Which element has the highest electronegativity?
(A) $F$
(B) $He$
(C) $Ne$
(D) $Na$

Hint: The one with the highest electronegativity is a non-metal that belongs to group 17. It is therefore the smallest halogen.


Complete step by step solution:
Let us start with the definition. Electronegativity is defined as the tendency of atoms to hold electrons close to themselves in a chemical bond. It is very important to understand that every element has this tendency but to a different extent.
The different factors that govern this are listed below:

- Size of the atom
As we go across a period the size of the atom decreases. This happens due to an increase in nuclear charge of the atom which is not compensated equivalently by the increment in electronic charge, resulting in the nucleus to draw the electrons in the valence shell closer ultimately reducing its overall size.
Talking about the groups, size of an atom increases as we go down the group. Adding a new shell at each step results in an increase in the diameter of the atom to which the increment in positive charge in the nucleus cannot cope up.
Electronegativity is inversely proportional to the size of atom i.e. the smaller the atom higher is its tendency to hold on to electrons. Therefore electronegativity decreases as we go down a group and increases as we go across a period.

- Block of the element
The periodic table contains four blocks, namely- s, p, d and f. These blocks are so named after the entry of the last electron belonging to the valence shell, meaning if the specified electron enters a d-orbital then it belongs to the d-block elements which occupy from group 3 to group 12. These orbitals have different shapes and are spaced at definite distances from the nucleus, s-orbital being the closest and f-orbital being the farthest and the others in between in a serial manner. The electrons in the s-orbitals are therefore held the strongest and it gradually weakens as we go to f-orbitals.

- The periodic table is designed in such a way so as to accommodate elements at places where their unique properties fit in just right. The table houses reactive metals in its first group and as we move further the metallic properties gradually change to metalloid in group 14 which then alter to reactive non-metallic at group 17 and then to the noble gases at group 18 which do not react at all.

- The idiosyncrasies of each element
The elements are unique in themselves and there are many such properties specific to them which influence their overall electronegativity.
Moving on to the options present, helium ($He$) and Neon ($Ne$) are noble gases. They do not react and as electronegativity is a reactive property these two elements do not meet the criteria to be the strongest. Sodium ($Na$) does have its last electron in the s-orbital but still fails because it is one of the most reactive metals, and metals are not electronegative but electropositive.

This takes us to the element in option (A) which is fluorine ($F$). It is placed in the group of the most reactive nonmetals, is at the right-most point just before the noble gases and therefore is the smallest in its period and is also the smallest in its group which contains other elements similar to its property.
 Fluorine ($F$) is the most electronegative in the periodic table.

Note: It is important to consider all the requirements mentioned above to decide an element’s electronegativity. Relying on any one of them is wrong.