Why does the element with lowest atomic number have the greatest atomic radii in each period?

Answer 1

Across the period, the total number of shells remain the same. But, atomic number, i.e., no. of protons and electrons goes on increasing. Protons (positively charged) present in the nucleus exert a nuclear charge towards the electrons (negatively charged) present around the nucleus, and pull the shells toward the nucleus. Hence, when no. of protons increases, nuclear charge also increases and hence it pulls the shells with greater force, and shells get closer to the nucleus. Hence, across the period, atomic size, or the radii go on decreasing.

Answer 2

The element with the lowest atomic number has the greatest atomic radius in each period because as you move across a period in the Periodic Table, the number of protons and electrons increases, resulting in a stronger positive charge in the nucleus that attracts the electrons more closely, leading to a smaller atomic radius.