Scientists come in two flavors, unifiers and diversifiers. Unifiers try to find the common threads underlying disparate phenomena. Diversifiers try to find more disparate phenomena for the unifiers to unify. Occasionally a diversifier may wear a unifier’s hat and consolidate what he knows and sometimes a unifier may take a break from his grand goal and revel in the details, but by and large the demarcation stands. As the history of science demonstrates, both diversifiers and unifiers are necessary for the creation of new ideas and growth of the scientific enterprise. But there are also certain periods and fields where one or the other type of scientist has been dominant. Physics provides a particularly interesting case where the goal of unification has driven the field for several hundred years. From Aristotle’s dream of seeing the world through the common lens of four ‘elements’ to modern string theorists’ dream of reducing the laws governing the universe to an abstract mathematical object, physics has always been particularly fruitful for unifiers. Yet there have been periods such as the fact-gathering era of the early nineteenth century when diversifiers have reigned. If physics has been principally driven by unification, chemistry has mainly been a diversifier’s game. For a long time, what was known as ‘chemistry’ consisted of the accumulation of facts about the nature of substances, including ordinary properties like color, smell and physical state combined with increasing knowledge of the transformation that these substances undergo. For all the scorn that they invoke, the alchemists were great diversifiers, carefully listing the fruits of their feverish labors to turn base metals into gold and creating much of the basic equipment that is a mainstay of today’s chemical laboratories. The first modern attempt at unification came at the end of the eighteenth century when Antoine Lavoisier classified substances into elements, compounds and mixtures. Lavoisier inaugurated a great age of unification in chemistry. His discoveries were followed about thirty years later by Friedrich Wöhler’s watershed synthesis of urea from common inorganic substances, an act that unified inorganic and biological chemistry. The synthesis of urea signaled the beginning of the science of organic chemistry and the beginning of the end for the regressive doctrine of vitalism. Wöhler’s discoveries were followed by the development of the structural theory of chemistry by scientists like Friedrich Kekule, Justig von Liebig, Archibald Couper and Alexander Butlerov which gave concrete shape to what until then had been mere placeholder names. Chemical substances could now be represented on paper as discrete collections of atoms making up molecules. The culmination of chemical unification in the nineteenth century came with Dimitri Mendeleev who put the classification of disparate elements on a firm footing based on atomic weights. Mendeleev also demonstrated how unification could be a potent tool for the prediction of unknown properties. The twentieth century has been a particularly striking example of how both unification and diversification play key roles in chemistry. The greatest act of chemical unification during this time was the success of Linus Pauling and other scientists in creating a theory of the chemical bond, a development that was directly based on the quantum mechanical revolution in physics. The work of quantum chemists made it possible to come up with common explanations for thousands of disparate chemical facts. Why are certain substances solids while others are liquids? Why do certain compounds dissolve in water while others don’t? What kind of bonds distinguish inorganic compounds from organic ones? What holds the structure of biological molecules together? Hundreds of such questions could be answered using the basic theory of chemical bonding combined with a potent tool – x-ray crystallography. The theory of bonding provided tantalizing explanations, but it was crystallography that allowed us to confirm the common provenance of molecules and the true nature of the chemical bond. A parallel thread of unification in organic chemistry was led by the American chemist Robert Burns Woodward who, through his spectacular syntheses of complex natural products, demonstrated the unifying role that a few good chemical principles can serve. Yet we saw that quantum chemistry did not do away with other fields of chemistry any more than quantum physics did away with other fields of physics. Diversifiers were still needed to do experiments. Chemistry is first and foremost an experimental science, and no amount of theorizing can diminish the value of the simple experiment revealing novel phenomena. The equations of quantum chemistry may be explanatory in principle, but in practice they are too complicated to explain or predict the most interesting chemical facts. We still have to experimentally determine the nature of the colors of a flower petal, the operating principle of the scent of ambergris, the drug staving off the cruel march of Alzheimer’s disease, the semiconducting material that would lead to the next breakthrough in electronics and the dye that could revolutionize the practice of solar energy. Theorists will aid all these discoveries but they will principally come from diverse experimenters. Another important aspect of chemistry is the ability to create diversity through unity. For instance, Woodward may have brought powerful unifying principles to bear on his syntheses, but the sheer diversity of the substances which he synthesized – ranging from cholesterol to vitamin B12 – is clear. Even Woodward’s predecessor Wöhler paradoxically initiated a push toward diversity; by demonstrating that biological substances could in fact be potentially made from simple inorganic ones, he opened a window into appreciating the astonishing variety of molecules that evolution has fashioned from a limited sampling of building blocks. This is in fact a recurring theme and here are two more examples: The common molecular features that enable us to probe molecular structure using Nuclear Magnetic Resonance (NMR) spectroscopy allow us to explore the subtle differences between molecules. In another case, you can use a single kind of reaction such as palladium catalyzed Suzuki cross-coupling to create libraries of diverse molecules. Chemistry is a particularly striking example of a science where diversity and unity piggyback on each other’s successes. The range of diverse activities in chemistry is also apparent in the number of chemical specialties that have sprouted up in the last few decades. Their practitioners have given them fancy names like chemical biology, neurochemistry, nanochemistry and astrochemistry. There are unifying themes between all of these – as well as, one suspects, some branding of old wine in new bottles – but the practitioners of these disciplines consider themselves to be distinct enough to engage separate field of research. Diversity in chemistry is alive and kicking, certainly at the level of departments, conferences and funding. There is another, deeper sense in which chemistry more than physics is a world of diversity. The equations of quantum mechanics do not help us understand the workings of brain chemistry not only because they are too complicated to solve in real time but because they deal with a different level of abstraction. One of the great philosophical paradigms of the twentieth century has been the discovery of emergent phenomena as a distinctive aspect of physical and biological systems. This paradigm demonstrated how, as we build up from atoms to molecules to cells to people, every level contains its own fundamental laws that cannot be directly mapped on to their underlying platforms. Quantum chemistry is quantum physics, but it’s more than that. And biochemistry is certainly chemistry, but in heralding the transition from nonliving matter to life, it shows itself capable of achieving something more than what simple chemistry can. Diversity has a dominant role in allowing chemistry to account for emergent phenomena. Diversifiers can provide both the theoretical and experimental wherewithal to navigate the contours of these multiple levels of understanding, but when it comes to actually uncovering the raw facts of emergence, at this point in history experiments are far ahead of theory. At some point we will have a concrete theoretical framework that accounts for the chemical transition between living and nonliving matter for instance, but until then experiments must lead the way. Chemistry has integrated itself in the working of the world at multiple levels, but at each level it demands separate explanatory frameworks that have lives of their own. One of the enduring challenges for chemists is how to use their knowledge of fundamental chemical principles to capture diversity at various levels of problem solving. Using their tools, diversifiers will illuminate corners of the tantalizing darkness. Unifiers can then find connections between these lonely spots which will reveal the grand edifice.