What is the periodic table of the elements?

Kate S. asks, "Dr. Volt, can you please explain the periodic table of the elements? Why does each element have a different atomic mass?"

An excellent topic, Kate! Let's start with the question of atomic mass and the related atomic number. Each element has a unique atomic number which gives the number of protons (positive charge) found in its nucleus. The simplest element, hydrogen, has only one proton and thus an atomic number of 1. Atomic mass takes into account both the number of protons in an atom, but also the number of neutrons (no charge). Atomic mass, however, is not necessarily the same for all atoms of an element, because many elements have different isotopes, which means they have a different number of neutrons. For example, most carbon atoms have an atomic mass of 12 - 6 protons and 6 neutrons. But some carbon atoms have more neutrons, for a total of 8. These atoms then have an atomic mass of 14, but still an atomic number of 6. Carbon-14, as it is called, is radioactive and will ultimately decay, and thus is not found in as large of quantities as everyday Carbon-12.

So why is the periodic table arranged as it is? To start with, in addition to the protons and neutrons in its nucleus, an atom also consists of one or more electrons which orbit the nucleus. The electrons are negatively charged, and so to balance out the atom's total charge an atom will normally have the same number of electrons and protons. The chemical properties of a particular element are greatly affected by the way that the electrons are arranged. Quantum mechanics tells us that no two electrons in an atom may exist in the exact same state, and this leads to the electrons arranging themselves in different "shells," or layers. The first electron shell can hold up to 2 electrons. Hydrogen, with just 1 electron, thus has its electron shell half full, while Helium, with 2 electrons has its shell completely full. The third element, Lithium, has its first shell filled with 2 electrons, followed by 1 electron in the next shell, and so on with all the other elements.

The number of electrons in an elements outer shell are a major facto in determining the element's chemical properties, as atoms "like" to have their outer electron shell full. Thus an atom with just 1 electron in the outer shell will bond most easily with an atom whose outer shell is missing 1 electron. The first atom can then share the electron with the second, and they both then are "happy" with what looks to them like a full outer shell. Elements such as Neon, however, where the outer electron shell is completely full, are chemically inert as they don't feel the need to share their electrons with anyone else.

So as you move horizontally across the periodic table, this corresponds to the filling of the electron shells: the first element in a line has just 1 electron in the other shell, the last in a row has a full outer shell. The elements in each vertical line on the table are thus chemically similar because they have the same number of electrons in their outer shell. There are other factors, of course, which also affect the chemical properties of an element, but it is the filling of the electron shells which gives the periodic table its basic shape.