A researcher is measuring changes in the density of the upper atmosphere over time. Which units should be used for the density measurements?

lbm/ft³ or psi/ft

Explanation:

The description in the question shows the density to measured is that of gases (upper atmosphere). The density of a gas can be described as the mass of the gas divided by the volume of the gas at a certain temperature and pressure. It's unit can be represented as lbm/ft³ or psi/ft.

NOTE:

"lbm" means "lean body mass" which is a unit for mass or weight and can be described as the pound mass

"psi" means "pound per square inch"; is also a unit for mass or weight

since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.