PROOF THAT OXYGEN IS DIVALENT and that
HYDROGEN AND OXYGEN ARE DIATOMIC
Dalton's thoughts. John Dalton in the early 1800s thought that water had the formula HO. Since 1
gram of hydrogen reacted with 8 grams of oxygen, then the respective atomic weights were indicated to
be H = 1 and O = 8.
The divalency of oxygen, shown by Avogadro's hypothesis. It is known that two volumes of
hydrogen gas and one volume of oxygen gas combine to form water. From Avogadro's hypothesis
(equal volumes of gas contain equal numbers of molecules) it can be concluded that water must have
two atoms of hydrogen and one atom of oxygen (or conceivably some multiple). Hence, oxygen is
divalent (valence of 2) while hydrogen has a valence of 1:
2H + O º H2O
This would indicate that ½ gram of hydrogen atoms reacted with 8 grams of oxygen atoms. Since the
atomic weight of hydrogen was defined as H = 1, then the respective atomic weights of hydrogen and
oxygen would be H = 1 and O = 16 (the values accepted today).
The diatomic nature of hydrogen and oxygen, as shown by Avogadro's hypothesis. It is observed
that when two volumes of hydrogen gas and one volume of oxygen gas combine, two volumes of water
gas are formed. If hydrogen and oxygen were monatomic (as shown in the equation above), then only
one volume of water would be formed. The only explanation is that hydrogen and oxygen are diatomic:
2H2 + O2 º 2H2O
Proving the divalency of oxygen using organic chemistry. By the 1800s a new compound was
known as "ether," which was believed to be ethyl oxide, EtO (where Et = the ethyl group). Alcohol
(ethyl alcohol, or grain alcohol) itself was believed to be the hydrate of ethyl oxide (ether), EtO-HO. A
variety of alcohol and ether could be formed from the distillation of wood, which were respectively
known as "wood alcohol" (methyl alcohol) and "wood ether"; the formula of wood ether was believed
to be MeO (where Me = the methyl group) and of wood alcohol to be the hydrate MeO-HO. At this
time it was believed that the atomic weights of hydrogen and oxygen were H = 1 and O = 8.
In 1851 a British scientist Alexander W. Williamson showed that a "mixed" ether (ethyl methyl ether,
MeEtO) could be synthesized by a clever reaction:
MeO- K+ + EtI º MeOEt + KI
The existence of "mixed" ethers demanded that oxygen had two bonds. Hence, ethyl alcohol = EtOH,
methyl alcohol = MeOH, and ethyl ether was actually diethyl ether, EtOEt; and the atomic weights of
hydrogen and oxygen must be H = 1 and O = 16.