In the first row you will find that atomic weight of sodium (Na) is equal to the average of relative atomic weight of Li and K.
• Can you establish the same relationship with the set of elements given in the remaining rows?
• Find average atomic weights of the first and third elements in each row and compare it with the atomic weight of the middle element.
• What do you observe?
Döbereiner’s attempts gave a clue that atomic weights could be correlated with properties of elements. It made chemists look at elements in terms of groups of elements with similar chemical and physical properties. This eventually led to rigorous classification of elements and the modern periodic table of elements.
i. All the known elements at that time could not be arranged in the form of triads.
ii. The law failed for very low mass or for very high mass elements.
In case of F, Cl, Br, the atomic weight of Cl is not an arithmetic mean of atomic weights of F and Br
iii. As the techniques improved for measuring atomic masses accurately, the law was unable to remain strictly valid.
| Think and discuss |
| • What relation about elements did Döbereiner want to establish? • The densities of calcium (Ca) and barium (Ba) are 1.55 and 3.51 gcm-3 respectively. Based on Dobereiner’s law of triads can you give the approximate density of strontium (Sr)? |
John Newlands was a British chemist. Newlands (1865) found that when elements were arranged in the ascending order of their atomic weights they appeared to fall into seven groups. Each group contained elements with similar chemical properties. Based on these observations, Newlands proposed the law of octaves.
The law of octaves states that when elements are arranged in the ascending order of their atomic weights they fall into a pattern in which their properties repeat at regular intervals. Every eighth