Looking across period 4 of the periodic table, potassium (atomic number 19) is followed by calcium (atomic number 20), which is followed by scandium (atomic number 21). which element should have the largest atomic radius? a) potassium b)calcium c)scandium d)all three have the same atomic radius.

While staying in the same period, if we move from left to right across the period, the atomic radius decreases. The reason is, in a period the number of shells remain the same and the number of electrons and protons increase as we move across the period to the right. The increased electrons and protons attract each other with greater force and hence the atomic size decreases. 

So the element on the left most will have the largest atomic radius. So the correct ans is Potassium. Potassium will have the largest atomic size among Potassium, Calcium and Scandium.