Question

What are isoelectronic species, give an example?

Answer 1

Mass number is equal to the sum of neutrons and protons that are present in the nucleus of an atom. Atomic number is equal to the total number of protons present in an atom or total number of electrons present in the orbits surrounding the nucleus in an atom.

Complete answer:
It is known that the Mass number of Ne is 20 . That is it has 10 protons or electrons and 10 neutrons.
As the atomic number of neon is 10 that is it contains 10 protons and 10 electrons from the definition of atomic no.
It is given that the mass number of oxygen is 16.
So it contains 8 protons or electrons and 8 neutrons .
As atomic number of oxygen is 8 from periodic table and atomic number is equal to the total number of protons present in an atom or the total number of electrons revolving around a nucleus.
Now in case of ${{O}_{2}}^{-}$ we will have 10 electrons and 8 protons as 2 electrons are added to the atom.
Now from the above information we can conclude that the total number of electrons in the valence shell of neon and oxide ions is the same that is 8.
They have an unequal number of protons and neutrons.
They have different mass numbers so they have different numbers of protons plus neutrons .
They can't be isomers because they have different molecular formulas.
So the only option that matches is isoelectronic. Isoelectronic can be defined as 2 molecules or atoms having equal number of valence electrons present in their orbit.

Note:
Some examples of isoelectronic species are carbon monoxide and nitrogen because they have equal numbers of electrons that both contain 10 valence electrons.