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450 BCE
Empedocles
Empedocles was the first to propose the concept that all matter is made of four elements; earth, air, fire and water. He thought that when they are mixed precisely (with specific amounts of the required elements) they create new and diverse substances. He also corroborated that although it may be invisible, air is still matter since it consumes space, which some think he may have proved by a simple experiment using a clepsydra. -
450 BCE
The Clepsydra Test
Empedocles used a clepsydra which has two holes; one on the top and one underneath. When he lowered the bottom of the clepsydra into the water, he saw the device was filled with water. When he restarted the experiment and plugged the top hole with his finger, the clepsydra remained visibly “empty” as it was submerged into water. This experiment essentially proved that air is matter because it takes up space, also proving that the clepsydra was not truly empty and was instead full of air. -
400 BCE
Democritus
Democritus suggested that all matter is composed of small particles which he called ‘atoms’ (which means uncuttable). These atoms cannot be dissected any smaller as they are indivisible. They come in a range of sizes, are in constant motion and have large separating spaces in between each other. He concluded that all elements and matter are made of many different types of atoms, but had no experimental evidence. -
350 BCE
Aristotle
Despite recent theories during his own time, Aristotle still strongly believed in Empedocles’ four-elements theory and rejected the theory of the atom. He also believed that each of the four elements had their own respective quality; dry, wet, hot and cold. His own opinions and writings were very influential during those times and Empedocles’ original theory was accepted for almost 2000 more years. -
Period: 500 to 600
[Unknown Alchemists]
Many alchemists thought that what were once cheap metals, ripened into luxurious substances, as if they had “bloomed” like plants. So, for many centuries they attempted to “grow” precious substances (such as gold), from cheap, inferior metals. Although their theory was quite ineffective, they also developed the chemical symbols and many of the laboratory tools that are still known and used today. -
Robert Boyle
Robert Boyle did not approve of Empedocles’ four-element model. He instead believed that air was not an element, rather more of a mixture. His definition of an element became widely known across the population, replacing Empedocles’ theory. He defined an element as a pure substance that cannot be chemically broken down into simpler substances. -
Joseph Priestley, Antoine Lavoisier, Henry Cavendish (Late 1700s)
Joseph Priestley still thought that water was an element and oxygen was not. But he was the first to isolate oxygen, which Antoine Lavoisier and Henry Cavendish benefited from. After experimenting, Lavoisier concluded that air is a mixture of multiple substances, one of which is oxygen. Meanwhile, Cavendish was experimenting by mixing metal and acid. He created a flammable gas which was lighter than air but also burned in the isolated oxygen. It was unknown that Cavendish had created hydrogen. -
Period: to
John Dalton
The theories had been merged into one and it had been concluded that all matter is made of elements. John Dalton had altered Democritus’ original theory saying that all matter is made of tiny particles called atoms (which each have their own mass). All atoms of an element are identical but atoms of different elements are different and atoms are rearranged to form new substances, but cannot be created or destroyed. Compounds are created when atoms of different elements link to form molecules. -
Michael Faraday
Michael Faraday discovered that an electric current can cause chemical changes to compounds in a solution. If the atoms were to become electrically charged, they would become ions (which are essentially just charged atoms). Michael Faraday altered Dalton’s atomic model even further, stating that all matter must contain positive and negative charges, and opposite charges attract and like charges repel. Also, that as a result of electrical attraction, naturally, atoms combine to form molecules. -
J. J. Thomson
J.J. Thomson discovered that negative particles are typically smaller and lighter (called electrons), while positive particles are usually heavier (called protons). His new model (aka the “raisin-bun” model) states that atoms contain particles called electrons (which have both a small mass and an electric charge as well as a sphere of positive charge surrounding them). The model also says that electrons are embedded in the sphere of charge, making the atoms either neutral or uncharged. -
Another scientist named H. Nagaoka modeled the atom as a large positive sphere (with protons) surrounded by a ring of negative electrons (neutrons).
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Ernest Rutherford (From McGill University in Montreal)
Rutherford designed experiments to test both Thompson and Nagaoka’s models. For his first experiment, he aimed positively charged alpha particles at a thin sheet of gold. Based on Thompson’s model, he predicted that the particles would pass straight through the thin sheet of gold foil. While most of them passed through as predicted, a handful of particles bounced back off the gold foil. He compared the astonishing results to bullets firing back and bouncing off a delicate tissue. -
Rutherford's Final Model
To further explain how the alpha particles bounced off the sheet of gold, Rutherford created a final model, which states that an atom has a tiny, dense positive core called the nucleus. This deflected the alpha particles off the gold in the experiment. He also said that the nucleus is surrounded mostly by empty space, containing rapidly moving negative electrons. This explains why in the experiment the majority of the alpha particles passed through the gold sheet easily.
