Answer:
Explanation:
The atomic radius represents the distance between the nucleus and the electrons of the valence layer (the outermost, where the valence electrons are located). It is possible to determine the size of the atom by means of the atomic radius.
The effective nuclear charge or Zef is defined as that attractive force exerted by the nucleus on the outermost electron of the atom. The higher the value, it indicates that the force of nucleus-electron attraction is greater, so that electrons are closer to the atomic nucleus, reducing the atomic radius.
When moving from top to bottom in a GROUP, atoms have more energy levels (one more for each period). Each level is more distant than the previous ones and the electrons of the levels that remain in the middle (internal electrons) "plug" or "shield" the nuclear charge. This causes that the electrons of the last energy level are not so attracted to the nucleus, so the atomic radius is increasing.
In other words, when descending in a group of the periodic table increases the effective nuclear charge, but the number of electrons added with respect to the previous element the same group increases in a complete layer (sometimes 8 electrons, if there are no orbitals d , and 18 electrons if any). The effect of adding new electronic layers is predominant, so they are increasingly distancing themselves from the atomic nucleus and the atomic radius is greater when descending in a group.
Taking all of the above into account, Ba and Mg are in the same group. But Barium is lower in the group than Magnesium, thus explaining the largest size.
