MCAT Expertise
The effective nuclear charge, Zeff, can explain all periodic trends as well as chemical properties.
NONMETALS
Nonmetals are found on the upper right side of the periodic table. The metals claim that the nonmetals are jealous of them for their shiny hair and sparkly personalities. The nonmetals scoff back, yet quietly steal electrons from them. Nonmetals are generally brittle in the solid state and show little or no metallic luster. They have high ionization energies, electron affinities, and electronegativities; have small atomic radii; and are usually poor conductors of heat and electricity. Nonmetals are less unified in their chemical and physical properties than are the metals. They are separated from the metals by a diagonal band of elements called the metalloids.
METALLOIDS
The metalloids are also called the semimetals because they possess characteristics that are between those of metals and nonmetals. The electronegativities and ionization energies of the metalloids lie between those of metals and nonmetals. Their physical properties, such as densities, melting points, and boiling points, vary widely and can be combinations of metallic and nonmetallic characteristics. For example, silicon has a metallic luster but is brittle and a poor conductor. The particular reactivity of the metalloids is dependent upon the elements with which they are reacting. Boron (B), for example, behaves as a nonmetal when reacting with sodium (Na) and as a metal when reacting with fluorine (F). The elements classified as metalloids form a “staircase” on the periodic table and include boron, silicon, germanium, arsenic, antimony, tellurium, and polonium.
Real World
Metalloids share some properties with metals, and others with nonmetals. For instance, metalloids make good semiconductors due to their electrical conductivity.
The Chemistry of Groups
ALKALI METALS
The alkali metals, Group IA (Group 1), possess most of the classic physical properties of metals, except that their densities are lower than those of other metals (as is true of lithium). The alkali metals have only one loosely bound electron in their outermost shells, and their Zeff values are very low, giving them the largest atomic radii of all the elements in their respective periods. The very low Zeff values also result in low ionization energies, low electron affinities, and low electronegativities, and these atoms easily lose one electron to form univalent cations. They react very readily with nonmetals, especially halogens.
Key Concept
Alkali and alkaline earth metals are both metallic in nature because they both lose electrons easily from the s-orbital of their valence shells.
ALKALINE EARTH METALS
The alkaline earth metals, Group IIA (Group 2), also possess many properties characteristic of metals. They share most of the characteristics of the alkali metals, except that they have slightly higher effective nuclear charges and so have slightly smaller atomic radii. They have two electrons in their valence shell, both of which are easily removed to form divalent cations. Together, the alkali and alkaline earth metals are called the active metals because they are so reactive that they are not naturally found in their elemental (neutral) state.
HALOGENS
The halogens, Group VIIA (Group 17), are highly reactive nonmetals with seven valence electrons. They are rather “desperate” to complete their octets by each gaining an additional electron. The halogens are highly variable in their physical properties. For instance, the halogens range from gaseous (F2 and Cl2) to liquid (Br2) to solid (I2) at room temperature. Their chemical reactivity is more uniform, and due to their very high electronegativities and electron affinities, they are especially reactive toward the alkali and alkaline earth metals. Fluorine has the highest electronegativity of all the elements. The halogens are so reactive that they are not naturally found in their elemental state but rather as ions (called halides).
MCAT Expertise
Halogens are seen often on the MCAT. Remember that they only need one more electron to become “noble” (have that full valence shell).
NOBLE GASES
The noble gases, Group VIIIA (Group 18), are also known as the inert gases because they have very low chemical reactivities as a result of their filled valence shells. They have high ionization energies, little or no tendency to gain or lose electrons, and no real electronegativities. They are essentially snobby elements, as they refuse to mingle with the hoi polloi. After all, they already have everything they need. The noble gases have low boiling points, and all exist as gases at room temperature.
TRANSITION METALS
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.