Dobereiner’s Triads

Dobereiner arranged a group of three elements with similar properties in the order of increasing atomic masses and called it a triad. He showed that the atomic mass of the middle element is approximately the arithmetic mean of the other two. But, Dobereiner could identify only the following three triads from the elements known at that time.

Newlands’ Law of Octaves

Assumptions and Limitations:

1. The law was applicable for elements with atomic masses up to 40.

2. Properties of new elements discovered did not fit into the law of octaves.

3. In a few cases, Newlands placed two elements in the same slot to fit elements in the table.

4. He also grouped unlike elements under the same slot.

Chapter 5

Periodic Classification of Elements

Early classification of elements.

  • Classification means identifying similar species and grouping them together.
  • Lavoisier divided lements into two main types known as metals and non- metals.

Doberiner’s law of triads :- 

Doberiner tried to arrange the elements with similar properties. He showed that when the three elements in a triad were written in the order of increasing atomic masses ; the atomic masses ; the atomic mass of the middle element was roughly the average of the atomic masses of other two elements.

i.e atomic masses Li, Na and K are 7,23, and 39 respectively , tus the mean of 1st and 3rd elements is 23 and the atomic mass of middle element is 23.

Limitation :-  He could identify only a few such triads and so the law could not gain importance.

For example, Fe, Co, Ni, all the three elements have nearly equal atomic mass and thus does not follow this law.

Newland’s law of octaves :-

 He found that every eighth element had properties similar to that of the first. HP compared this to the octaves found in music . he called it ‘law of octaves’.

For Example. The properties of lithium and sodium were found to be the same sodium is the eighth  element after lithium.

 Limitation :-

(i) law of octaves was applicable only upto calcium as after calcium every eigth element did not possess properties similar to that of first.

(II)    According  to him , only 56 elements exist in nature and no more elements would be discovered in the future. But later on several new element  were discovered wose properties did not fit into law of octaves. 

(III)    In order to fit new elements into his table newland  adjust two elements in the same column, but put some unlike elements under the same column. 

(IV)    Thus, newland’s  classification was not accepted.