How does atomic size increase on periodic table?

Hint: We know that atomic size is described by the atomic radius. Atomic radius is defined as the distance from the centre of the nucleus to the outermost shell containing the electrons. The atomic radius is measured in three forms namely, covalent radii, van der Waals radii and metallic radii.

Complete step by step answer:
Let's discuss the variation of atomic radius in the periodic table. We will start by understanding the variation in a period. Along a period (moving from left to right of the periodic table), the atomic radii of the elements generally decreases. That means, the size of fluorine (group 17) is smaller than the lithium (Group 1).
Now, we will understand the reason for the above phenomenon. When we move from left to right in a period, there is a gradual increase of nuclear charge by one unit and at the same time an electron is added in the electronic shell. Due to the increase of nuclear charge from left to right, there is more and more attraction of electrons towards the nucleus. This results in a decrease of the atomic size of the elements.
Now, we discuss the variation of atomic size in a group. There is an increase of atomic radius of elements when we move down a group. This is because on moving down a group, the number of electron shells increases. Therefore, the increase of atomic size is expected to take place. At the same time, nuclear charge also increases. As a result, there must be a decrease of atomic size . However, the effect of increase in the electron shells is more than the effect of increase in nuclear charge. This results in the increase of the atomic size down a group.

Note: It is to be noted that the atomic radius of the last element in each period which is a noble gas element is quite large. For example, the atomic radius of neon is 160 pm. Actually, it is van der Waals radius while the rest of the elements have covalent radii. Therefore, no comparison can be made.