Atomic radius of an element is not possible to measure in its isolated state. This is because it is not possible to determine the location of the electron that surrounds the nucleus. However, we can measure the distance between the nuclei of adjacent atoms in a solid. From this we can estimate the size of the atom by assigning half of this distance to the radius of each atom. This method is best suited to elements such as the metals that exist in the solid state. More than 75 percent of the elements are metals and atomic radii of metals are called metallic radii. Another way of estimating the size of an atom is to measure the distance between the two atoms in covalent molecules. The size of a chlorine atom is estimated by measuring the length of the covalent bond between two chlorine atoms in a Cl 2 molecule. Half of this distance is taken as atomic radius which is called as the covalent radius of chlorine atom.

Atomic radius is measured in ‘pm’ (pico meter) units. 1 pm = 10-12m

Variation of atomic radii in group

Atomic radii increase from top to bottom in a group (column) of the periodic table. As we go down in a group, the atomic number of the element increases. Therefore to accommodate more number of electrons, more shells are required. As a result the number of shells in a group top to bottom increases. The distance between the nucleus and the outer shell of the atom increases. Hence the atomic size increases with atomic number when we go down the group.

Variation of atomic radii in period

Atomic radii of elements decrease across a period from left to right. As we go to right, electrons enter into the same main shell or even inner shell in case of ‘d’ block and ‘f’ block elements. Therefore, there should be no change in distance between nucleus and outer shell but nuclear charge increases because of the increase in the atomic number of elements in period. Hence, the nuclear attraction on the outer shell electrons increases. As a result the size of the atom decreases.


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