Question

Why is flourine highly electronegative?
Why is Fr least electronegative?

Answer 1

Fluorine is the most electronegative element on the periodic table. Fluorine's atomic electron configuration is 1s^22s^22p^5 .
Fluorine is the most electronegative element because it has 5 electrons in it's 2P shell. The optimal electron configuration of the 2P orbital contains 6 electrons, so since Fluorine is so close to ideal electron configuration, the electrons are held very tightly to the nucleus. And also it has the largest nuclear charge and the least atomic shell, so it has the largest nuclear pull force. On other word, fluoride has the strongest ability to pull or catch an electron to form negative ion.

Francium is least elecronegative elememt.
This is due to the distance of its electrons from the nucleus and its atomic number. As it is very large, it rarely attract electrons.