Class 11 Chemistry: Classification of
Elements and Periodicity in Properties
1. Historical Background
The attempt to organize elements started early to make the study of chemistry
systematic.
• Dobereiner’s Triads: Elements arranged in groups of three (triads). The
atomic mass of the middle element was approximately the arithmetic mean
of the other two.
• Newlands’ Law of Octaves: Arranging elements in increasing order of
atomic mass, every eighth element resembled the first, similar to musical
notes.
• Mendeleev’s Periodic Table: Based on atomic mass. He left gaps for
undiscovered elements and accurately predicted their properties.
2. Modern Periodic Law
"Physical and chemical properties of elements are a periodic function of their
atomic number."
Developed by Henry Moseley, this replaced atomic mass with atomic number as the
basis for classification.
3. The Modern Periodic Table
Organized into 18 Groups (vertical) and 7 Periods (horizontal). Elements are
classified into s, p, d, and f blocks based on the last electron's orbital.
Elements and Periodicity in Properties
1. Historical Background
The attempt to organize elements started early to make the study of chemistry
systematic.
• Dobereiner’s Triads: Elements arranged in groups of three (triads). The
atomic mass of the middle element was approximately the arithmetic mean
of the other two.
• Newlands’ Law of Octaves: Arranging elements in increasing order of
atomic mass, every eighth element resembled the first, similar to musical
notes.
• Mendeleev’s Periodic Table: Based on atomic mass. He left gaps for
undiscovered elements and accurately predicted their properties.
2. Modern Periodic Law
"Physical and chemical properties of elements are a periodic function of their
atomic number."
Developed by Henry Moseley, this replaced atomic mass with atomic number as the
basis for classification.
3. The Modern Periodic Table
Organized into 18 Groups (vertical) and 7 Periods (horizontal). Elements are
classified into s, p, d, and f blocks based on the last electron's orbital.
