Isotopes differ by the number of neutrons present in the nucleus. Ions are formed by the loss or gain of protons. The average atomic mass represents the relative abundance of different isotopes of an element.
All matter is composed of extremely small particles called atoms.
All atoms of a given element are alike, in mass and other properties. Compounds are formed when atoms of different elements unite in fixed proportions. A chemical reaction involves a rearrangement of atoms. No atoms are created or destroyed. I is a salt, II is a base, and III is an acid.