I am one of the elements. I am a soft representative metal, and you can cut me with a knife, but watch out because I am highly reactive. I have the smallest atomic number of any element in my period. Within my group I have the largest atomic radius of the elements that have no d-electron shells filled. Who am I

Potassium

Explanation:

From the given riddle. Potassium is the right answer. This because on the periodic table, potassium has the smallest atomic number of any element in its period with the electronic configuration 1s2 2s2 2p6 3s2 3p6 4s1, it has the largest atomic radius within its group because the radius has no d-electron shell filled. Potassium is a soft metal but contains high reactivity.

that's what i got you can count the number of atoms