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450 BCE
Empedocles' Proposal on The Four Elements
In approximately 450 B.C, Greek Scholar, Empedocles, proposed a theory that suggests that all matter is composed of four "elements", earth, air, fire and water. The theory suggests that these elements mixed together is the base of all substances. Being one of the earliest models of matter, it led to new discoveries for humanity. For example, Empedocles' experimenting demonstrated that air is indeed "something", although invisible, because it takes up space, meaning it must be a form of matter. -
400 BCE
Democritus Suggests The Atomic Theory
In approximately 400 B.C, Greek scholar, Democritus, proposes a theory that all matter was made of tiny particles that could not be broken down any further. These particles were named atoms, which comes from the Greek word "atomos" meaning indivisible. This was a very revolutionary concept at the time, as it goes against the previous model from Empedocles, as this theory is an atomic theory while the previous model was elemental. This is theory is used today to describe matter and substances. -
350 BCE
Aristotle Brings The Revival of Empedocles' Theory
In around 350 B.C, the Greek philosopher, Aristotle, was a believer in Empedocles' theory that suggested that all substances were made of air, earth, water, and fire, despite the latest developments Democritus suggested with the atomic theory. He was an extremely influential writer, so much so that his writings on Empedocles' theory sparked a revival of the theory's acceptance, which lasted for approximately 2000 years. -
Period: 500 to
The Alchemist Theory on Growth in Metals
In these times, many alchemists believed that metals were capable of evolving, like plants. For example, they performed numerous experiments on cheap metals such as iron and lead, attempting to turn them into gold. These alchemists also believed in the Empedocles four-element theory of 450 B.C rather than the atomic theory. While this proved that it is indeed impossible to make gold from lead, we did gain some new developments, such as lab equipment & the division between elements and compounds. -
Boyle Gives "Element" a New Meaning
In 1650, English scientist, Robert Boyle, who did not believe in the "four-element theory", proposed to give the word "element" a different definition. His version of the word described pure substances that cannot be broken down chemically into simpler substances. This was a huge development in humanity's knowledge on matter, as we can now have a specific category that we can classify certain substances in. Boyle, with this theory in mind, also believed that air is not an element but a mixture. -
New Findings on Gas
In the 1700s, oxygen was finally isolated scientifically by Joseph Priestly, which lead to Antoine Lavoisier classifying it as an element. Lavoisier also concluded that air must be a mixture of two elements, one being oxygen. Meanwhile, experiments of mixing metal with acid, performed by Henry Cavendish, resulted in gasses lighter than air, what we know today as hydrogen. He later discovered that it burns in some of Priestly's oxygen to create water, proving that water is not an element. -
Dalton's Atomic Theory For Matter
In 1808, when the idea that matter was made of elements was generally accepted, English chemist, John Dalton, published a theory on the difference between elements and other non-elements. This theory included that;
-All matter is made of atoms, particles that are too small to see
-Each element has its own type of atom, with its own specific mass
-Compounds are created when atoms of different elements link together to become molecules
-Atoms can't be created, destroyed, or subdivided chemically -
Filling The Gaps in Dalton's Model
Dalton's model had some questions unanswered, leaving out matter's ability to generate positive and negative charges. This led to a modified model, with negatively charged particles that could be separated and shared to other atoms. Michael Faraday then finds that electric current could cause chemic changes in some compounds in solution, so;
-Matter must contain negative and positive charges
-Opposite charges attract and like ones repel
-Atoms link to form molecules due to electrical attractions -
Thomson Develops The Model Further
In 1904, J.J Thomson makes a new model to include his discovery of very light negative particles, electrons. Also, he experimented with beams of much heavier positive particles, what we know today as protons. Thomson's model was named the "raisin-bun" model;
-Atoms contain particles called electrons
-Electrons have a small mass and negative charge
-The rest of the atom is a sphere with a positive charge
-The electrons are embedded in this sphere, so the resulting atoms are neutral or uncharged -
Rutherford Creates The Nuclear Model
In 1911, Ernest Rutherford of McGill University experimented the previous "raisin-bun" model. He aimed a radiation named alpha particles, (very small positively charged particles) at a thin sheet of gold foil. The majority of the particles passed through, as expected, however, a small amount bounced back. Leading to the nuclear model, stating;
-An atom has a small, dense, positive core called the nucleus
-The nucleus is mostly surrounded by emptiness, containing rapid moving negative electrons
