De rerum natura, which was rediscovered in the 15th century, helped fuel a 17th-century debate between orthodox Aristotelian views and the new experimental science. The poem was printed in 1649 and popularized by Pierre Gassendi, a French priest who tried to separate Epicurus’s atomism from its materialistic background by arguing that God created atoms. Show
Soon after Italian scientist Galileo Galilei expressed his belief that vacuums can exist (1638), scientists began studying the properties of air and partial vacuums to test the relative merits of Aristotelian orthodoxy and the atomic theory. The experimental evidence about air was only gradually separated from this philosophical controversy.  Anglo-Irish chemist Robert Boyle began his systematic study of air in 1658 after he learned that Otto von Guericke, a German physicist and engineer, had invented an improved air pump four years earlier. In 1662 Boyle published the first physical law expressed in the form of an equation that describes the functional dependence of two variable quantities. This formulation became known as Boyle’s law. From the beginning, Boyle wanted to analyze the elasticity of air quantitatively, not just qualitatively, and to separate the particular experimental problem about air’s “spring” from the surrounding philosophical issues. Pouring mercury into the open end of a closed J-shaped tube, Boyle forced the air in the short side of the tube to contract under the pressure of the mercury on top. By doubling the height of the mercury column, he roughly doubled the pressure and halved the volume of air. By tripling the pressure, he cut the volume of air to a third, and so on. This behaviour can be formulated mathematically in the relation PV = P′V′, where P and V are the pressure and volume under one set of conditions and P′ and V′ represent them under different conditions. Boyle’s law says that pressure and volume are inversely related for a given quantity of gas. Although it is only approximately true for real gases, Boyle’s law is an extremely useful idealization that played an important role in the development of atomic theory. Soon after his air-pressure experiments, Boyle wrote that all matter is composed of solid particles arranged into molecules to give material its different properties. He explained that all things are
In France Boyle’s law is called Mariotte’s law after physicist Edme Mariotte, who discovered the empirical relationship independently in 1676. Mariotte realized that the law holds true only under constant temperatures; otherwise, the volume of gas expands when heated or contracts when cooled. Forty years later Isaac Newton expressed a typical 18th-century view of the atom that was similar to that of Democritus, Gassendi, and Boyle. In the last query in his book Opticks (1704), Newton stated:
By the end of the 18th century, chemists were just beginning to learn how chemicals combine. In 1794 Joseph-Louis Proust of France published his law of definite proportions (also known as Proust’s law). He stated that the components of chemical compounds always combine in the same proportions by weight. For example, Proust found that no matter where he obtained his samples of the compound copper carbonate, they were composed by weight of five parts copper, four parts oxygen, and one part carbon. 3) Compounds are made of molecules. (Democritus had the idea of atoms combining but in a mechanical sense.)
4) All atoms or molecules of a pure substance are identically alike. (I don't know if Democritus shared this idea.)
5) In chemical reactions, atoms are only rearranged, neither being created nor destroyed. (Democritus didn't know of chemical reactions but he did have the idea of atoms being able to combine.) Early humans easily distinguished between materials that were used for making clothes, those that could be shaped into tools, or those that were good to eat. Then they gave these things the names, such as "fur," "stone," or "rabbit." However, these people did not have our current understanding of the substances that made up those objects. Empedocles, a Greek philosopher and scientist who lived on the south coast of Sicily between 492 BCE and 432 BCE, proposed one of the first theories that attempted to describe the things around us. Empedocles argued that all matter was composed of four elements: fire, air, water, and earth. The ratio of these four elements affected the properties of the matter. Stone was thought to contain a high amount of earth, while a rabbit was thought to have a higher ratio of both water and fire, thus making it soft and giving it life. Empedocles's theory was quite popular, but it had a number of problems. For example, regardless of how many times you break a stone in half, the pieces never resemble any of the core elements of fire, air, water, or earth. Despite these problems, Empedocles's theory was an important development in scientific thinking because it was among the first to suggest that some substances that looked like pure materials, like stone, were actually made up of a combination of different "elements." The atom is proposedA few decades after Empedocles, Democritus (460 BCE - 370 BCE), who was also Greek, developed a new theory of matter that attempted to overcome the problems of his predecessor. Democritus's ideas were based on reasoning rather than science, and drew