Question

What is the most reactive element?

Answer 1

Hint : We know that the number of electrons in the outermost shell of an atom and its tendency to lose electrons determines its reactivity. For example, Noble gases have low reactivity because they have full electron shells.

Complete Step By Step Answer:
The elements are generally soft, reactive metals with low melting points. They easily react with water to form an alkaline metal hydroxide solution and hydrogen. Reactivity of elements increases down the group. Alkali metals are among the most reactive metals in the periodic table and this is due to their larger atomic radii and low ionization energies. They tend to donate their electrons in reactions and have an oxidation state of $+1.$
As we know that reactivity increases down the group, so, you must be thinking that Francium is looking more reactive. But it is not the case, Francium has a heavier nucleus than Cesium and it causes radioactivity. Due to that heavy nuclei the interactions between the nucleus and the electron is more and hence it requires high energy to remove an electron from the outermost shell. There is no relation of radioactivity and tendency to forming compounds. Down to the Francium the reactivity must be slightly decreased due to the heavy nuclei.
Alkali metals (situated far away from transitional metals and noble gases) are the most reactive element group. Cesium is second from the bottom of this group, with six electron shells, so it fits all the characteristics of a reactive atom, therefore making it the most reactive element.

Note :
Remember that all the alkali metals react vigorously with halogens to produce salts, as the halogens needs one electron to complete its octet and alkali metals can readily lose electrons to have an oxidation state of $+1.$