The ancient idea that matter is made up of tiny, indivisible pieces has led scientists in directions that Leucippus and Democritus could never have imagined. Today’s world of human-made substances depends on our knowledge of atoms, the subatomic particles within those atoms, and the forces that hold them together or break them apart.
Those ancient Greek philosophers began with two simple questions: What is matter made of, and why do different kinds of matter behave so differently from each other? When they imagined dividing everyday substances into smaller and smaller bits until the pieces were indivisible atoms, they also decided that the atoms of each substance would have a particular shape and texture. For instance, they concluded that water atoms would be round and smooth, while rock atoms would be hard and sharp or gritty.
The presumed atoms were much too tiny for the Greeks to test their ideas. Besides, the notion of testing theories by observation, a cornerstone of modern science, was not yet part of human culture. Toward the end of Democritus’ life, Greek philosophy entered what is often called a “golden era,” where great thinkers such as Socrates, Aristotle, and Plato used their powerful minds and logic to deduce what they believed to be the truth about the natural world.
Aristotle was so brilliant in many fields that his ideas were rarely questioned. For nearly two thousand years, most people accepted his conclusion that all the world’s matter was made of four elements: earth, air, fire, and water. The idea of atoms all but disappeared. Today we know that both Democritus and Aristotle were right in general but wrong in detail.
Democritus. This sculpture depicts Democritus, the founder of an ancient Greek philosophical movement known as atomism. He proposed that matter is made of tiny indivisible particles, an idea that eventually led to the modern theory of atoms.
Democritus spoke of a limit to how small a piece of matter can be cut and still remain the same substance. That idea was right, but most substances are compounds and mixtures, not elements. The smallest possible piece of most substances is usually a molecule instead of an atom. Furthermore, molecules can be divided into their atoms, and even atoms are not indivisible.
Aristotle’s Elements. Atomism was largely abandoned for nearly two thousand years and replaced by the four elements of Aristotle: earth, air, fire, and water. This illustration from the Middle Ages shows the symbols for those elements plus a fifth, aether, which was thought to make up the heavens.
Aristotle was correct that all matter is made of combinations of particular elements, but not the four he wrote about. Water is a compound, earth and air are mixtures containing both elements and compounds, fire is a process that produces energy as atoms rearrange themselves into different compounds, and the number of natural elements is nearly one hundred, not four.
As you might imagine, the road from the philosophy of the ancient Greeks to today’s scientific knowledge of atoms is a long one with many interesting twists and stops along the way. It begins with an activity called alchemy, in which people tried to make certain substances out of others, often by heating things together. Most often, alchemists were searching for ways to turn less valuable metals into gold using techniques that we now know were doomed to failure. Gold is an element, and neither ancient alchemy nor modern chemistry can change one kind of atom into another.
Though many alchemists were fraud artists, others succeeded in developing a rudimentary knowledge of matter, extracting or purifying many useful elements and compounds from natural substances. By the seventeenth century, scientific thinking had begun to take hold, and alchemy was gradually transformed into the science of chemistry. Eighteenth-century chemists made a number of important discoveries, including facts about the behavior of gases, the processes of combustion and corrosion, and the relationship between electricity and matter. None of those phenomena were fully understood, but plenty of evidence and measurements were being gathered systematically and scientifically.
An Alchemist’s Laboratory. This drawing by sixteenth-century Flemish painter Jan van der Straet shows one of alchemy’s most common practices: distillation. This remains a common technique for separating substances in modern chemistry and industry.
As the nineteenth century dawned, English scientist John Dalton (1766–1844), already in his thirties, was turning his attention from meteorology to chemistry, hoping to gain a better understanding of the gases of the air. He soon realized that the ancient idea of atoms could explain many of the phenomena that others had observed in gases and chemical reactions. In 1810, Dalton published a book that revolutionized chemistry. Titled A New System of Chemical Philosophy, the book was based on the assumption that all matter was made of atoms.
