Atomic Theory and Structure Early Ideas about Matter: From Democritus to Dalton

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 theorized that all matter was composed of four elements: fire, air, water, and earth

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 proposed

A 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.

Democritus theorized that atomos were specific to the material that they made up

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 Mass

Mercury calx
Mercury calx

Many 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.

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