Explain the relationship between atomic structure and the position of elements in the periodic table?

The atomic size increases down the group. As the number of electrons increases quickly when you go down the group, new shells are added making the size bigger. When you move from left to right in the table, the size decreases. As the electrons increase one by one, the increasing protons in the nucleus create a greater electrostatic attraction. The electrostatic attraction dominates over the increasing number of electrons thereby pulling them closer to the nucleus and reducing the size.