The periodic table is a tabular method of displaying the elements in such a way, that the elements having similar properties occur in the same vertical column or group.
Dobereiner’s Triads
This classification is based on the atomic mass.
According to this, when elements are arranged in order of increasing atomic masses, groups of three elements, having similar properties are obtained.
The atomic mass of middle element of the triad being nearly equal to the average of the atomic masses of the other two elements.
For Example Li (6.9), Na (23), K (39).
Limitation
It fails to arrange all the known elements in the form of triads, even having similar properties.
Newland’s Law of Octaves
According to this ‘when elements are placed in order of increasing atomic masses, the physical and chemical properties of every 8th element are a repetition of the properties of the first element.’
Limitations
- Law of octaves was applicable only upto calcium (only for lighter elements).
- Newland adjusted two elements in the same slot (e.g.
Co and Ni), having different properties.
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For example; Co and Ni with Fluorine, Chlorine, Bromine and Iodine. - According to Newland, only 56 elements existed in nature and no more elements would be discovered in future.
Mendeleev’s Periodic Law
According to this “The physical and chemical properties of the elements are the periodic function of their atomic masses.”
The Modern Periodic Table
In 1913, Henry Moseley showed that the atomic number of an element is a more fundamental property than its atomic mass.
The topics discussed in this chapter are as follows:
- Early models of periodic table
- Trends in early models of periodic table
- Modern periodic table
- Periods and groups in modern periodic table
- Trends in modern periodic table
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NCERT Solutions for Chapter 5: Periodic Classification of Elements
Also Check
Chapter 1: Chemical Reactions and Equations
Chapter 2: Acids, Bases and Salts
Chapter 3: Metals and Non-Metals
Chapter 4: Carbon and its compounds
Chapter 6: Life Processes
Chapter 7: Control and Coordination
Chapter 8: How do organisms reproduce?
Chapter 9: Heredity and Evolution
Chapter 10: Light - Reflection and Refraction
Chapter 11: The Human Eye and the Colourful World
Chapter 12: Electricity
Chapter 13: Magnetic Effects of Electric Current
Chapter 14: Sources of Energy
Chapter 15: Our Environment
Chapter 16: Sustainable Management of Natural Resources
Check out Frequently Asked Questions (FAQs) for Chapter 5: Periodic Classification of Elements
What is achievement of Dobereiner’s law of triads?
State two reasons for rejecting law of octaves.
(i) The law did not extend properly beyond the element calcium.
(ii) The law did not provide any specific place for hydrogen.
