How does atomic size of elements vary in the periodic table?

Atomic size definition:

This is the atomic radius. It is measured from the center of the atom to the outermost electron shell of the atom.

Factors on which atomic size is dependent:

The atomic size (or radius) of elements is dependent on the atomic number and the number of shells of the atoms of elements:

• Atomic number: As the atomic number of elements increases, the atomic radius decreases. This is because the increase in atomic number means increase in the number of protons in the nucleus of the element. Increase in the number of protons means that the positive charge in the nucleus has also increased. The increase in the positive charge in the nucleus causes the outermost shell of electrons to be attracted towards it and thus the outermost shell is pulled towards the nucleus. This causes the decrease in the atomic radius of the atom.
• Number of shells : As the number of shells of atoms increases, the atomic size increases since the distance between the outer most shell and the nucleus also increases.

Variation across rows and down groups:

• On going from left to right in a row of the periodic table, the atomic radius of the elements decreases.
  Reason and explanation:
  The number of shells in the atoms of elements remains same, where as the atomic number increases one by one. Since the atomic number increases, the number of protons in the nucleus of that element also increases, and hence the attractive force between the outer electrons and the protons in the nucleus also increases. This causes a decreases in the atomic size of the elements, since the outermost shell is pulled closer to the nucleus by the increasing positive charge of the nucleus.
• On going down a group of the periodic table, the atomic radius of elements increases
  Reason and explanation:
  On moving down a group of the periodic table, the number of shells in each elements atoms increases one by one with each element. This causes an increase in the atomic size of the elements as we go down a group in the periodic table. The atomic number of elements also increases as we move down a group, and that should reduce the size of the atoms, but the decrease in atomic size caused by the increase in atomic number is slight as compared to the increase caused by increasing number of shells. Therefore, the atomic size of elements decreases as we move down a group of the periodic table.