Chemistry

• 1. Introduction

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  Chemistry is sometimes called the central science, and the nickname carries weight. It sits in an intermediate position between physics and biology. From that perch it explains how plants grow, how igneous rocks form, how atmospheric ozone is made, how medications work, and how to collect DNA evidence at a crime scene. The same body of knowledge describes the soil on the Moon and the degradation of environmental pollutants. It is the scientific study of the properties and behavior of matter, including the nature of the chemical bonds that hold compounds together. But this science once went by a different name and chased a different prize. Practitioners hunted for a way to turn lead and other base metals into gold. So how did a discipline tangled up with astrology, mysticism, and the quest for an elixir of eternal life become a quantitative science governed by named laws? What exactly is the smallest piece of matter that still behaves like itself? And why does heat pass between substances more easily than light? The answers run from Ancient Greece to a United Nations declaration, and from a single atom to the global chemical industry.

• 2. From Alchemy To Method

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  The word chemistry comes from a Renaissance modification of alchemy, the earlier set of practices that mixed metallurgy, philosophy, astrology, astronomy, mysticism, and medicine. Alchemy itself traces back through the Arabic al-kimiya to the Ancient Greek, and possibly to Kemet, the ancient name of Egypt in the Egyptian language. An alternate path derives it from a Greek word meaning cast together. In the Hellenistic world, alchemy first proliferated, blending magic and occultism with the study of natural substances. Its goals were to transmute elements into gold and to discover the elixir of eternal life. Distillation developed further in the early Byzantine period, where the most famous practitioner was the 4th century Greek-Egyptian Zosimos of Panopolis. The Arabic works attributed to Jabir ibn Hayyan brought a systematic classification of chemical substances, with instructions for deriving sal ammoniac from organic materials like plants, blood, and hair. Some Jabirian works, including the Book of Mercy and the Book of Seventy, were later translated into Latin under the name Geber. Robert Boyle, though skeptical of elements and still convinced of alchemy, helped raise the sacred art into an independent and philosophical discipline in his 1661 work The Sceptical Chymist. The crucial break came with the scientific method that chemists employed in their work. Antoine Lavoisier turned this body of knowledge into an established science, developing a law of conservation of mass that demanded careful measurement and quantitative observation.

• 3. The Anatomy Of Matter

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  An atom consists of a dense core called the atomic nucleus, surrounded by a space occupied by an electron cloud. The nucleus holds positively charged protons and uncharged neutrons, together called nucleons, while negatively charged electrons orbit it. The mass of a nucleon is approximately 1,836 times that of an electron, yet the radius of an atom is about 10,000 times that of its nucleus. A chemical element is a pure substance built from a single type of atom, defined by its number of protons, the atomic number represented by the symbol Z. Atoms of one element can carry different numbers of neutrons; these variants are isotopes. All atoms with 6 protons are carbon, but a carbon atom may have a mass number of 12 or 13. Move beyond single elements and you reach compounds, pure substances composed of more than one element, whose properties bear little similarity to those of the elements inside them. A molecule is the smallest indivisible portion of a pure substance that keeps its unique set of chemical properties. Yet many substances are not built from discrete molecules at all. Most of the solid material in the Earth's crust, mantle, and core is made of compounds without molecules. Table salt, diamond, metals, and silicate minerals like quartz and granite are organized instead around formula units or unit cells, the smallest repeating structure within the substance.

• 4. How Atoms Hold Together

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  An ionic bond forms when a metal loses one or more electrons to become a positively charged cation, while a non-metal gains those electrons to become a negatively charged anion. Sodium loses one electron to become Na+, chlorine gains it to become Cl-, and the electrostatic attraction between them produces sodium chloride, common table salt. A covalent bond works differently. There, one or more pairs of valence electrons are shared between two atoms, and the resulting electrically neutral group is a molecule. Atoms tend to share electrons so that each reaches a noble gas configuration of eight electrons in its outermost shell, a pattern called the octet rule. Hydrogen and lithium are exceptions; they need only two electrons to reach stability, following the duet rule and matching helium. For complicated compounds such as metal complexes, valence bond theory falls short, and chemists turn to molecular orbital theory instead. The noble gas elements, helium, neon, argon, krypton, xenon, and radon, stand apart entirely. They exist as lone atoms rather than bonding into molecules or networks. Some molecules carry one or more unpaired electrons, creating radicals, most of which are reactive. A few, like nitric oxide, can be stable.

