The mass of an atom is

distributed uniformly throughout the atomic sphere.

concentrated in the electrons

divided equally between the nucleus and the electrons.

concentrated in the nucleus.

The mass of an atom is... concentrated in the nucleus.  

This is evident by the fact that the mass number of the element is the number of protons and neutrons and the fact that the mass of an electron is very negligible

hurricanes form in warm water

oxygen, selenium, tellurium, and polonium.

they're all from the same group as sulfur (group 16).