Answer:

The species that is not stable is the one which has an incomplete octet or an odd number of electrons. Such species are highly reactive and can form unstable compounds. In this context, the following species are stable or unstable:

1. Stable species: A stable species is one that has a complete octet or a stable configuration of electrons. Such species are usually unreactive and do not readily form compounds. Examples of stable species include noble gases, such as helium, neon, and argon.

2. Unstable species: An unstable species is one that has an incomplete octet or an odd number of electrons. Such species are highly reactive and can form unstable compounds. Examples of unstable species include free radicals, such as hydroxyl radicals (OH•) and methyl radicals (CH3•).

3. Exceptions: There are some exceptions to the octet rule, such as compounds of boron and other elements in which the central atom has fewer than eight electrons in its valence shell. These species are still considered stable because they have a stable configuration of electrons.

In conclusion, the species that is not stable is the one that has an incomplete octet or an odd number of electrons. Such species are highly reactive and can form unstable compounds. It is important to note that there are exceptions to the octet rule, and some species can still be considered stable even if they have fewer than eight electrons in their valence shell.

Which species are you referring to?