Which of the following best explains the trends in atomic radii down a group? It increases because the electron cloud expands, covering more space. It decreases because protons attract valence electrons with greater force. It decreases because fewer energy levels are occupied. It increases because the number of shielding electrons decreases.

The first statement, "It increases because the electron cloud expands, covering more space" best explains the trend in atomic radii down a group. The atomic radius increases because more energy levels are being added that valence electrons can occupy which are further away from the attraction of the nucleus. This explains the bigger electron cloud.

The statement " It decreases because protons attract valence electrons with greater force" is wrong because increased electron shielding prevents the protons from strongly attracting valence electrons. As a matter of fact, the atomic radius increases as you move down a group.

The statement "It decreases because fewer energy levels are occupied" is incorrect because as mentioned earlier more energy levels are being occupied.

Likewise the statement "It increases because the number of shielding electrons decreases" is not correct because there is increased shielding of valence electrons.