The basic building block of all matter is the atom. Curiously, the idea of atoms was first proposed in the fifth century BCE, when the Greek philosophers Leucippus and Democritus proposed their existence in a surprisingly modern fashion. However, their ideas never took hold among their contemporaries, and it wasn't until the early 1800s that evidence amassed to make scientists reconsider the idea. Today, the concept of the atom is central to the study of matter. 3.1: Prelude to Atoms, Molecules, and Ions The angstrom unit is named after Anders Jonas Ångström, a nineteenth-century Swedish physicist. Ångström's research dealt with light being emitted by glowing objects, including the sun. Ångström studied the brightness of the different colors of light that the sun emitted and was able to deduce that the sun is composed of the same kinds of matter that are present on the earth. By extension, we now know that all matter throughout the universe is similar to the matter that exists on our own planet. 3.2: Atomic Theory Chemistry is based on the modern atomic theory, which states that all matter is composed of atoms. Atoms themselves are composed of protons, neutrons, and electrons. Each element has its own atomic number, which is equal to the number of protons in its nucleus. Isotopes of an element contain different numbers of neutrons. Elements are represented by an atomic symbol. The periodic table is a chart that organizes all the elements. 3.3: Molecules and Chemical Nomenclature Molecules are groups of atoms that behave as a single unit. Some elements exist as molecules: hydrogen, oxygen, sulfur, and so forth. There are rules that can express a unique name for any given molecule, and a unique formula for any given name. 3.4: Masses of Atoms and Molecules The atomic mass unit (u) is a unit that describes the masses of individual atoms and molecules. The atomic mass is the weighted average of the masses of all isotopes of an element. The molecular mass is the sum of the masses of the atoms in a molecule. 3.5: Ions and Ionic Compounds Ions form when atoms lose or gain electrons. Ionic compounds have positive ions and negative ions. Ionic formulas balance the total positive and negative charges. Ionic compounds have a simple system of naming. Groups of atoms can have an overall charge and make ionic compounds. 3.6: Acids An acid is a compound of the H+ ion dissolved in water. Acids have their own naming system. Acids have certain chemical properties that distinguish them from other compounds. 3.E: Atoms, Molecules, and Ions (Exercises) These are exercises and select solutions to accompany Chapter 3 of the "Beginning Chemistry" Textmap formulated around the Ball et al. textbook.