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450 BCE
Aristotle's View of the World
Aristotle thought that matter was made up of primarily 4 Elements: air, earth, water, and fire, and did not agree with Democritus' model of atoms or particles. He also envisioned another element called the Aether, which made up celestial bodies, like the Sun. Aristotle did not have proof for this depiction, but due to Aristotle's fame in many other fields, his believes went unquestioned for centuries. The only part of Aristotle's model that stuck to modern atomic theory is the idea of elements. -
442 BCE
Democritus' Atom
Democritus believed that all matter was made up of tiny solid particles called "atomos", the Greek for indivisible, and empty space, disagreeing with Aristotle's view of 4 Elements. He proposed that if you cut objects in half multiple times, it cannot be cut anymore at a point: the atomos, which had no internal structure, eternal, and specific to that object; i.e. stone atomos were different from glass in shape and size. Atomos differences determined the characteristics of the object formed. -
Lavoisier's Reactions
The standpoint in Lavoisier's time, was that combustion released a fire-like substance called phlogiston in the air. It was also believed that air was a single substance, agreeing with Aristotle's views. Lavoisier showed that when combining combusted metals with air, they gained weight, counteracting what phlogiston would explain. He hypothesized that air was made up of 2 substances, "oxygène", and other compound that does not support combustion. In this, the conservation of mass was discovered. -
Trait lmentaire de Chimie
In this book by Lavoisier, he showed that water was 85% oxygène and 15% hydrogen. Lavoisier also suggested that combustion is just the combining of substances with oxygen. Other scientists had problems with this theory, however, as this not explain the combustion of inflammable air. When Lavoisier added oxygen to this air, water was formed, showing that water was not an element, but comprised of elements. This radically opposed Aristotle's view of the world, and is the start of modern chemistry. -
Dalton's Conjectures
- All matter is made of atoms. Atoms are indivisible.
- All atoms of a given element are identical in mass and properties.
- Compounds are formed by a combination of two or more different kinds of atoms.
- A chemical reaction is the rearrangement of atoms.
- Atoms of different elements combine in simple whole-number ratios to form chemical compounds. (Law of Partial Pressures)
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Dalton and Lavoisier
Dalton used Lavoisier's Law of Conservation of Mass to make his Conjectures of Atomic Theory, along with his Law of Partial Pressures. This law, which he had no evidence of, was not accepted until 2 centuries later. Dalton conjectured this based on faith on the simplicity of nature, however, this was necessary for other experiments, as molecular composition is necessary for atomic weights and other properties to be calulated. As for the shape of atoms, he largely agreed with Democritus' sphere. -
The Atom Splits
Henri Becquerel, in his experiments on x-rays and light, placed photographic plates and uranium salts in a desk drawer. He was surprised to note that the uranium was emitting strong radiation independent of external influences. He also noticed that the radiation discharged electric bodies, leading to an electric current in the surrounding air. This showed that the atom could change form to emit amounts of energy. Therefore, atoms were divisible, disagreeing with many previous scientists. -
Becquerel's 3 Major Contributions
- Becquerel showed that beta particles, a component of electric and magnetic radiation, was identical to the recently discovered electron by Thomson, using deflections of it on uranium.
- The active medium in radioactive elements (in uranium, uranium X) lost its radioactivity over time, while the uranium, though inactive when prepared, eventually regained it.
- Becquerel's report on the burn caused by carrying radium in his pocket led to medical precautions in using radioactive elements.
