Döbereiner's Triads
Trial Elements and

Atomic Weights

Elemental Form Principal CompoundsSpecial Properties
(I) Cl, Br, I:

35.5, 80, 127

 Colored diatomic molecules:

Cl2 (yellow)

Br2 (brown)

I2 (violet)

Form simple salts containing -1 ions: Cl-,

Br-, I-. Form oxyanions containing one to four oxygen atoms: ClO4-, ClO3-, BrO3-, IO3-, ClO-, IO4-. Hydrogen compounds are molecular HCl, HBr, HI

Free elements react vigorously with electron donors to form negative ions Cl-, Br-, I-.

2Na + Cl2 Æ 2Na+ + 2 Cl-

I2 + S2- Æ 2I- + S

Salts (like NaCl) are very soluble in water. Halide salts of Li, Na, and K give neutral solutions. Hydrogen compounds are strong acids and ionize completely in water.

HBr + H2O Æ H3O+ + Br-

(II) S, Se, Te

32, 79, 127.6

 Colored crystalline nonmetals

(Te somewhat metallic):

S8 (yellow)

Se8 (red)

Form simple salts with -2 ions: S2-, Se2-, Te2-, and very smelly compounds with hydrogen: H2S, H2Se, H2Te. Form oxyanions with up to four oxygen atoms: SO32-,a SO42-, SeO42-. Form dioxides and trioxides SO2, SO3,a SeO2, TeO2, TeO3. Salts, except those with triads III and IV, are slightly soluble in water: CuS, ZnS, HgS. Soluble salts (Na2S) give basic solutions:

S2-+ H2O Æ HS- + OH-

Hydrogen compounds are weak acids.

(III) Ca, Sr, Ba

40, 88, 137

 Reactive metalsForm salts containing +2 ions: Ca2+, Sr2+, Br2+ in BaSO4, CaCO3, SrCl2, and so on. Salts give bright colors in flame: Ca (orange), Sr (red), Ba (green). Sulfates and carbonates are insoluble. Metals replace hydrogen slowly from water.
(IV) Li, Na, K

7, 23, 39

 Very reactive metalsForm salts containing +1 ions: Li+, Na+, K+ in Li2CO3, NaCl, K3PO4, and so on. Almost all salts are soluble: metals and salts give brightly colored flames: Li (red), Na (yellow), K (purple). Metals react violently with water to produce hydrogen and soluble ionic hydroxides:

2Na + 2H2O Æ
H2 + 2Na+ + 2OH-

a Note the importance of charge: SO32- is very different from SO3 (no charge).