on the teachings of two Greek philosophers who came before him: Leucippus and Anaxagoras. Democritus knew that if you took a stone and cut it in half, each half had the same properties as the original stone. He reasoned that if you continued to cut the stone into smaller and smaller pieces, at some point you would reach a piece so tiny that it could no longer be divided. Democritus called these infinitesimally small pieces of matter atomos, meaning 'indivisible'. He suggested that atomos were eternal and could not be destroyed. Democritus theorized that atomos were specific to the material that they made up, meaning that the atomos of stone were unique to stone and different from the atomos of other materials, such as fur. This was a remarkable theory that attempted to explain the whole physical world in terms of a small number of ideas. Ultimately, though, Aristotle and Plato, two of the best-known philosophers of Ancient Greece, rejected the theories of Democritus. Aristotle accepted the theory of Empedocles, adding his own (incorrect) idea that the four core elements could be transformed into one another. Because of Aristotle's great influence, Democritus's theory would have to wait almost 2,000 years before being rediscovered. In the 17th and 18th centuries CE, several key events helped revive the theory that matter was made of small, indivisible particles. In 1643, Evangelista Torricelli, an Italian mathematician and pupil of Galileo, showed that air had weight and was capable of pushing down on a column of liquid mercury (thus inventing the barometer). This was a startling finding. If air – this substance that we could not see, feel, or smell – had weight, it must be made of something physical. But how could something have a physical presence, yet not respond to human touch or sight? Daniel Bernoulli, a Swiss mathematician, proposed an answer. He developed a theory that air and other gases consist of tiny particles that are too small to be seen, and are loosely packed in an empty volume of space. The particles could not be felt because unlike a solid stone wall that does not move, the tiny particles move aside when a human hand or body moves through them. Bernoulli reasoned that if these particles were not in constant motion, they would settle to the ground like dust particles; therefore, he pictured air and other gases as loose collections of tiny billiard-ball-like particles that are continuously moving around and bouncing off one another. Law of Conservation of MassMany scientists were busy studying the natural world at this time. Shortly after Bernoulli proposed his theory, the Englishman Joseph Priestley began to experiment with red mercury calx in 1773. Mercury calx, a red solid stone, had been known and coveted for thousands of years because when it is heated, it appears to turn into mercury, a silver liquid metal. Priestley had observed that it does not just turn into mercury; it actually breaks down into two substances when it is heated, liquid mercury and a strange gas. Priestley carefully collected this gas in glass jars and studied it. After many long days and nights in the laboratory, Priestley said of the strange gas, "What surprised me more than I can well express was that a candle burned in this air with a remarkably vigorous flame." Not only did flames burn strongly in this gas, but a mouse placed in a sealed container of this gas lived for a longer period of time than a mouse placed in a sealed container of ordinary air. Priestley's discovery revealed that substances could combine together or break apart to form new substances with different properties. For example, a colorless, odorless gas could combine with mercury, a silver metal, to form mercury calx, a red mineral. Priestley called the gas he discovered dephlogisticated air, but this name would not stick. In 1778, Antoine Lavoisier, a French scientist, conducted many experiments with dephlogisticated air and theorized that the gas made some substances acidic. He renamed Priestley's gas oxygen, from the Greek words that loosely translate as 'acid maker'. While Lavoisier's theory about oxygen and acids proved incorrect, his name stuck. Lavoisier knew from other scientists before him that acids react with some metals to release another strange and highly flammable gas called phlogiston. Lavoisier mixed the two gases, phlogiston and the newly renamed oxygen, in a closed glass container and inserted a match. He saw that phlogiston immediately burned in the presence of oxygen, and afterwards he observed droplets of water on the glass container. After careful testing, Lavoisier realized that the water was formed by the reaction of phlogiston and oxygen, and so he renamed phlogiston hydrogen, from the Greek words for 'water maker'. Lavoisier also burned other substances such as phosphorus and sulfur in air, and showed that they combined with air to make new materials. These new materials weighed more than the original substances, and Lavoisier showed that the weight gained by the new materials was lost from the air in which the substances were burned. From these observations, Lavoisier established the Law of Conservation of Mass, which says that mass is not lost or gained during a chemical reaction. Comprehension Checkpoint Elements are used up when they fuel chemical reactions, so resulting substances have less mass.
