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"???HUH???? If you get it, please tell me." JA

Well, it's both simple, and complicated.

In 1811, Avogadro published an article in Journal de physique that clearly drew the distinction between the molecule and the atom. He pointed out that Dalton had confused the concepts of atoms and molecules. The "atoms" of nitrogen and oxygen are in reality "molecules" containing two atoms each. Thus two molecules of hydrogen can combine with one molecule of oxygen to produce two molecules of water.

Avogadro suggested that:
equal volumes of all gases at the same temperature and pressure contain the same number of molecules
which is now known as Avogadro's Principle.

It was this concept that allowed researchers to begin dealing with large numbers of atoms, (a necessity in any case), and yet have a clear understanding of how the individual atoms were reacting.

Initially, they weren't entirely sure whether they were dealing with atoms or molecules, but thanks to Avogadro, they knew how many were involved in a particular volume. It was for this reason that the term "mole" became popular. It is a term that denotes a particular number of things. It's more than a dozen, and it's more than a trillion, or even a decillion, but it sounds friendlier.

This led to a clearer understanding of Atomic Number, Atomic Weight, and eventually developed into a recognition that there were isotopes.

All of this was necessary as a precursor to understanding how the Sun got its energy. One can make a back-of-the-envelope calculation of how much energy the Sun sends out to this distance; the radius of Earth's orbit. Since the Sun sends this energy in all directions, the numbers grow enormous. What could possibly provide so much energy?

So, in essence, Avogadro and his "moles" did the spadework for another peculiar fellow named Albert Einstein.