Periodic Table

At this point it should be clear what makes one element different from another (differing numbers of protons), but what makes them similar? What allowed Mendeleev to arrange the elements into a periodic table whereby elements with similar chemistry were placed one under the other?

The last piece of atomic sub-structure needed to fully explain the arrangement of the elements of the periodic table is the electron configuration. It is the arrangement of the electrons around the nucleus that determines the degree and type of reactivity an element will exhibit. At the time that Mendeleev assembled the periodic table, electrons as well as the sub-atomic structure of the atom were yet to be discovered. Their discovery revealed the underlying principles upon which the periodic table is based.

Elements that appear in the same group have the same valence shell electron configuration. Electrons are found in shells, or energy levels, around the nucleus of the atom. The farther the shell is from the nucleus, the higher the energy of its electrons. The valence shell is the outermost shell of an atom. The number of shells in an atom of an element can be determined by noting the number of the period in which that element is found. For example, potassium (K) is in Period 4, which means it has four electron shells. The valence shell is the fourth shell, and the farthest from the nucleus. Sodium is in Period 3, which means it has three shells. The valence shell is the third shell, which is closer to the nucleus than the fourth. By this reasoning it is easy to see that the valence electrons in potassium must be of higher energy than the valence electrons in sodium. It is the electrons in the valence shell that are involved in chemical reactions. Electrons below the valence shell are considered core electrons and are not important when determining reactivity. To be able to fully use the table, subshells, which help describe the locations of an atom's electrons, need to be briefly explained.

As just discussed, the outermost electron shell of an atom is the same as the period in which an element is found. The shell can be thought of as a street name in the "address" of the electrons. Each of the families of elements belongs to a particular subshell, which can be thought of as the house number in the "address" of the electrons. We will not go into detail on the physical meaning of subshells, we only need to know that the valence electrons of the main group metals (Groups 1 and 2) are in the s subshell. The valence electrons of the main group nonmetals (Groups 13 through 18) are in the p subshells, the transition metal valence electrons are in the d subshells, while the inner transition metal valence electrons are in the f subshells. The energy of the subshells increases, within the same shell, from s, then p, then d, and finally f.

Each subshell contains orbitals, which are like the rooms of the house at the particular "address" of the electrons. Each orbital holds a maximum of two electrons. There is only one orbital in each s subshell, three orbitals in each p subshell, five orbitals in each d sub-shell, and seven orbitals in each f subshell. Therefore, the s subshell can hold two electrons, the p subshell can hold six electrons, the d subshell can hold 10 electrons, and the f subshell can hold 14 electrons.

To illustrate how these electronic properties are relevant to the periodic table let us look at the first three elements Figure 2. Size representation of the atomic radii of the main-group elements. Illustration by Hans & Cassidy. Courtesy of Gale Group. of Group 16, oxygen, sulfur, and selenium. Each of these elements has a valence shell electron configuration of s²p⁴, which means there are two electrons in the s subshell and four electrons in the p subshell. Although they are each in different periods, their electronic structure is the same and we expect them to have similar chemistry. We are all familiar with the compound water which has the formula H₂O. Likewise there are the compounds H₂S and H₂Se. In a similar fashion, if you are told that the Group 15 element nitrogen, with the valence shell electronic configuration s²p³, forms the compound NH₃ (called ammonia), can you infer the formula of the compound that forms between phosphorus and hydrogen? By analogy to NH₃ we expect the compound to have the formula PH₃. It is by this same type of reasoning that Mendeleev predicted the existence of the unknown elements.

Group 18 in the periodic table, called the noble gases, are all very unreactive elements. They do not easily combine, if at all, with other elements. This indicates that there is some special stability to the electron configuration s²p⁶ which the noble gases possess. When an element has the full shell configuration s²p⁶ it is referred to as having an octet, or eight valence electrons. Much of the reactivity of the elements can be described as an attempt to achieve an octet. Since the noble gases naturally have an octet configuration, they do not need to react with other atoms to achieve this stable structure.

In the ionic compound sodium chloride (NaCl) we find a positively charged sodium atom (Na⁺) and a negatively charged chlorine atom (Cl^-). If we look at the valence shell electron configuration of each ion we can see that a chlorine atom, by gaining an extra electron, goes from a s²p⁵ (Group 17) configuration to the stable s²p⁶ (Group 18) configuration of the chloride ion. The chloride ion is referred to as being isoelectronic (having the same electronic configuration) with argon (Ar). The sodium atom (s¹) can lose an electron to become a sodium cation with the stable s²p⁶ configuration, making it isoelectronic with neon (Ne).