Question

Give the scientific reason for the following:
In the atmosphere, oxygen is present as ${O_2}$ but not as $O$.

Answer 1

The molecular oxygen i.e. ${O_2}$ is the most encountered and stable ${3^{rd}}$ form of the oxygen which forms about $21\%$ of the Earth’s atmosphere. The oxygen that we breathe is ${O_2}$. While singular atomic oxygen exists, it would kill you if you breathe it.

Complete answer:
When we see the electronic configuration of oxygen, it has $6$ electrons in its outermost shell. Its electronic configuration is $1{s^2},2{s^2},2{p^6}$. According to Bohr’s model, in general, atoms are most stable, least reactive, when their outermost electron shell is full. Thus, atom like to have a full outer shell in order to attain stability. This shell capacity is given by Bohr’s rule, according to which the first four shells full capacity is given as:
${1^{st}}$ shell= $2$ electrons.
${2^{nd}}$ shell= $8$ electrons
${3^{rd}}$ shell= $8$ electrons
For oxygen to have a full outer shell it must have $8$ electrons in it. But it only has $6$ electrons in its valence shell. Each oxygen atom is actively seeking to get more electrons to complete its valence shell. If no other atom except oxygen atoms are available, each oxygen atom will try to get extra valence electrons from another oxygen atom. So if one oxygen atom merges with another, they share electrons, giving both a full outer shell and ultimately being virtually unreactive.

Note:
Oxygen itself does not have enough electrons to be stable so it shares electrons with another oxygen atom. This creates diatomic oxygen molecules, each one consisting of two oxygen atoms sharing electrons with one another. Since this is the most common form of oxygen, its formula is written as ${O_2}$ not as $O$.