What metal has the largest atomic radius in period 4 (78 points)

The element potassium(K) with atomic number 19 has the largest atomic radius ( around 225 picometers ).

Explanation:

There is the general expected decrease from left to right with increasing nuclear charge without quantum level expansion or increase in shielding and so pulling the outer electrons closer. The 3d block of metals (Z = 21 to 30) tend to be similar and in fact tend to go through a small minimum at Mn/Fe.

The potassium been on the left most part of period , has largest radius due to least nuclear pulling effect.