16. C
In the first period, all elements have only an s-orbital. In the second period onwards, a 2p-orbital is present. In the third period, we find 3s-, 3p-, and 3d-orbitals. Though 3d appears to be part of the fourth period, it still shares the same principal quantum number as 3s and 3p (n = 3) and is therefore still applicable.
Chapter 3: Bonding and Chemical Interactions
The Maillard reaction is one of the more important chemical processes that occur in the process of cooking and baking. (Yes, here we go with food preparation, again. This is, in fact, relevant to your MCAT preparation.) The reaction mechanism itself is one with which you are (or will be) closely familiar from your studies of organic chemistry: a nucleophilic reaction between the amino terminus of the peptide chain of a protein and the carbonyl functionality of a sugar to form an N-substituted glycosylamine. This compound then undergoes a complex series of rearrangements and other reactions to produce a set of compounds that gives cooked food its pleasing brown color and delectable flavor. For no food preparation is this reaction more important than for meat (or meat substitute for all you vegetarians out there).
When the surface of the meat (or meat substitute) comes into contact with the hot surface of a pan or grill, the proteins and sugars on the meat exterior begin interacting through the Maillard reaction. The pan must be sufficiently hot to bring the exterior of the meat to a temperature of around 155°C (310°F), which is the optimal temperature for the reaction to occur. Of course, those willing to shell out $50 or more for a steak at a fine steakhouse are expecting perfection and good taste through and through, not just on the surface. So how does a grill master achieve the impossible: generating very high heat for the exterior but not overcooking the interior? The answer, or at least part of it, is this: drying the meat. When meat that has a lot of water on its exterior surface hits the hot pan, the first process that takes place is the boiling of the water. Boiling is a phase change from liquid to gas and occurs at a constant temperature; water’s boiling point is 100°C (212°F). Because this temperature is considerably lower than that necessary for the Maillard reaction, no browning will occur and the flavor compounds so sought after will not form. Rather, the meat will essentially steam, and the end product will be a lifeless, overcooked hunk of toughened proteins unworthy of its hefty price tag. The lesson here is, if you want a tasty steak, always dry your meat!
Of course, the real lesson is the topic of discussion for this chapter: bonding and chemical interactions. We will not actually address complex chemical bonding, such as that which takes place in the Maillard reaction, in this chapter. (We cannot stress enough, however, that the nucleophilic mechanism by which many reactions, like the Maillard reaction, proceed will be tested in the MCAT’s Biological Sciences section.) Rather, this chapter will address the basics of chemical bonding and interactions. Here, we will investigate the nature and behavior of covalent and ionic bonds. We will also review a system by which bonding electrons are accounted for, Lewis structures, and go over the main principles of valence shell electron pair repulsion (VSEPR) theory. Finally, we will recount the various modes of interaction between molecules, the intermolecular forces.
Bonding
The atoms of most elements, except for a few noble gases, can combine to form molecules. The atoms in most molecules are held together by strong attractive forces called chemical bonds, which are formed via the interaction of the valence electrons of the combining atoms. The chemical and physical properties of the resulting compound are usually very different from those of the constituent elements. For example, elemental sodium, an alkali metal, is so reactive that it can actually produce fire when reacting with water (the reaction is highly exothermic), and elemental chlorine gas is so toxic that it was used for chemical warfare during World War I. However, when an atom of sodium and an atom of chlorine react, the produced ionic compound, sodium chloride, is safe for us to eat. You may know it better as common table salt!
Bridge
Electronegativity (which we learned about in the last chapter) is a property that addresses how an individual atom acts within a bond and will help us understand the quality of the molecules formed from atoms with different electronegativities.
Why do atoms join together to form compounds? Why do the sodium atom and the chlorine atom form sodium chloride? For many molecules, the constituent atoms have bonded according to the octet rule, which states that an atom tends to bond with other atoms until it has eight electrons in its outermost shell, thereby forming a stable electron configuration similar to that of the noble gases. However, this is not a hard and fast rule, more a “rule of thumb,” and as we suggested in the first chapter, there are more elements that can be exceptions to the rule than there are elements that follow the rule without exception. These “exceptional” elements include hydrogen, which can only have two valence electrons (achieving the configuration of helium); lithium and beryllium, which bond to attain two and four valence electrons, respectively; boron, which bonds to attain six valence electrons; and all elements in period 3 and below, which can expand the valence shell to include more than eight electrons by incorporating d-subshell orbitals. For example, in certain compounds, chlorine can form seven covalent bonds, thereby holding 14 electrons in its valence shell.
Mnemonic
The octet rule is kind of like the English language, with its eight parts of speech. Nobles generally speak properly, using all eight parts as they are intended. Most people aren’t nobles, however. Civilians might desire to be so sophisticated, but unless they find someone to share some wisdom with, they’ll speak in slang, perhaps neglecting some prepositions or adverbs.
A simple way to remember all the exceptions is as follows:
• Hydrogen is excused from the octet rule because it doesn’t have enough “space” for eight electrons, it only has the one s-subshell (which can hold a maximum of two electrons).
• Lithium, beryllium, and boron are just lazy—they have enough room because they have both s- and p-orbitals to hold a total of eight electrons, but they’d rather not put in all the hard work to get all eight.