Final answer:
The atoms arranged from smallest to largest atomic radius are Be, Mg, Ca, Ba, and Mg has a smaller atomic radius than Na. Atomic size decreases from left to right in a period due to increased nuclear charge.
Step-by-step explanation:
The question asks to arrange the atoms in order from smallest to largest atomic radius among Be, Mg, Ca, and Ba. Based on the periodic table, atomic radius increases as you move down a group. Therefore, the order from smallest to largest atomic radius will be Be, Mg, Ca, Ba. When comparing Mg and Na, which are in the same period but different groups, Mg will have a smaller atomic radius due to greater effective nuclear charge, so the order would be Mg before Na. For the atomic size trend within a row of the periodic table, the atomic radius decreases from left to right due to an increase in protons, which causes a stronger attraction of electrons towards the nucleus, effectively reducing the atomic radius.