Periodic Classification of Elements

Why Classification is Needed?

With the increasing number of discovered elements, it became essential to classify them based on similarities in their properties for easier study and understanding.

Early Attempts of Classification

• Dobereiner’s Triads: Elements were grouped in triads where the atomic mass of the middle element was the average of the other two. Example: Li, Na, K.
• Newlands’ Law of Octaves: Every eighth element had similar properties to the first, when arranged by increasing atomic mass.
• Mendeleev’s Periodic Table: Elements were arranged in increasing atomic mass. It left gaps for undiscovered elements and predicted their properties.

Modern Periodic Table

The modern periodic table is based on Moseley’s Periodic Law: “The physical and chemical properties of elements are the periodic function of their atomic numbers.”

• Elements are arranged in increasing order of atomic number.
• It consists of 7 periods (horizontal rows) and 18 groups (vertical columns).
• Groups 1, 2, and 13–18 are called representative elements.
• Groups 3–12 are transition elements.

Classification in the Periodic Table

• Metals: Found on the left and center of the table; good conductors, malleable, ductile.
• Non-metals: Found on the right side; poor conductors, brittle.
• Metalloids: Have properties of both metals and non-metals; lie along the zig-zag line (e.g., B, Si, As).

Trends in the Modern Periodic Table

• Atomic Size: Decreases across a period, increases down a group.
• Ionization Energy: Increases across a period, decreases down a group.
• Electronegativity: Increases across a period, decreases down a group.
• Metallic Character: Decreases across a period, increases down a group.

Merits of Modern Periodic Table

• Based on atomic number, which is more fundamental than atomic mass.
• Clearly separates metals, non-metals, and metalloids.
• Provides a systematic arrangement for prediction of properties.
• Accommodates new elements without disturbing the structure.

Limitations of Mendeleev’s Table

• Position of hydrogen was ambiguous.
• No fixed place for isotopes.
• Some elements with higher atomic masses were placed before lighter ones (e.g., Co and Ni).

Previous Year Questions

• SSC CGL 2019: Who gave the modern periodic law?
• Henry Moseley
• HSSC 2021: Which group contains noble gases?
• Group 18
• RRB NTPC 2020: What is the number of periods in the modern periodic table?
• 7

Conclusion

The periodic classification of elements simplifies the study of chemistry by grouping similar elements together and predicting their behavior. The modern periodic table is an invaluable tool in understanding chemical reactions, element properties, and trends.