By 1829, Döbereiner had found other groups of three elements
(hence "triads") whose physical properties were similarly related. 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.
| Triad name | Elements and atomic masses | ||
|---|---|---|---|
|
Element 1 Mass |
Element 2 Mean of 1 and 3 Actual mass |
Element 3 Mass |
|
| Alkali-forming elements |
Lithium 6.94 |
Sodium 23.02 22.99 |
Potassium 39.10 |
|
Alkaline-earth-forming elements |
Calcium 40.1 |
Strontium 88.7 87.6 |
Barium 137.3 |
| Salt-forming elements |
Chlorine 35.470 |
Bromine 80.470 78.383 |
Iodine 126.470 |
| Acid-forming elements |
Sulfur 32.239 |
Selenium 80.741 79.263 |
Tellurium 129.243 |
From Wikipedia