I started Chapter 3 - Basic Chemistry Made Easy on p 78 of "The Chemistry of Essential Oils Made Simple" today.
Dr. Stewart modifies the definition of chemistry as follows: "Chemistry is the study of electrical exchanges between atoms [which are mass] involving only their shells of electrons and is not directly concerned with activities involving their nuclei."
Matter is made up of mass (or weight) and occupies space. People are matter. In contrast, energy does not have mass, and does not occupy space. Light is energy. You could shine a ton of flashlights on the same spot on a wall without any interference from any of the beams of light, but you cannot fit a ton of people in one chair. Physics is the study of energy.
Matter is composed of electrons, protons and neutrons. Electrons have a negative electrical charge and weigh nothing. Protons have a positive electrical charge, and weigh just less than neutrons. For chemistry purposes, neutrons and protons are generally given the same weight.
Though electrons weigh less in mass than protons, the negative charge is as strong as the positive charge of protons, and there are always the same number of electrons and protons in any element, which keeps the elements stable. Basically, x-x=0 (no charge, same as neutrons).
A man named Lavoiser who is normally called the "Father of Chemistry" wrote a law that states, "Nothing is lost. Nothing is created. Everything is transformed." After the discovery of natural radioactivity, the laws of conserving energy and conserving matter were changed to state that matter and energy are equivalent to each other. This is why Einstein's formula works: E=mc2 (Energy=mass x speed of light squared).