Question

What do the following abbreviations stand for?
A.O
B.2O
C.${O_2}$
D.$3{O_2}$

Answer 1

Michael Sendivogius isolated oxygen before 1604, although it is widely assumed that Carl Wilhelm Scheele in Uppsala in 1773 or earlier and Joseph Priestley in Wiltshire in 1774 independently identified the element. Priestley is frequently accorded first priority because his work was the first to be published. Priestley, on the other hand, referred to oxygen as "dephlogisticated air" and did not consider it a chemical element. Antoine Lavoisier, who first discovered oxygen as a chemical element and properly described its role in combustion, created the word oxygen in 1777.

Complete answer:
Oxygen is a chemical element with the atomic number 8 and the symbol O. It is a highly reactive nonmetal and an oxidising agent that quickly produces oxides with most elements and other compounds. It belongs to the chalcogen group in the periodic table. By mass, oxygen is the third most plentiful element in the universe, behind hydrogen and helium. Two atoms of the element bond together at normal temperature and pressure to create dioxygen, a colourless and odourless diatomic gas with the formula${O_2}$. Diatomic oxygen gas makes up 20.95 percent of the Earth's atmosphere, however this has fluctuated significantly throughout time. Oxygen, in the form of oxides, makes up about half of the Earth's crust.​
Hence the answer is
(i) O - one atom of oxygen
(ii) 2O - two separate atoms of oxygen
(iii) ${O_2}$- one molecule of oxygen
(iv)$3{O_2}$ - three molecules of oxygen

Note:
Atoms are solitary, non-magnetic particles. Molecules are made up of two or more atoms that are linked together. A molecule is made up of atoms, which are the simple building units of matter.' Covalent (chemical) bonds link one or more atoms to form molecules. The smallest component unit of a chemical element is the atom. Molecules are a collection of two or more atoms linked together by chemical bonds.