Group 1 elements have only one electron in their outer shell, which is the electron given up to take part in various chemical reactions. Elements are always trying to achieve an octet configuration, which means 8 electrons in their outer shell. Group 1 metals are generally quite reactive as they readily give up this one valence electron to achieve a stable octet. Francium is the last element of Group 1, which means it also has the greatest number of electron shells. As the number of shells of an atom increases, there is less attraction between the outermost valence electron and the nucleus due to more shielding. Since the pull of the electron towards the nucleus decreases, the element becomes more reactive as it can more easily give up it’s electron to achieve an octet structure. Hence, francium is the most reactive Group 1 element as it has the greatest number of electron shells.
Shielding effect: electrons can shield themselves from the pull of the nucleus. The greater the number of electron shells, greater the distance between the valence electrons and the nucleus, so there is less attraction between the two. The more shielding that occurs, the further the valence shell can spread out.
Helpful Content:
https://www.bbc.co.uk/bitesize/guides/z3xn82p/revision/3
https://www.quora.com/Why-are-caesium-and-francium-the-most-reactive-metals
https://www.toppr.com/ask/en-ae/question/why-is-francium-the-most-reactive-metal/
https://www.angelo.edu/faculty/kboudrea/periodic/periodic_main1.htm