Compare diameter of magnesium, calcium, strontium, and barium atoms. What is the relationship between the elements' atomic numbers and diameters of their atoms? Why?

The elements may be arranged in order of increasing atomic diameters as such:

Mg < Ca < Sr < Ba

The atomic diameter increases with atomic number. This relationship exists because when we move down a group, an entire electron shell is added to the atom to create the next. This means that there are now more electrons present between the nucleus of the atom and the outermost shell of the atom. This is known as shielding. The increased shielding means that the attractive force of the nucleus on the outermost electrons also decreases.