Question

What is the relation between Ne atom (mass number 20) and ${{O}^{2-}}$ ion(mass number 16)?
A. They contain the same number of protons.
B. They contain the same number of neutrons.
C. They contain same number of protons plus neutrons
D. They are isoelectronic
E. They are isomers

Answer 1

Mass number is equal to the sum of protons and neutrons present in the nucleus of an atom.
Atomic no 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 given that the Mass no if Ne is 20 .
That is it has 10 protons or electrons and 10 neutrons.
As atomic no 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 of oxygen is 16.
So it contains 8 protons or electrons and 8 neutrons .
As atomic no of oxygen is 8 from periodic table and atomic no is equal to the total no of protons present in an atom or the total no of electrons revolving around a nucleus.
Now in the 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 no of electrons in the valence shell of neon and oxide ions is the same that is 8.
They have unequal no of protons and neutrons.
They have different masses so they have different 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 no of valence electrons present in their orbit.

So the correct option is D.

Note:
Some examples of isoelectronic species are carbon monoxide and nitrogen because they have equal no of electrons that both contain 10 valence electrons.