Dalton explained that each element was made of a particular kind of atom, and all of its atoms were identical to each other. He also wrote that atoms of different elements have different properties, including their weight. When atoms join to form compounds, it is always in small numbers of whole atoms—no fractional atoms allowed.
John Dalton (1766–1844). Dalton is considered the father of modern chemistry for his 1810 breakthrough book that introduced the concepts of elements, compounds, atoms, and molecules.
Discovering that Water is H2O
How did scientists discover that a water molecule is formed from two hydrogen atoms and one oxygen atom—which gives it the chemical formula H2O? They did so by running experiments with combining gases.
In 1808, French chemist Joseph Louis Gay-Lussac (1778—1850) described what happened when two gases reacted to form another gas. From performing numerous experiments, he found that the volumes of the reacting gases at the same temperature and pressure were always in the ratio of simple whole numbers. For instance, when hydrogen burned in oxygen to form water, for every 2 cubic meters (528 gallons) of hydrogen burned, it took 1 cubic meter (264 gal) of oxygen. The result was 2 cubic meters (528 gal) of water vapor. He proposed that discovery as a law of nature.
When Italian scientist Amadeo Avogadro (1776–1856) read about Gay-Lussac’s proposed law of gases, he tried to understand why that could be so. When he thought about the nature of gases in containers, he envisioned lots of molecules colliding with each other and with the container walls. Because the molecules were so far apart, it didn’t matter what kind of molecules they were. He concluded that any gas having the same temperature and pressure and occupying the same volume had the same number of molecules. In 1811, he proposed that as a law of nature.
Combining the two laws, it became clear that Gay-Lussac’s law of whole number volumes was also a law of whole number of atoms or molecules. Thus a molecule of water had to contain two hydrogen atoms and one oxygen atom. Other experiments showed that hydrogen molecules were two hydrogen atoms combined, or H2, and, likewise, oxygen molecules were O2. Therefore Gay-Lussac’s experiment showed this reaction:
Joseph Louis Gay-Lussac (1778–1850). French chemist Gay-Lussac discovered that the volumes of gases reacting at the same temperature and pressure are always in the ratio of small whole numbers. He proposed this as a law of nature.
2H2 + O2 → 2H2O
(2 hydrogen molecules and 1 oxygen molecule produce 2 molecules of water)
Using those simple rules, Dalton was able to determine the atomic mass, or atomic weight, of different elements. He assigned hydrogen, the lightest element, one unit of atomic weight, and determined the atomic weight of other atoms from that. For instance, he knew that water was a compound of hydrogen and oxygen, with eight times as much oxygen by weight. Assuming that a water molecule had one atom of each element, he set the atomic weight of oxygen at eight units. Later on, when more research showed that water molecules had two atoms of hydrogen and one of oxygen, scientists corrected that result, setting the atomic weight of oxygen at sixteen.
Close, but Incorrect. By assuming water was a compound of one hydrogen atom and one oxygen atom, Dalton drew the incorrect conclusion that the atomic mass of oxygen was eight. Later experiments showed the correct formula was H2O, and thus the atomic mass of oxygen was sixteen.
Following Dalton’s method, scientists studying chemical reactions gradually identified more compounds and the elements that composed them, and they determined the atomic weights of each element. Though no one had detected individual atoms, Dalton’s idea of elements and compounds, and atoms and molecules, had given chemistry a new basic vocabulary. As is often the case with scientific breakthroughs, atomic theory also opened up a wealth of new questions, including: How many elements are there, and is there a way to classify them to better understand their chemical behavior?
Mendeleyev and the Periodic Table
By 1869, a total of sixty-three elements were known, and scientists were having a hard time keeping track of them. They could see hints of similarities and patterns among the elements, such as in their melting or boiling points, their densities (how much each cubic centimeter weighs), the way they combined with other elements, and the properties of the compounds they formed. Still, no one had come up with a successful way to classify them—until Russian chemist Dmitry Ivanovich Mendeleyev (1834—1907) had a dream.
