Tlen

Tlen is the Polish word for the chemical element oxygen. In scientific contexts, tlen denotes the element with the symbol O and atomic number 8, a colorless, odorless gas that is diatomic (O2) at standard conditions. Oxygen makes up about 21% of Earth’s atmosphere and is the third most abundant element by mass in the universe. It is essential for most forms of life on Earth and plays a key role in combustion and oxidation reactions. Oxygen is highly reactive and forms oxides with many elements; it is the most plentiful element in the Earth's crust by mass.

In terms of occurrence and production, oxygen occurs freely in the atmosphere and dissolved in water. Industrially, it is produced mainly by fractionally distilling liquefied air or by other separation methods, and it is widely used in metal production, chemical synthesis, medical therapies, and environmental applications such as water treatment and waste processing. Its medical form is supplied in concentrated or medical-grade batches for oxygen therapy, and high-purity oxygen is used in various industrial processes.

The discovery and naming of oxygen are well documented in the history of chemistry. Priestley and Scheele independently identified the gas in the late 18th century, and the name oxygen was coined by Antoine Lavoisier, who helped establish its central role in chemical reactions. In Polish, the term tlen serves as the standard everyday and scientific word for this element.

Tlen also refers to a defunct Polish online messaging service and client known as Tlen.pl, which operated in the early 2000s as a local chat network and partner of the Onet.pl portal. The service offered real-time messaging and contact management before being discontinued as newer communication platforms emerged.