• 5. Energy And Reaction

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  Chemical reactions are not possible unless the reactants surmount an energy barrier known as the activation energy. The speed of a reaction at a given temperature relates to that barrier through Boltzmann's population factor, the probability that a molecule carries enough energy to react. This exponential dependence on temperature is the Arrhenius equation, and the necessary energy can arrive as heat, light, electricity, or mechanical force in the form of ultrasound. A reaction is called exothermic when it releases heat to the surroundings and endothermic when it absorbs heat instead. Gibbs free energy, which folds in entropy, predicts whether a reaction is feasible; a reaction proceeds only when the total change in free energy is negative, and reaches equilibrium when that change is zero. Heat tends to move between substances more easily than light. The phonons that carry vibrational and rotational energy hold much less energy than the photons involved in electronic transfer, and those vibrational levels sit more closely spaced. So ultraviolet radiation does not pass from one substance to another as readily as thermal or electrical energy. The characteristic energy levels of each substance leave a fingerprint. Chemists read it through spectroscopy, using IR, microwave, NMR, and ESR methods, and the same approach identifies the composition of stars and distant galaxies by analyzing their radiation spectra.

• 6. Acids, Bases, And Redox

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  Arrhenius theory offers the simplest account of acids and bases: an acid produces hydronium ions when dissolved in water, and a base produces hydroxide ions. The Bronsted-Lowry theory reframes this around a moving particle, defining an acid as a substance that donates a positive hydrogen ion and a base as the substance that receives it. A third view, Lewis acid-base theory, rests on bond formation, where an acid accepts a pair of electrons and a base provides one. Strength is measured two ways. The pH scale tracks hydronium ion concentration on a negative logarithmic scale, so a low pH means high acidity. The acid dissociation constant, Ka, measures how readily a substance donates hydrogen ions, with higher values marking stronger acids. Redox reactions tell a parallel story about electrons. They cover every reaction in which an atom's oxidation state changes through gaining electrons, which is reduction, or losing them, which is oxidation. A substance that strips electrons from another is an oxidizing agent, while a reductant transfers electrons away and is itself oxidized, earning the name electron donor. Strictly, oxidation and reduction refer to a change in oxidation number, and the actual transfer of electrons may never occur. Oxidation is best defined as an increase in oxidation number, and reduction as a decrease.

• 7. Building The Modern Science

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  Air turned out to be a crowd. In the decades after Boyle, what had been treated as a single thing was found to be many different gases. The Scottish chemist Joseph Black and the Flemish Jan Baptist van Helmont discovered carbon dioxide, which Black called fixed air, in 1754. Henry Cavendish discovered hydrogen, while Joseph Priestley and, independently, Carl Wilhelm Scheele isolated pure oxygen. The phlogiston theory of combustion, advanced by Georg Ernst Stahl, was overturned by Lavoisier, called the chemical analogue of Newton. John Dalton then proposed that all substances are made of indivisible atoms with varying atomic weights. Electrochemistry advanced through Jons Jacob Berzelius and Humphry Davy, made possible by Alessandro Volta's voltaic pile; Davy discovered nine new elements, including the alkali metals, by extracting them from their oxides with electric current. The periodic table took shape in the 1860s through Dmitri Mendeleev and, independently, Julius Lothar Meyer, with the noble gases added later by William Ramsay working with Lord Rayleigh. The interior of the atom opened up at the turn of the twentieth century. J.J. Thomson of the University of Cambridge discovered the electron in 1897, Becquerel and the Curies investigated radioactivity, and Ernest Rutherford at the University of Manchester found the proton and transmuted the first element by bombarding nitrogen with alpha particles. His students Niels Bohr, Henry Moseley, and Otto Hahn carried the work forward, with Hahn fathering nuclear chemistry and discovering nuclear fission. In 2011 the United Nations declared the International Year of Chemistry, marking the 100th anniversary of Marie Curie's Nobel Prize in Chemistry.

• 8. The Many Chemistries

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  Analytical chemistry asks a single question of any sample: what is it made of, and how is it structured. Its standardized methods reach into nearly every other branch, excluding only purely theoretical chemistry. Biochemistry studies the chemicals and reactions inside living organisms, spanning medicinal chemistry, neurochemistry, molecular biology, forensics, plant science, and genetics. Inorganic chemistry, once the study of metals and minerals, now centers on compounds of metals and main group elements, overlapping with organic chemistry in the sub-discipline of organometallic chemistry. Organic chemistry examines compounds built on a carbon skeleton, organized by functional groups. Physical chemistry probes the energetics and dynamics of chemical systems, drawing on thermodynamics, kinetics, electrochemistry, and spectroscopy, and it uses calculus to derive its equations. Theoretical chemistry reasons from mathematics and physics, and since the end of the Second World War, the rise of computers has grown computational chemistry into a field of its own. These divisions are not walls. The discipline keeps spinning off interdisciplinary fields, from astrochemistry and geochemistry to green chemistry and medicinal chemistry. All of it feeds an industry of real economic weight; the global top 50 chemical producers in 2013 had sales of US$980.5 billion with a profit margin of 10.3%.