Class 11 Chemistry Chapter 3 Classification of Elements and Periodicity in Properties – Revision Notes

The periodic table’s history runs from Doebereiner’s triads through Newlands and Mendeleev to Moseley’s modern law based on atomic number, which this chapter traces before describing the table’s 18 groups and s, p, d, and f blocks. It then covers how atomic radius, ionization enthalpy, and electronegativity trend across periods and down groups.

Last Updated: September 23, 2026

History

• Doebereiner → Newlands → Mendeleev → Modern periodic law (Moseley, atomic number).

Structure

• 18 groups, 7 periods; s, p, d, f blocks.

Periodic Trends

• Atomic radius: decreases across period, increases down group.
• Ionization enthalpy: increases across period, decreases down group.
• Electronegativity: increases across period, decreases down group.

One-Line Summary

The modern periodic table organizes elements by atomic number, revealing periodic trends in atomic radius, ionization enthalpy, electron gain enthalpy, and electronegativity across periods and groups.

Frequently Asked Questions

What is the modern periodic law, and how does it differ from the original law given by Mendeleev?
The modern periodic law states that the properties of elements are a periodic function of their atomic number, correcting the earlier law given by Mendeleev which was based on atomic mass and led to a few inconsistencies in element ordering.

Why does atomic radius generally decrease across a period but increase down a group?
Across a period, nuclear charge increases while electrons are added to the same shell, pulling electrons closer, whereas down a group, a new shell is added each time, increasing the distance between the nucleus and outer electrons.

Written by Satish

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