Matter, the stuff that everything is made of, is formed from atoms. The heart of an atom, the nucleus, is composed of protons and neutrons and is surrounded by a cloud of electrons. For example, consider an atom of the gas helium, which consists of two protons, two neutrons, and two electrons (Figure 2.1).
1 We now know that protons, neutrons, and electrons are formed from fundamental particles called quarks, of which there are six flavors: up, down, charm, strange, top (or truth), and bottom (or beauty). Quarks are so weird that they have been referred to as “The dreams that stuff is made from,” and they are way beyond the scope of this book.