Modern atomic theoryPriestley, Lavoisier, and others had laid the foundations of the field of chemistry. Their experiments showed that some substances could combine with others to form new materials, other substances could be broken apart to form simpler ones, and a few key "elements" could not be broken down any further. But what could explain this complex set of observations? John Dalton, an exceptional British teacher and scientist, put together the pieces and developed the first modern atomic theory in 1803. To learn more about Priestley's and Lavoisier's experiments and how they formed the basis of Dalton's theories, try the interactive experiment Dalton's Playhouse, linked to below. Interactive Animation:Dalton's Playhouse Dalton made it a regular habit to track and record the weather in his hometown of Manchester, England. Through his observations of morning fog and other weather patterns, Dalton realized that water could exist as a gas that mixed with air and occupied the same space as air. Solids could not occupy the same space as each other; for example, ice could not mix with air. So what could allow water to sometimes behave as a solid and sometimes as a gas? Dalton realized that all matter must be composed of tiny particles. In the gas state, those particles floated freely around and could mix with other gases, as Bernoulli had proposed. But Dalton extended this idea to apply to all matter – gases, solids, and liquids. Dalton first proposed part of his atomic theory in 1803 and later refined these concepts in his classic 1808 paper A New System of Chemical Philosophy (which you can access through a link under the Resources tab). Dalton's theory had four main concepts:
Comprehension Checkpoint An element is made up of
Some of the details of Dalton's atomic theory require more explanation. Elements: As early as 1660, Robert Boyle recognized that the Greek definition of element (earth, fire, air, and water) was not correct. Boyle proposed a new definition of an element as a fundamental substance, and we now define elements as fundamental substances that cannot be broken down further by chemical means. Elements are the building blocks of the universe. They are pure substances that form the basis of all of the materials around us. Some elements can be seen in pure form, such as mercury in a thermometer; some we see mainly in chemical combination with others, such as oxygen and hydrogen in water. We now know of approximately 116 different elements. Each of the elements is given a name and a one- or two-letter abbreviation. Often this abbreviation is simply the first letter of the element; for example, hydrogen is abbreviated as H, and oxygen as O. Sometimes an element is given a two-letter abbreviation; for example, helium is He. When writing the abbreviation for an element, the first letter is always capitalized and the second letter (if there is one) is always lowercase. Atoms: A single unit of an element is called an atom. The atom is the most basic unit of matter, which makes up everything in the world around us. Each atom retains all of the chemical and physical properties of its parent element. At the end of the nineteenth century, scientists would show that atoms were actually made up of smaller, "subatomic" pieces, which smashed the billiard-ball concept of the atom (see our Atomic Theory I: The Early Days module). Compounds: Most of the materials we come into contact with are compounds, substances formed by the chemical combination of two or more atoms of the elements. A single "particle" of a compound is called a molecule. Dalton incorrectly imagined that atoms "hooked" together to form molecules. However, Dalton correctly realized that compounds have precise formulas. Water, for example, is always made up of two parts hydrogen and one part oxygen. The chemical formula of a compound is written by listing the symbols of the elements together, without any spaces between them. If a molecule contains more than one atom of an element, a number is subscripted after the symbol to show the number of atoms of that element in the molecule. Thus the formula for water is H2O, never HO or H2O2. Comprehension Checkpoint The formula for water can be written as either H2O or HO2.
Law of Definite ProportionsThe idea that compounds have defined chemical formulas was first proposed in the late 1700s by the French chemist Joseph Proust. Proust performed a number of experiments and observed that no matter how he caused different elements to react with oxygen, they always reacted in defined proportions. For example, two parts of hydrogen always reacts with one part oxygen when forming water; one part mercury always reacts with one part oxygen when forming mercury calx. Dalton used Proust's Law of Definite Proportions in developing his atomic theory. The law also applies to multiples of the fundamental proportion, for example: In both of these examples, the ratio of hydrogen to oxygen to water is 2 to 1 to 1. When reactants are present in excess of the fundamental proportions, some reactants will remain unchanged after the chemical reaction has occurred. The story of the development of modern atomic theory is one in which scientists built upon the work of others to produce a more accurate explanation of the world around them. This process is common in science, and even incorrect theories can contribute to important scientific discoveries. Dalton, Priestley, and others laid the foundation of atomic theory, and many of their hypotheses are still useful. However, in the decades after their work, other scientists would show that atoms are not solid billiard balls, but complex systems of particles. Thus, they would smash apart a bit of Dalton's atomic theory in an effort to build a more complete view of the world around us. SummaryTracking the development of our understanding of the atomic structure of matter, this module begins with the contributions of ancient Greeks, who proposed that matter is made up of small particles. The module then describes how Lavoisier's Law of Conservation of Mass and Proust's Law of Definite Proportions contributed to Dalton's modern atomic theory. What is the difference between Democritus and Dalton's idea?The primary difference between Democritus and Dalton lies in their manner of working - one was a philosopher, while the other was an empirical scientist.
How were Democritus's ideas similar to Daltons?-Democritus believed that atoms could not be created, destroyed, or further divided. What did Dalton Believe about matter? -Atoms cannot be created, divided into smaller particles, or destroyed.
What is the difference between the ideas of the atoms during Democritus time and Dalton's time?The key difference between Democritus and Dalton atomic theory is that the Democritus atomic theory is an ancient theory that scientists later refined and elaborated whereas Dalton atomic theory is a comparatively modern, scientific theory that we cannot discard due its important statements.
What did Democritus and Dalton both believe about the atom?Both Democritus and Dalton suggested that matter is made up of atoms. Dalton's atomic theory stated that atoms separate, combine, or rearrange in chemical reactions. Dalton's atomic theory stated that matter is mostly empty space. Dalton was correct in thinking that atoms could not be divided into smaller particles.
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How did Democritus idea of matter atoms compare with that of Daltons?
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