Click here to purchase the entire book in PDF format. Introduction Everything, everywhere is made of molecules, which in turn are made of atoms (Except in the case of the elements in which the molecules are atoms). Atoms can be thought of as being made of two things, electrons that orbit a nucleus. The nucleus is made of protons and neutrons. Each of these three particles (the electrons, neutrons and protons) has a specific charge or electrical capacity. Electrons have a negative charge, protons have a positive charge, and neutrons have no charge. As a result, the electrons, which orbit around the nucleus like planets around the sun, don't go flying off into the next atom because their negative charge attracts to the positive charge of the protons. Just as gravity ensures that the planets keep orbiting around the sun and don't go flying off into space, charge keeps the electrons orbiting the nucleus. There is a slight difference between the orbits of the planets and the orbits of the electrons. In the case of the solar system, every planet maintains a unique orbit - each being a different distance from the sun, forming roughly concentric ellipses from Mercury out to Pluto (or sometimes Neptune). In an atom, the electrons group together into what are called valence shells. A valence shell is much like an orbit that is shared by a number of electrons. Different valence shells have different numbers of electrons which all add up to the total number in the atom. Different atoms have a different number of electrons, depending on the substance. This number can be found up in a periodic table of the elements. For example, the element hydrogen, which is number 1 on the periodic table, has 1 electron in each of its atoms; copper, on the other hand, is number 29 and therefore has 29 electrons. Figure 2.1: The structures of helium and copper atoms showing the arrangement of the electrons in valence shells orbiting the nucleus. Each valence shell likes to have a specific number of electrons in it to be stable. The inside shell is ``full'' when it has 2 electrons. The number of electrons required in each shell outside that one is a little complicated but is well explained in any high-school chemistry textbook. Let's look at a diagram of two atoms. As can be seen in the helium atom in Figure 2.1, all of the valence shells are full, the copper atom, on the other hand, has just one lonely electron in its outermost shell. This difference between the two atom structures give the two substances very different characteristics. In the case of the helium atom, since all the valence shells are full, the atom is very stable. The nucleus holds on to its electrons very tightly and will not let go without a great deal of persuasion, nor will it accept any new stray electrons. The copper atom, in comparison, has weakly-held electron that can be nudged out of place. The questions are, how does one ``nudge'' an electron, and where does it go when released? The answers are rather simple: we push the electron out of the atom with another electron from an adjacent atom. The new electron takes its place and the now-free particle moves to the next atom to push out its electron. So essentially, if we have a wire made of a long string of copper atoms, and we add some electrons to one end of it, and give the electrons on the other end somewhere to go, then we can have a flow of particles through the metal.