Notes 5. Periodic Classification of Elements - Newlands Law of Octaves | Class 10 Science - Toppers Study

Notes 5. Periodic Classification of Elements - Newlands Law of Octaves | Class 10 Science - Toppers Study

Chapter 5. Periodic Classification of Elements

What is periodic classification of elements? 

The periodic classification of elements is a tabular arrangement of the chemical elements, ordered by their atomic number (number of protons in the nucleus), electron configurations, and recurring chemical properties. The periodic table is a chart that shows how elements are related to one another.

• The classification of elements mean keeping them systemically into various groups on the basis of their properties. 
• First time, elements are classified into metals and non-metals.  

Dobereiner’s Triads: 

What are Dobereiner's Traids?

Dobereiner made a triads of three elements to arrange the elements with similar properties into groups. the three elements in a triad were written in the order of increasing atomic masses; the atomic mass of the middle element was roughly the average of the atomic masses of the other two elements. 

Example of Dobereiner's Triads

For example: lithium (Li), sodium (Na) and potassium (K). 

Dobereiner discovered three such triads are as follow:

1. lithium (Li), sodium (Na) and potassium (K). 
2. Calcium (Ca) Strontium (Sr) and Barium (Ba).
3. Chlorine (Cl) Bromine (Br) and Iodine (I).

These elements are arranged downwards in order of increasing atomic masses, They will show this law.

According to this law, when three elements in a triad were written in order of increasing atomic masses, the atomic mass of  the middle element was roughly the average of the atomic masses of the other two elements. Such a triad is known as Dobereiner's triads. 

Limitation/Backwards of Dobereiner's Law:

1. Döbereiner could identify only three triads from the elements known at that time.
2. Hence, this system of classification into triads was not found to be useful.

Newland’s Law of Octaves: 

Every eighth element had properties similar to that of the first. This law is known as Newland’s Law of Octaves.

Limitation of Newland’s Law of octaves:

(i) It was found that the law of octaves was applicable only up to calcium.

(ii) No more elements would be discovered in the future instead of 56 known elements, it was assumed by Newlands. But did not match the properties of new discovered elements to this law.

(iii) Newland’s Law of Octaves worked well with lighter elements only.