Atoms of which element have the greatest tendency to gain electrons?
The winner would be fluorine, F.
The tendency to gain electrons can be stated quantitatively either by giving the electron affinity of the element, the amount of energy released when the atom gains one electron, or by stating its electronegativity, which is a derived quantity (as opposed to being directly measurable) which indicates the atoms ability to draw bonding electrons toward itself.
In either case, the highest element on the list would be fluorine. This is largely due to the fact that it has a valence shell (n=2) that is close to a nucleus that carries a substantial positive charge. This places the
Fluorine is also the most electronegative element, which means it is the best element at attracting electrons to itself.