Characteristics of doberbeiner's traids
Answers
Dobereiner’s law of triads states that, the atomic mass of the middle element of a triad is the arithmetic mean of the atomic masses of the other two elements. Example: In the triad of lithium, sodium and potassium. The atomic mass of lithium is 7 and the atomic mass of potassium is 39. The average of masses of lithium and potassium gives atomic mass of sodium 23. Drawbacks: All the known elements could not be arranged in the form of triads. This law did not hold good for elements with very low or very high atomic mass. Example: The arithmetic mean of the atomic masses of fluorine 19 and bromine 80, which comes to 49.5, varies significantly from the atomic mass of chlorine, which is 35.5.
Explanation:
In the history of the periodic table, Döbereiner's triads were an early attempt to sort the elements into some logical order by their physical properties. In 1817, a letter reported Johann Wolfgang Döbereiner's observations of the alkaline earths; namely, that strontium had properties that were intermediate to those of calcium and barium.[1] By 1829, Döbereiner had found other groups of three elements (hence "triads") whose physical properties were similarly related.[2] He also noted that some quantifiable properties of elements (e.g. atomic weight and density) in a triad followed a trend whereby the value of the middle element in the triad would be exactly or nearly predicted by taking the arithmetic mean of values for that property of the other two elements.
Predicted vs actual atomic mass of the central atom of each triadTriad name[2]Elements and atomic masses[2][3]Element 1
MassElement 2
Mean of 1 and 3
Actual massElement 3
MassAlkali-forming elementsLithium
6.94Sodium
23.02
22.99Potassium
39.10Alkaline-earth-forming elements
[atomic masses verification needed]Calcium
40.1Strontium
88.7
87.6Barium
137.3Salt-forming elementsChlorine
35.470Bromine
80.470
78.383Iodine
126.470Acid-forming elementsSulfur
32.239Selenium
80.741
79.263Tellurium
129.243-
[atomic masses verification needed]Iron
55.8Cobalt
57.3