Kiseonik / Oxygen (O)

Kiseonik je hemijski element koji se označava simbolom O i ima atomski broj 8. Pripada halkogenim nemetalima. On je najčešći i najrasprostranjeniji element u Zemljinoj kori sa udjelom od oko 49%.  Takođe čini i 21% Zemljine atmosfere. U elementarnom obliku kiseonik se pretežno javlja kao O2 – molekularni kiseonik ili dikiseonik. On je bezbojan gas bez mirisa i ukusa. Neophodan je za sagorijevanje i koroziju. Potreban je za život gotovo svih živih bića na Zemlji. Stvaraju ga biljke u procesu fotosinteze, ali ga i same troše za disanje, mada u manjoj mjeri odnosu na količinu koju proizvode fotosintezom.  U visokim koncentracijama kiseonik je za većinu živih bića otrovan. Metastabilni, vrlo reaktivni alotropni oblik kiseonika sa tri atoma kiseonika O3 naziva se ozon. 

Švedsko-njemački hemičar Karl Vilhelm Šile 1774. i engleski hemičar Džozef Pristli 1771, nezavisno jedan od drugog, otkrili su i izolovali kiseonik u sklopu proučavanja procesa sagorijevanja.

Kisik se veže sa mnogim elementima, stvarajući okside i druga jedinjenja. 

Oxygen is a chemical element denoted by the symbol O and has the atomic number 8. It belongs to chalcogenic non-metals. It is the most common and widespread element in the Earth's crust with a share of about 49%. It also makes up 21% of the Earth's atmosphere. In its elemental form, oxygen predominantly occurs as O2 - molecular oxygen or dioxygen. It is a colorless gas without smell and taste. It is necessary for combustion and corrosion. It is necessary for the life of almost all living beings on Earth. It is created by plants in the process of photosynthesis, but they also consume it for respiration, although to a lesser extent in relation to the amount they produce by photosynthesis. In high concentrations, oxygen is poisonous to most living things. A metastable, highly reactive allotropic form of oxygen with three oxygen atoms O3 is called ozone.

The Swedish-German chemist Carl Wilhelm Schiele in 1774 and the English chemist Joseph Priestley in 1771, independently of each other, discovered and isolated oxygen as part of the study of the combustion process.

Oxygen binds to many elements, forming oxides and other compounds.