Why are fewer organisms found as you move up the food chain?

Are there choices we should be choosing from? Or do you just want an explanation?

Because as you move higher up the food chain there is less prey for predators to eat, which limits the size of the predator population. The populations of predators higher on the food chain are totally dependent on the population of their prey, which are below them on the food chain.

Yes you are correct use the scientific method

You use the atomic number to find out how many protons and neutrons there are in the atom. you use the atomic mass to find out how many protons and electrons there are in the atom.