Johann Wolfgang Dobereiner

As the number of elements increased, chemists inevitably began to find patterns in their properties. In 1829 Johann Wolfgang Dobereiner discovered the existence of families of elements with similar chemical properties. Because there always seemed to be three elements in these families, he called them triads. Each of the vertical columns in Table 7.1 represents one of these triads.

A.N.: Insert Table 7.1 in the margin here

TABLE 7.1

Dobereiner's Triads

Li Ca S Cl Mn

Na Sr Se Br Cr

K Ba Te I Fe

Dobereiner's triads grouped elements with similar chemical properties. Consider lithium, sodium, and potassium, for example.

1. These elements all react with water at room temperature.

2. They react with chlorine to form compounds with similar formulas: LiCl, NaCl, and KCl.

3. They combine with hydrogen to form compounds with similar formulas: LiH, NaH, and KH.

4. They form hydroxides with similar formulas: LiOH, NaOH, and KOH.

Dobereiner also found patterns in the physical properties of the elements in a triad. He noted, for example, that the atomic weight of the middle element in each triad is about equal to the average of the atomic weights of the first and third elements. The atomic weight of sodium (22.99 g/mol), for example, is remarkably close to the average of the atomic weights of lithium (6.94 g/mol) and potassium (39.10 g/mol). Dobereiner also found that the density of the middle element in most triads is roughly equal to the average of the densities of the other elements. The density of strontium (2.60 g/cm³), for example, is close to the average of the densities of calcium (1.55 g/cm³) and barium (3.51 g/cm³).