Chapter 5. Periodic Classification of Elements

• 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: 

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. 

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

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. 

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.