0.3 The structure of an atom  (Page 3/6)

The relevant observation seems unrelated to the previous observations. In this case, we examine the frequencyof x-rays emitted by atoms which have been energized in an electrical arc. Each type of atom (each element) emits a fewcharacteristic frequencies of x-rays, which differ from one atom to the next. The lowest x-ray frequency emitted by each element isfound to increase with increasing position in the periodic table.

Most amazingly, there is an unexpected relationship between the frequency and the relative mass of eachatom. Let’s rank order the elements by atomic mass, and assign an integer to each according to its ranking in order bymass. In the Periodic Table, this rank order number also corresponds to the element’s position in the Periodic Table.For example, Hydrogen is assigned 1, Helium is assigned 2, etc. If we now plot the lowest frequency versus the position number in theperiodic table, we find that the frequency increases directly as a simple function of the ranking number. This is shown here , where we have plotted the square root of the x-ray frequency as afunction of the ranking number. After a single correction, there is a simple straight-line relationship between these numbers. (Thesingle correction is that the rankings of Argon and Potassium must be reversed. These elements have very similar atomic masses.Although Argon atoms are slightly more massive than Potassium atoms, the Periodic Law requires that we place Argon beforePotassium, since Argon is a member of the inert gas group and Potassium is a member of the alkali metal group. By switching theirorder to correspond to the Periodic Table, we can maintain the beautiful relationship shown here .)

Why is this simple relationship a surprise? The integer ranking of an element by mass would not seem to be aphysical property. We simply assigned these numbers in a listing of the elements which we constructed. However, we have discovered thatthere is a simple quantitative relationship between a real physical quantity (the x-ray frequency) and the ranking number we assigned.Moreover, there are no "breaks" in the straight line shown here , meaning that all of the elements in our mass list must beaccounted for. Both observations reveal that the ranking number of each atom must also be a real physical quantity itself, directlyrelated to a structural property of each atom. We now call the ranking number the atomic number , since it is a number which uniquely characterizes each atom.

Furthermore, we know that each atom must possess an integer number of positive charges. Since the x-ray datademonstrates a physical property, the atomic number, which is also an integer, the simplest conclusion is that the atomic number fromthe x-ray data is the number of positive charges in the nucleus. Since each atom is neutral, the atomic number must also equal thenumber of electrons in a neutral atom.

We now know a great deal about the structure of an atom. We know that the atom has a nuclear structure, we knowthat the positive charges and mass of the atom are concentrated in the nucleus, and we know how many protons and electrons each atomhas. However, we do not yet know anything about the positioning and movement of the electrons in the vast space surrounding thenucleus.