How does atomic radius change from top to bottom in a group in the periodic table.

Hint:Behavior of atoms or compounds can be known when we talk about the size of atoms. Atomic radius is a means to express the atomic size. This helps us know the fitting of molecules and how some of them fit together while others have parts that get too crowded under certain conditions. Edge of the atom’s orbital helps to understand the atom's size.

Complete step-by-step answer:To measure the atomic radii, the distance between nuclei of two identical atoms bonded together is measured. We can define atomic radius as one-half the distance between the nuclei of identical atoms which are together and bonded. We can measure it for elements with its units in picometers which is equal to \[{10^{ - 12\;}}\] meters.
For example, when we take out the internuclear distance between two hydrogen atoms in a \[{H_2}\] molecule, we find that it comes out to be \[74pm\]. Thus by this we understand that the atomic radius of a hydrogen atom is \[74/2 = {\text{ }}37pm\]
\[\left( {r = d/2} \right)\]
When we go down the group, the atomic radius of atoms generally increases. This is because there is an increase in the positive nuclear charge. The number of principle energy levels which are occupied also increases.
Higher principal energy levels consist of orbitals which are larger in size than the orbitals from lower energy levels. The effect of the greater number of principal energy levels outweighs the increase in nuclear charge and so atomic radius increases down a group.

Note: Atoms are bonded in metals by metallic bonds. Some of them are bonded with covalent bonds in molecules, some are attracted to one another in ionic crystals. We can see covalent molecules in which two like atoms are held together by a single covalent bond. Atomic radius patterns are observed throughout the periodic table.