What is an Atom?
The atom was once thought to be the basic building block from which all else was created. It is the smallest particle of an element that still has the characteristics of that element. Helium atoms, for example, are used to fill balloons because they are lighter than air. One helium atom will rise up through the air, but because it is so tiny, many are needed to conquer the Earth's gravity. Today, physicists know that atoms are made from even smaller parts called elementary particles. Figure 3.1 represents a typical atom:
Protons, Neutrons, and Electrons
The nucleus, at the centre of an atom, is made up of elementary particles called protons and neutrons. Travelling around the nucleus, in a variety of ways, are electrons. An Electron Orbital Path is the space through which electrons travel as they tour the nucleus. Each Electron Orbital Path has a limit to the number of electrons allowed on it. Figure 3.1 shows blurry electrons because both their position and precise momentum can never be known at the same time (this is the Heisenberg Uncertainty Principle). These paths are known as shells.
The shell an electron follows depends on its energy. All the electrons in a specific shell have exactly the same amount of energy. To move from one shell to another, an electron must either gain or release a fixed amount of energy. A fancy way to say this is: electrons are restricted to quantized orbits. These shells are not necessarily fixed in size, but defined in terms of probability. Although there is a chance that an electron could be thousands of kilometres away, it is more likely to be close to the nucleus around which it travels.
Although useful to picture electrons orbiting a nucleus as planets encircle the Sun, it is not entirely accurate.
The next section illustrates these concepts with a race analogy.
Electron Orbital Race
Imagine a race with the following rules:
- runners on their track must run single file, without passing;
- runners must remain on their track while they have the same physical fitness;
- a runner that gets a boost of energy must jump to an outer track;
- a runner that loses energy must jump to an inner track; and
- there are many tracks, shown simplified in Figure 3.2.
Figure 3.2. Atom Analogy.
In this analogy, a runner gains energy by eating fruit. When this happens, the runner instantly obtains a new level of fitness, and must jump to a new track according to the rules. Runners can never be moderately fit, or somewhat lazy, but always lose or gain specific amounts (discrete units) of fitness.
Quantum Leap
The word quanta means discrete units. It is analagous to the difference between a ladder and a slide; you can stand anywhere on a slide, but only on the rungs of a ladder. Now substitute the words track and runner with shell and electron. The word quantized describes the small, discrete, leaps that electrons make from shell to shell, as though ascending or descending a ladder.
The act of an electron jumping between two shells is called a quantum leap.
Force Carrier
Electrons do not eat apples to make a quantum leap (their mouths are too small). They do, however, revolve around the nucleus as close as possible because it uses less energy; this is known as their ground state. If an electron gets extra energy, it must leap to a higher shell. After a short period of time, it will spontaneously release the extra energy, and leap back to its former shell. Just as a runner eats fruit for more energy, electrons get energy from a force carrier to make quantum leaps.
