Flexi Says: Francium is the largest element in the periodic table. The atomic radius of atoms generally decreases from left to right across a period. The atomic radius of atoms generally increases from top to bottom within a group. As the atomic number increases down a group, there is an increase in the positive nuclear charge. However, there is also an increase in the number of electron shells. The effect of the increased number of electron shells outweighs the increase in nuclear charge and so atomic radius increases down a group. So according to these concepts, Last element of group 1, Francium (Fr) is the largest element.