What element has a complete outermost energy level and is found in the 3rd period

What is the subscript for oxygen in its molecular formula

1) describe the physical layout of the ocean floor ? 2) explain how the dumbo octopus swims differently than other octopus species and why this would be an advantage in the aphonic zone . 3) why are the types of organisms that live at each underwater hot vent so dramatically different ?

Which of these features are formed when hot groundwater is forced out through cracks in the earth's surface?