Oxygen and its oxidation states

Valency and oxidation states of oxygen


Oxy­gen is one of the most im­por­tant chem­i­cal el­e­ments. Oxy­gen mol­e­cules are present in a huge num­ber of chem­i­cal com­pounds which peo­ple use in ev­ery­day life.

Gen­er­al char­ac­ter­is­tics of oxy­gen

Oxy­gen was of­fi­cial­ly dis­cov­ered as a chem­i­cal el­e­ment in 1774 by the British chemist Joseph Priest­ley. The sci­en­tist was con­clud­ing an ex­per­i­ment on the break­down of mer­cury ox­ide, which was in a her­met­i­cal­ly sealed con­tain­er, and ob­tained a gas that sup­port­ed com­bus­tion. How­ev­er, the dis­cov­ery went un­no­ticed, and Priest­ley be­lieved that he had ex­tract­ed a com­po­nent of the air, and not a new el­e­ment. Priest­ley shared the re­sults of the ex­per­i­ment with the French chemist An­toine Lavoisi­er, who re­al­ized the true na­ture of the sci­en­tif­ic dis­cov­ery. In 1775, Lavoisi­er es­tab­lished that the “com­po­nent of the air” was an in­de­pen­dent chem­i­cal el­e­ment, and called it in French oxygène, which means “form­ing acids” in Greek. Lavoisi­er orig­i­nal­ly be­lieved that oxy­gen was a com­po­nent of all acids. Lat­er this the­o­ry was re­ject­ed.


Oxy­gen is a col­or­less gas with­out smell or taste. Here you’ll find in­ter­est­ing ex­per­i­ments with gas­es. The chem­i­cal for­mu­la of the el­e­ment is О₂.

The mol­e­cule con­sists of 2 oxy­gen atoms con­nect­ed by a co­va­lent bond. This bond is quite sta­ble. In a nor­mal state, the den­si­ty of the el­e­ment is 1.429 kg/m3. 1 liter of oxy­gen weighs less than 1.5 grams. In pure form, this gas is heav­ier than air.

Liquid oxygen [Wikimedia]

At a tem­per­a­ture of be­low -183 °C, oxy­gen be­comes a liq­uid of a light blue col­or, and at -218.35 °C it moves into a crys­talline state. Oxy­gen crys­tals are dark blue.

There are mol­e­cules that con­sist of three atoms, О₃, an al­lotrop­ic mod­i­fi­ca­tion known as ozone.

Va­len­cy and ox­i­da­tion states of oxy­gen

The va­len­cy of a chem­i­cal el­e­ment is its abil­i­ty to form a cer­tain num­ber of bonds with atoms of oth­er el­e­ments. The va­len­cy of an oxy­gen atom is (2). For ex­am­ple, the wa­ter mol­e­cule H₂O forms as a re­sult of the co­va­lent bond aris­ing be­tween 1 atom of oxy­gen and 2 atoms of hy­dro­gen.


Sub­stances ob­tained by the in­ter­ac­tion of a chem­i­cal el­e­ment with oxy­gen are called ox­ides. The ox­i­da­tion state of oxy­gen in ox­ides is equal to (-2).

Ox­i­da­tion states of oxy­gen

The ox­i­da­tion state is equal to the charge in the atom in the com­plex sub­stance, on the con­di­tion that all com­mon elec­tron pairs are dis­placed to atoms of more elec­troneg­a­tive el­e­ments, i.e. the ox­i­da­tion state is cal­cu­lat­ed based on the as­sump­tion that all co­va­lent bonds have be­come ion­ic. This as­sump­tion is ar­bi­trary, as the ox­i­da­tion state is a for­mal val­ue which does not re­flect the real ra­tios of the charges of the atoms in the mol­e­cule. This con­cept is used in cer­tain log­i­cal con­structs (cal­cu­la­tion of co­ef­fi­cients in equa­tions, clas­si­fi­ca­tion of com­pounds, etc.).

There are sev­en ox­i­da­tion states of oxy­gen, which de­pend on the el­e­ment that the sub­stance in­ter­acts with:

  • in ox­ides – (-2);
  • in per­ox­ides – (-1);
  • in su­per­ox­ides – (-1/2);
  • in in­or­gan­ic ozonides (true for ozone) – (-1/3);
  • in salts of a cation of oxy­gen– (+1/2);
  • in oxy­gen monoflu­o­ride – (+1);
  • in oxy­gen di­flu­o­ride – (+2).