The three components of atoms are electrons, protons, and neutrons. For the purposes of chemistry, each of these is indivisible. Some of the basic properties of these particles are well known. The masses and charges of these particles are (SOURCE):

The masses of the proton and neutron are thus almost the same and about 1840 times greater than the mass of the electron. The charges on the electron and proton are identical but of opposite sign, while the neutron is uncharged. We will use the following notations for the particles sometimes to save space: electron (e^–), proton (p⁺), and neutron (n).

Electrons have another property in addition to mass and charge that has no direct analogy in normal human experience, although it has a name that would suggest otherwise: electrons have "spin." Electron spin can have two values, that we will call "up" and "down." But in fact there is really nothing up or down about electron spin. We will find that electron spin plays several very important roles in atoms and in molecules.

Protons and neutrons comprise the nuclei of atoms. Coulomb's law tells us that two protons are expected to repel each other, so there is clearly another force at work that holds nuclei together. The protons and neutrons of a nucleus are bound together with the so-called nuclear force or nuclear binding energy (see this link for more information). Some nuclei are unstable and fall apart (fission) into two smaller nuclei, a nucleus and an electron, or into a smaller nucleus and high energy electromagnetic radiation (a gamma ray photon). Unstable nuclei are called radioactive because radiation or an energetic particle is produced.

The smallest atomic nucleus is a single proton, with a charge of +1. We use "Z" for nuclear charge, hence Z = 1. We call this the hydrogen nucleus, H, or ¹H, where the superscript "1" placed before the atomic symbol indicates the total number of protons and neutrons in the nucleus. The next heaviest nucleus consists of one proton and one neutron. It is represented as ²H. It is also known as the deuterium nucleus or D. Both ¹H (H) and ²H (D) are stable and do not decay. Nuclei with the same number of protons and various numbers of neutrons are known as isotopes. H is the most common form of hydrogen nucleus; only 26 D nuclei exist in the universe for every million H nuclei (SOURCE). The third isotope of hydrogen is ³H, also known as tritium or T. Its nucleus has one proton and two neutrons. Unlike the ¹H and ²H nuclei, it is unstable, with a half-life of about 12 years (SOURCE). Four more nuclei of H have been created in laboratories, ⁴H, ⁵H, ⁶H, and ⁷H, but they are all extremely unstable and fall apart almost immediately.

Nuclei for Z = 1-18 are listed in the following table, along with their names. Some of the known isotopes are listed, and the most common isotope is indicated with red font (SOURCE).

For the purposes of the module, we'll assume that we're always dealing with the most common isotope for a given element. But isotope effects do matter in chemistry. Changing to a less common isotope can alter chemical reaction rates (especially for H and D) and characteristic spectral signatures. One field where this is exploited is astrochemistry. For many of the cases where a given molecule has been detected where all of the nuclei are the most common isotope there are corresponding detections for rarer isotopes.

Let's summarize what we've learned so far with the first box:

It's now time to start building atoms from nuclei and electrons. To built atoms, we will add electrons to nuclei one at a time. We will begin by adding a single electron to a number of different nuclei, starting with ¹H. Even with very simple systems like these, the behavior we will see is remarkably different from anything we observe in our normal day-to-day lives, so we will devote some time to characterizing the differences between quantum and "normal" behavior.