For main-group elements, the atomic radius increases going down a group because: 

• A. Effective nuclear charge decreases.
• B. The mass of an element increases.
• C. The highest principal quantum number of the valence orbitals increases.
• D. Electron affinity increases.
• E. Both effective nuclear charge and the principal quantum number of valence orbitals increases.

Answer: (A)

Verified step by step guidance

1. Understand that atomic radius refers to the size of an atom, typically measured from the nucleus to the outer boundary of the surrounding cloud of electrons.
2. Recognize that as you move down a group in the periodic table, each element has an additional electron shell compared to the one above it. This is due to the increase in the principal quantum number \( n \), which represents the energy level of the valence electrons.
3. Consider the concept of effective nuclear charge, which is the net positive charge experienced by valence electrons. It is calculated as the total positive charge of the nucleus minus the shielding effect of inner-shell electrons.
4. Note that while the nuclear charge increases with more protons as you move down a group, the increase in electron shielding due to additional inner shells means the effective nuclear charge experienced by the outermost electrons does not increase significantly.
5. Conclude that the increase in the principal quantum number \( n \) and the relatively constant effective nuclear charge result in a larger atomic radius as you move down a group, as the outermost electrons are further from the nucleus.