Chapter 2: The Periodic Table
The pharmacological history of lithium is an interesting window into the scientific and medical communities’ attempt to take advantage of the chemical and physical properties of an element for human benefit. By the mid-1800s, the medical community was showing great interest in theories that linked uric acid to a myriad of maladies. When it was discovered that solutions of lithium carbonate dissolved uric acid, therapeutic preparations containing the lithium carbonate salt became popular. Even nonmedical companies tried to profit from lithium’s reputation as a cure-all by adding it to their soft drinks.
Eventually, fascination with theories of uric acid wore off, and lithium’s time in the spotlight seemed to be coming to an end. Then, in the 1940s, doctors began to recommend salt-restricted diets for cardiac patients. Lithium chloride was made commercially available as a salt (sodium chloride) substitute. Unfortunately, lithium is quite toxic at fairly low concentrations, and when medical literature in the late 1940s reported several incidents of severe poisonings and multiple deaths—some associated with only minor lithium overdosing—U.S. companies voluntarily withdrew all lithium salts from the market. Right around this time, Australian psychiatrist John Cade proposed the use of lithium salts for the treatment of mania. Cade’s clinical trials were quite successful. In fact, his use of lithium salts to control mania was the first instance of successful medical treatment of a mental illness—and lithium carbonate became commonly prescribed in Europe for manic behavior. Not until 1970 did the U.S. Food and Drug Administration finally approve the use of lithium carbonate for manic illnesses.
Lithium (Li) is a chemical element with atomic number 3. It is an alkali metal, very soft, and under standard conditions it is the least-dense solid element, with a specific gravity of 0.53. Lithium is so reactive that it does not naturally occur on earth in its elemental form, being found only in various salt compounds.
Why would medical scientists pay attention to this particular element? What would make medical scientists believe that lithium chloride would be a good substitute for sodium chloride for patients on salt-restricted diets? The answers lie in the periodic table.
The Periodic Table
In 1869, Russian chemist Dmitri Mendeleev published the first version of his periodic table, in which he showed that ordering the known elements according to atomic weight produced a pattern of periodically recurring physical and chemical properties. Since then, the periodic table of the elements has been revised, using the work of physicist Henry Moseley, to organize the elements on the basis of increasing atomic number rather than atomic weight. Using this revised table, the properties of certain elements that had not yet been discovered were predicted. Experimentation later confirmed a number of these predictions. The periodic table puts into visual representation the principle of the periodic law: The chemical and physical properties of the elements are dependent, in a periodic way, upon their atomic numbers.
The modern periodic table arranges the elements into periods (rows) and groups (columns), also known as families. There are seven periods, representing the principal quantum numbers n = 1 through n = 7. Each period is filled sequentially, and each element in a given period has one more proton and one more electron (in the neutral state) than the element to its left. Groups or families include elements that have the same electronic configuration in their valence shell, which is the outermost shell, and share similar chemical properties. The electrons in the valence shell, known as the valence electrons, are the farthest from the nucleus and have the greatest amount of potential energy of all the electrons in the atom. Their higher potential energy and the fact that they are held less tightly by the nucleus allows them to become involved in chemical bonds with other elements (by way of the valence shells of the other elements); the valence shell electrons largely determine the chemical reactivity and properties of the element.
The Roman numeral above each group represents the number of valence electrons. The Roman numeral is combined with the letter A or B to separate the elements into two larger classes. The A elements are known as the representative elements and include groups IA, IIA, IIIA, IVA, VA, VIA, VIIA, and VIIIA. The elements in these groups have their valence electrons in the orbitals of either s or p subshells. The B elements are known as the nonrepresentative elements and include the transition elements, which have valence electrons in the s and d subshells, and the lanthanide and actinide series, which have valence electrons in the s, d, and f subshells. For the representative elements, the Roman numeral and the letter designation determine the electron configuration. For example, an element in Group VA will have five valence electrons and a valence electron configuration of s2p3. The MCAT does not require you to know the corresponding association between Roman numerals and valence electron configuration for the nonrepresentative elements, however. The use of Roman numerals and letters to identify a particular family is confusing, because European and North American scientists traditionally have used the Roman numeral–letter system in different ways. In light of this, IUPAC developed and recommends a group identification system using Arabic numbers, 1–18, starting with the alkali metals on the left and ending with the noble gases on the right.
Periodic Properties of the Elements
We hope that it goes without saying that the MCAT will not expect you to have memorized the entire periodic table. Those of you with biology backgrounds may need the services of a Sherpa to find any element beyond the fourth period. The even less adventurous among you may never have ventured past chlorine! Fortunately, the periodic table is a guide unto itself, sort of a self-referencing GPS for all the elements. Remember, the modern table is organized in such a way to represent visually the periodicity of chemical and physical properties of the elements. The periodic table, then, can provide you with a tremendous amount of information that otherwise would have to be memorized. While you do not need to “memorize” the periodic table for the MCAT (or ever), you absolutely need to understand the trends within the periodic table that will help you predict the chemical and physical behavior of any element you encounter on the MCAT (and in your medical career).