1.2. Atomic Structure
An electron is one particle of atomic structure. A simplified model of an atom is shown in Figure 1-1. The model represents an atom consisting of three basic particles: protons, neutrons, and electrons. The protons and neutrons are coupled tightly together at the center, or nucleus, of the atom, and the electrons rotate in concentric circles around the nucleus.3 This model is called a planetary model because the electrons resemble planets orbiting around the sun. This is typical of what the world’s understanding of atomic structure was about 100 years ago. We now know that an atom is much more complex than this. Still, this simplified model is very useful for our understanding of the basic nature of current flow.
Figure 1-1. Planetary model of an atom.
Protons and neutrons are very similar to each other, with one exception. Each proton has one unit of positive charge, whereas neutrons have no charge. Electrons each have one unit of negative charge. All stable elements in nature must be charge neutral, so in any element (atom) there must be an equal number of protons and electrons.
The number of protons (and therefore the number of electrons) in an atom is called the atomic number. The atomic number is what distinguishes one element from another in nature. For example, hydrogen has an atomic number of 1. An atom of hydrogen has a single proton and a single electron. Helium has an atomic number of 2. A single helium atom has 2 protons and 2 electrons. Copper has an atomic number of 29, so it contains 29 protons and 29 electrons.
The atomic weight (sometimes called atomic mass) of an atom is approximated by the sum of the number of protons and the number of neutrons in the nucleus of the atom. Hydrogen has an atomic number of 1 and an atomic weight of 1 because it has no neutrons. The atomic weight of helium is 4 (recall that the atomic number is 2). An atom of helium has 2 protons and 2 neutrons. The atomic weight of copper is 64; it has 29 protons and 35 neutrons.4
The Periodic Table (of the Elements) is the primary way we display information about atomic structure and the identification of the various elements. Anyone who has taken chemistry in school has seen a periodic table (at least I hope so). A search for “periodic table” on the Web will turn up millions of hits. A major advantage of Web-based tables (over their text-based counterparts) is that Web-based tables are frequently animated, greatly helping our understanding of the information they convey.
What is most important for us to understand is how the electrons of an atom are organized around the nucleus. We think of electrons as orbiting around the nucleus in concentric spheres (sometimes called bands or shells). But there is a very definite order in how this happens. Each sphere has a maximum number of electrons it can hold. And the spheres must be filled in order. That is, each inner sphere must be filled to capacity before electrons can begin to fill the next sphere. The first sphere can contain two electrons. A hydrogen atom has 1 electron in this sphere. A helium atom has 2 electrons in this sphere, filling it. Lithium (with an atomic number of 3) has 2 electrons filling the inner sphere and 1 electron in the next sphere.
The outermost sphere (or band) of an element is called the valence band. It is the nature of this valence band that is important to us and to current flow. Electrons, being negatively charged, are naturally attracted to protons with their positive charge. Their energy level in their various bands is what keeps them from collapsing into the nucleus. This is very analogous to the gravitational attraction of planets to the sun. Planets would collapse into the sun if it weren’t for their rotational energy in circulating around the sun. If the valence band of an element has a single electron in it, that electron, being relatively “farther away” from the nucleus, is, relatively speaking, more loosely attached to the atom. We sometimes (not altogether appropriately) refer to it as a “free” electron. On the other hand, when a valence band is completely filled with electrons, those electrons are relatively tightly held by the nucleus.
Let’s go back to the idea that current flow is the flow of electrons. Elements that hold the electrons loosely in the valence band—those with only a single electron in the valence band, for example—give up those electrons fairly easily. These elements, therefore, act like conductors. Electrons can move relatively freely through such conductors without much external energy being applied. On the other hand, elements that hold their electrons very tightly—those whose valence bands are more fully occupied—do not allow the free flow of electrons. Therefore, they are the opposite of conductors; they are insulators.
We intuitively know that copper, silver, and gold are excellent conductors of current. These elements have two characteristics that make them good conductors: They are solid at room temperature and they each have a single electron in their valence band.
When the atoms of a conductor element are formed into a conducting wire or trace, they cluster together in a crystalline structure. Each element has its own special way of combining with other similar elements, but with gold, silver, and copper, the structure is such that it is not immediately clear which atomic nucleus “owns” which valence band electron. The nuclei can share, or trade, these valence electrons with very little effort. So if there is a force that tends to pull or push electrons in a particular direction, the electrons can shift from one nucleus to an adjacent one with relative ease. This process is illustrated in Figure 1-2. Some force is moving the electrons from left to right. Some electrons move from one nucleus to the next, while some jump over several nuclei before settling into another valence band. Studies have suggested that the typical transition of electrons among atoms in a copper structure when current flows is approximately four atoms. But what is most important to observe is that when current flows, it is not a single electron that flows from one end of a conductor to the other. All electrons tend to shift in the same direction. This is analogous to a train with many cars entering and leaving a long tunnel. The cars enter and leave the tunnel at the same rate, but it may be a considerable amount of time before an individual car that enters the tunnel leaves it again at the other end.
Figure 1-2. Electrons can travel through a conductor from one nucleus to another.