Atoms are so small that it is impossible to see an atom even with a powerful optical microscope. The size of a single atom, therefore, cannot be directly measured. However, techniques have been developed which can measure the distance between the centers of two adjacent atoms of any element. Half of this distance is considered to be the radius of the atom. In the periodic Table, the atomic radius increases from top to bottom within a group due to increase in atomic number. This is because of the addition of an extra shell of electron in each period. In a period, however, as the atomic number increases from left to right the atomic radius decreases. This gradual decrease in the radius is due to increase in the positive charge in the nucleus. As the positive nuclear charge increases, the negatively charged electrons in the shells are pulled closer to the nucleus. Thus the size of the outermost shell becomes gradually smaller. This effect is quite remarkable in the elements of longer periods in which "d" and "f" subshells are involved. For Example, the gradual reduction in the size of Lanthanides is significant and called Lanthanide Contraction. It is all about atomic radius.