Kaplan MCAT General Chemistry Review

The transition elements, Groups IB to VIIIB (Groups 3 to 12), are all considered metals and as such have low electron affinities, low ionization energies, and low electronegativities. These metals are very hard and have high melting and boiling points. They tend to be quite malleable and are good conductors due to the loosely held electrons that are progressively filling the d subshell orbitals in the valence shell. One of the unique properties of the transition metals is that many of them can have different possible charged forms, or oxidation states, because they are capable of losing various numbers of electrons from the s- and d-orbitals of the valence shell. For instance, copper (Cu), in Group 1B (Group 11), can exist in either the +1 or the +2 oxidation state, and manganese (Mn), in Group VIIB (Group 7), can have the +2, +3, +4, +6, or +7 oxidation state. Because of this ability to attain different positive oxidation states, transition metals form many different ionic and partially ionic compounds. The dissolved ions can form complex ions either with molecules of water (hydration complexes) or with nonmetals, forming highly colored solutions and compounds (e.g., CuSO₄·5H₂O), and this complexation may enhance the relatively low solubility of certain compounds. For example, AgCl is insoluble in water but quite soluble in aqueous ammonia due to the formation of the complex ion [Ag(NH₃)₂]⁺. The formation of complexes causes the d-orbitals to split into two energy sublevels. This enables many of the complexes to absorb certain frequencies of light—those containing the precise amount of energy required to raise electrons from the lower to the higher d sublevel. The frequencies not absorbed (known as the subtraction frequencies) give the complexes their characteristic colors.

MCAT Expertise

Transition metals are present in biological systems and are therefore often seen on the MCAT (think iron in hemoglobin). You don’t need to memorize them, but be able to use your knowledge from these first two chapters to understand how the transition metals ionize and act.

Conclusion

Now that we have completed our review of the periodic table of the elements, commit to understanding (not just to memorizing) the trends of physical and chemical properties that will allow you to answer quickly the questions on the MCAT. You will find, as you progress through the chapters of this book, that your foundational understanding of the elements will help you develop a richer, more nuanced understanding of their general and particular behaviors. Topics in general chemistry that may have given you trouble in the past will be understandable from the perspective of the behaviors and characteristics that you have reviewed here.

CONCEPTS TO REMEMBER

The periodic table of the elements organizes the elements according to their atomic numbers and reveals a repeating pattern of similar chemical and physical properties. Elements in the same row are in a period, while those elements in a column are in a group. Elements in the same period have the same principal energy level, n. Elements in the same group have the same valence shell electron configuration.

The valence electrons are those located in the outer shell and/or are available for interaction (bonding) with other atoms. The representative elements have their valence electrons in either s- or s- and p-orbitals. The nonrepresentative elements (the transition elements) have their valence electrons either in s- and d- or in s-, d-, and f-orbitals.

Effective nuclear charge (Z_eff) is the net positive charge experienced by electrons in the valence shell. Z_eff increases from left to right across a period, with little change in value from top to bottom in a group. Valence electrons become increasingly separated from the nucleus as the principal energy level, n, increases from top to bottom in a group. These two trends are the basis for all the other trends exhibited by the elements in the periodic table.

Atomic radius decreases from left to right across a period and increases from top to bottom in a group.

Ionization energy (IE) is the amount of energy necessary to remove an electron from the valence shell. It increases from left to right across a period and decreases from top to bottom in a group.

Electron affinity is the amount of energy released when an atom gains an electron in its valence shell. It increases from left to right across a period and decreases from top to bottom in a group.

Electronegativity is a measure of the attractive force that an atom in a chemical bond will exert on the electron pair of the bond. It increases from left to right across a period and decreases from top to bottom in a group.

There are three general classes of elements:

—The metals, located on the left and middle of the periodic table, including the active metals and the transition metals