We began these first two chapters with the statement that ours was a universe mainly of hydrogen and helium, which at the same time are the simplest and the oldest two elements. These two elements illustrate in miniature most of the chemical principles that we will encounter with the heavier elements. The other elements, like hydrogen and helium, are built from positively charged nuclei containing protons and neutrons, surrounded by enough negatively charged electrons to neutralize the positive charge of the protons.

The number of protons, or the atomic number, determines the chemical behavior of an atom because it determines the number of electrons that surround a neutral atom; and the gain, loss, and sharing of electrons is responsible for an atom's chemical properties.

The number of neutrons usually is equal to or slightly greater than the number of protons. Neutrons have little effect on chemical properties of an atom, except for those that are influenced by mass.

Atoms with the same atomic number but different numbers of neutrons are called isotopes. The total number of neutrons and protons in the nucleus is the mass number of the atom, and the actual mass in amu is the atomic weight relative to that of carbon-12 as exactly 12 amu. Observed atomic weights usually are averages of the weights of several naturally occurring isotopes.