Priestley and Lavoisier had made this gas first by heating mercury oxide (HgO) and subsequently by heating many other oxides. In fact, Antoine Lavoisier used these reactions to establish conservation of matter during chemical reactions.Oxygen is the second most abundant element in the atmosphere after nitrogen. It is approximately 21% by volume. It is a colorless and odorless gas at room temperature. Some of its important physical properties are listed below  in Table 1.Its symbol is O and as the pure element, it occurs in nature as diatomic molecule O2 and triatomic molecule O3 or ozone (in the upper crust of the atmosphere). Its atomic number is 8. It has three stable isotopes namely 16O, 17O, and 18O. Besides, it has ten radioactive isotopes – 12O, 13O, 14O, 15O, 19O, 20O, 21O, 22O, 23O, and 24O . It has 8 protons in its nucleus and therefore 8 electrons in different orbits around its nucleus. Therefore its electronic configuration is 1s22s22p4. Inbox notation, the electronic configuration will be the following.It can be seen that oxygen needs two electrons to complete its octave and hence to attain a stable electronic configuration. This is the reason why it is highly electronegative. It is the second most electronegative element only next to fluorine and hence always remains in negative oxidation states except in its pure forms O2 and O3 (oxidation number is zero), in OF2 (oxidation number is +2) and in O2F2 (oxidation number is +1). It is bivalent because it needs two electrons to complete its octet. It is highly reactive and forms readily oxides with most of the elements barring noble metals like Au (gold), Pt (Platinum), etc. and inert gases. The reaction with oxygen is an exothermic process and therefore self-sustaining. however, to initiate the reaction some initial heating is required to dissociate the double bond in O2.This compound is liquid at room temperature. Its freezing point is 0 oC and the boiling point is 100 oC. This is found in abundance on the planet Earth, mostly in the form of liquid.