Aprincipal quantum number refers to

A principal quantum number refers to the energy level of the orbital.

Explanation:

A principal quantum number refers to the energy level of the orbital, the first level being the one with the lowest energy. This orbital refers to the average distance of the electron to the nucleus, this mean when the principal quantum number increases the energy of that orbital increases too but the probability of finding the electron near the nucleus decreases.

This quantum number can take  positive integers from 1 to 7

plants need sunlight, water, and carbon dioxide to make food. plants use the raw materials of water, nutrients they absorb from soil, and carbon dioxide to make sugars the plant can use. energy from the sun and chlorophyll are also needed.

acid rain does not usually kill trees directly. instead, it is more likely to weaken the trees by damaging their leaves, limiting the nutrients available to them, or poisoning them with toxic substances slowly released from the soil. the main atmospheric pollutants that affect trees are nitrates and sulphates.