Why is helium not the most electronegative elemnt?

Electronegativity describes an atoms relative tendency to attract a pair of electrons in a molecule. It can be described as a property of an atom within a molecule. As helium is a noble gas it will not react to form molecules and will thus not be in a situation where it would attract a pair of electrons within a molecule.

Because it has never formed #"He"_2#, so it did not have a Pauling electronegativity assigned to it. In fact, #"Ne"# is the most electronegative element!

There are, however, other electronegativity scales which do not depend on forming a homonuclear dimer, and #"He"# does have an electronegativity then.

Other scales are shown here:

A more modern electronegativity chart is shown here.

• A history of Pauling's derivation is shown here.

• [You may want to read this article...](https://pubs.acs.org/doi/pdf/10.1021/ar00169a001)

By definition, #"electronegativity"# is defined as the ability of an atom involved in a chemical bond to polarize electron density towards itself. There are various scales, of which the Pauling scale was the earliest, and still very widely used.

I think you meant to ask why helium has the HIGHEST ionization energy of any element...i.e. the energy required for the reaction...

#X(g) + Delta rarr X^+(g) + e^(-)#...

...and such ionization reactions SPECIFY gaseous products and gaseous reactants. Now for helium, we gots a closed shell configuration...i.e. a nuclear charge of #+2#, and #"2 electrons"#, that are conceived to reside in the first electronic shell (the atomic radius is also smaller than that of the hydrogen atom, for which #Z=1#). And so let us interrogate some data...

![kemkorner.blogspot.com]()

And thus helium has the highest ionization energy, and forms NO conventional compounds. Why are the ionization energies of each alkali metal so disproportionately low?