0.3 Structure of an atom  (Page 4/5)

Observation 2: x-ray emissions from atoms

It is interesting to know that each atom has a nuclear structure, particularly since we now know that the positive and negative charges in the atom are separated into different parts of the atom. The interaction of these positive and negative charges must be what determines the properties of each type of atom, including the types of molecules that they tend to form. Even though each atom is neutral, it might be possible for the positive charges on one atom to interact with the negative charges on another atom, and vice versa. To show this, we would need to know how many positive and negative charges there are in each atom. It seems reasonable to assume that these numbers are different for atoms of different elements, but we don’t know that without making more observations.

At this point, we know that atoms contain electrons, and that there are an integer number of these. We also know that each atom contains an equal number of positive charges, all of which are in the nucleus. It makes sense that, since the positive charge is an integer, there must be particles of positive charge in the nucleus, and we will now call these particles “protons.” This doesn’t make our life any easier, but it does clarify our language.

Our problem now boils down to finding out how many protons and how many electrons there are in each atom of each type. These integers are the same number for each atom, of course, since each atom is neutral. Finding this integer for each element seems quite difficult, and the experiment which reveals the number to us is a strange one which doesn’t seem related to the question at all.

When materials are placed in an electrical discharge, they commonly emit light or “electromagnetic radiation.” Not all of this light is visible. Some of this light is high-energy light called ultraviolet radiation, and some of it is even higher energy radiation called x-rays. We can tell the difference between different types of radiation by the “frequency” of the radiation, a property that tells us how rapidly the electromagnetic field of the radiation oscillates. Different frequencies of radiation can be separated by passing the radiation through different materials. This is how a prism works, for example. We can also separate the different frequencies with something called a “diffraction grating,” which can be either a solid with parallel grooves or a transparent solid with closely spaced lines. As common examples of simple diffraction gratings, compact discs and DVDs have grooves in parallel to each other and can be used to produce a visible rainbow. A white light source will reveal the different colors of light, each with its own frequency, when the light is reflected off of the surface of the disc.

This separation of light into different frequencies is very useful in a type of experiment called “spectroscopy.” There are many types of spectroscopy, but in most cases, a sample of a material is energized in some way, often in an electrical discharge, and the sample emits light. This is an everyday observation. For example, heated objects tend to glow, which means that they are emitting light. In spectroscopy, the light which the sample emits is separated by a prism or a grating and a wonderful result occurs. Each different element or compound has its own characteristic set of frequencies of light which are emitted. In other words, only certain frequencies of light are emitted by each substance, and the set of frequencies for each substance is unique to that substance. Spectroscopy has great value in science, because we can identify that a sample contains a particular compound or element just by looking at the frequencies of light that the sample emits. This is how we know the composition of distant stars, for example.