Mendeleyev was a professor at St. Petersburg University, known for his thorough knowledge of the elements and their properties. He also owned a small country estate, and his neighbors relied on his advice about farming and cheese making. So from time to time he would schedule a visit to the country. Traveling by train, he would often occupy his mind by playing “patience” with a deck of cards. Like most solitaire games, the object of patience was to build an arrangement of the cards from the highest to the lowest within each suit (spades, hearts, diamonds, and clubs).
Before one such trip in early 1869, Mendeleyev had been working continuously for three days, trying to discover a way to classify the elements that would make sense of all the fragments of patterns he and others had noticed. He decided to make a set of cards, one for each element, listing the known properties of each. He arranged them in order of increasing atomic weight. Just before he was to leave to catch his train, the weary professor fell asleep and dreamed of playing patience with his deck of element cards. When he woke, he knew what he had to do. The atomic weights were like the order of the cards. All he needed was to figure out how many groupings there were (nature had no reason to choose four suits like in playing cards) and how many cards were in each.
Some Elements Discovered by 1869
Mendeleyev’s periodic table was built on many centuries of discovery. Here, in alphabetical order, are some of the elements he knew well.
Aluminum (Al): Discovered in 1825 by Hans Christian Oersted in Denmark.
Boron (B): Discovered in 1808 by J. L. Gay-Lussac and L. J. Thenard as well as Sir Humphry Davy.
Calcium (Ca): Discovered in 1808 by Sir Humphry Davy in London, England.
Carbon (C): Discovered in prehistoric times.
Hydrogen (H): Discovered in 1766 by Henry Cavendish in London, England.
Iron (Fe): Discovered by ancient civilizations.
Lithium (Li): Discovered in 1817 by J. A. Arfwedson in Sweden.
Nitrogen (N): Discovered in 1772 by Daniel Rutherford in Edinburgh, Scotland, as well as in the early 1770s by Carl Wilhelm Scheele in Sweden, Henry Cavendish, and Joseph Priestly in England.
Oxygen (O): Discovered independently around 1772 by Carl Wilhelm Scheele in Sweden and 1774 by Joseph Priestley in England.
Zinc (Zn): Known in India and China before 1500 and to the Greeks and Romans before 20 bce.
The First Periodic Table. This diagram shows Russian chemist Dmitry Mendeleyev’s first published periodic table of the sixty-three known chemical elements at that time. Later research discovered additional elements that fit into some of the gaps in this chart and had properties very similar to Mendeleyev’s predictions.
By the time Mendeleyev arrived at his destination, the arrangement had begun to fall into place. Though he had used other properties to develop his arrangement, he discovered that the groupings seemed to follow a chemical property called “valence.” Valence accounted for how many of one atom would combine with how many of another. For example, the elements called the alkali metals—lithium, sodium, potassium, rubidium, and cesium—all fell into alignment, as did the elements called halogens—fluorine, chlorine, bromine, and iodine. As atomic weight would increase, the atoms would follow a pattern: going from one valence to the next to the next, and periodically starting over again. Mendeleyev thereby called his arrangement the periodic table of the elements.
Valence in Chemical Compounds. In Mendeleyev’s time, scientists were developing rules for the formation of molecules from atoms based on a property called valence, which was either a positive or negative small whole number for each atom. In one common usage, a compound has a total valence of zero. For example, in water, hydrogen has a valence of 1 and oxygen has a valence of -2. That gives H2O a total valence of 0. Likewise, common table salt, sodium chloride, is formed from a sodium atom (Na) with valence of 1 and a chlorine atom (Cl) with a valence of -1.
The table was not perfect, and it had a few gaps. Mendeleyev claimed that the gaps represented elements not yet discovered. He predicted not only that they would be found, but also what compounds they would form and what their atomic weights, density, and other properties (such as melting or boing points) would be. Mendeleyev was right, and with those discoveries, the periodic table of the elements was established as one of the great ideas of chemistry.
Mendeleyev’s discovery of the periodic table is considered one of the great achievements of science. But it was far from a complete description of the nature of matter. It led to many important new questions, especially this one: What makes the properties of atoms periodic? Answering that would require the discovery of subatomic particles.