Kaplan MCAT General Chemistry Review

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).

MCAT Expertise

Don’t try to memorize the periodic table. You will have access to it on Test Day. Do understand its configuration and trends so that you can use it efficiently to get a higher score!

A few basic facts to keep in mind, before we examine the trends in detail: First, as we’ve already mentioned, as you move from element to element, left to right across a period, electrons and protons are added one at a time. As the “positivity” of the nucleus increases, the electrons surrounding the nucleus, including those in the valence shell, experience a stronger electrostatic pull toward it. This causes the electron cloud, the “outer boundary” of which is defined by the valence shell electrons, to move closer and bind more tightly to the nucleus. This electrostatic attraction between the valence shell electrons and the nucleus is known as the effective nuclear charge (Z_eff), which is a measure of the net positive charge experienced by the outermost electrons. For elements within the same period, then, Z_eff increases from left to right.

Second, as one moves down the elements of a given group, the principal quantum number increases by one each time. This means that the valence electrons are increasingly separated from the nucleus by a greater number of filled principal energy levels, which can also be called “inner shells.” The result of this increased separation is a reduction in the electrostatic attraction between the valence electrons and the positively charged nucleus. These outermost electrons are held less tightly as the principal quantum number increases. As you go down a group, the increase in the shielding effect of the additional insulating layer of inner shell electrons negates the increase in the positivity of the nucleus (the nuclear charge). Thus, the Z_eff is more or less constant among the elements within a given group. In spite of this, the valence electrons are held less tightly to the nucleus due to the increased separation between them.