern Periodic Table
- Total elements: 118
- Periods (horizontal rows): 7
- Groups (vertical columns): 18
- Division: s-block, p-block, d-block, and f-block elements
Classification:
- s-block: Groups 1 & 2 (alkali and alkaline earth metals)
- p-block: Groups 13–18 (includes metals, metalloids, and non-metals)
- d-block: Transition elements (Groups 3–12)
- f-block: Lanthanides and actinides (inner transition elements)
Periodic Properties of Elements
1. Atomic Radius
- Decreases across a period (due to increasing nuclear charge).
- Increases down a group (due to addition of shells).
2. Ionization Enthalpy
Energy required to remove an electron.
- Increases across a period (nucleus holds electrons tightly).
- Decreases down a group (shielding effect increases).
3. Electron Gain Enthalpy
Energy released when an atom gains an electron.
- Becomes more negative across a period.
- Less negative down a group due to larger atomic size.
4. Electronegativity
The tendency of an atom to attract electrons.
- Increases across a period, decreases down a group.
- Fluorine is the most electronegative element (4.0 on the Pauling scale).
5. Metallic and Non-Metallic Character
- Metallic character decreases across a period and increases down a group.
- Non-metallic character shows the opposite trend.
Significance of the Periodic Table in NEET
Understanding periodic trends helps predict:
- Chemical reactivity of elements
- Formation of ionic and covalent bonds
- Oxidation states and electron configurations
- Acidic and basic nature of oxides
NEET frequently includes questions on:
- Trends in ionization energy and atomic radius
- Identification of elements based on electronic configuration
- Comparative analysis of groups and periods
Thus, mastering periodicity builds a strong base for inorganic and physical chemistry.
NEET Preparation Tips for This Chapter
- Revise trends visually — use arrows and color codes for each property.
- Memorize group-wise patterns like alkali metals and halogens.
- Solve previous NEET questions to understand the application of concepts.
- Refer to reliable platforms like MBBS Advisor for NEET 2025 notes, topic-wise strategies, and counselling support.
- Practice mock tests regularly to strengthen conceptual clarity.
Conclusion
The Classification of Elements and Periodicity in Properties chapter is the backbone of inorganic chemistry. By understanding periodic trends, students can predict element behavior, bond formation, and chemical reactivity — all of which are crucial for NEET 2025. Consistent practice, smart visualization, and conceptual clarity will ensure success in this important topic.
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