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450 BCE
Empedocles' Theory
Empedocles (a Greek scholar) introduced his theory that everything is made up of "elements": earth, air, fire and water. Depending on how one combined these elements, different substances would be created. -
400 BCE
Atomic Model
The idea that matter is made up of tiny particles was first brought to light by a man named Democritus. Democritus' theory was that matter was made up of tiny particles that could not be broken down into simpler substances. He called these particles atoms, deriving from the Greek word "atomos", meaning "indivisible". Different elements made up of different atoms was a revolutionary theory at the time. -
350 BCE
Aristotle Believes in Empedocles' Theory
All the philosopher Aristotle had to do to get people to believe Empedocles' Four-Element Theory was to believe it himself. He influenced so many people in his writings that Empedocles's Theory was accepted for almost 2000 years. -
Period: 500 to
Alchemists Experiment
For centuries, alchemists experimented to find an answer to their question: do metals grow like plants, ripening into gold? To test this, they attempted to turn cheap metals into gold. While experimenting, they discovered many new substances, and developed chemical symbols, (recognized today as symbols for elements and compounds), as well as lab equipment that is in use today. After all of this, they still accepted the four-element model, finding no way to turn cheap metals into gold. -
Elements
Unfortunately for Empedocles, scientist Robert Boyle did not believe Empedocles' "four-element" Theory. Boyle gave a new definition for an element, stating that an element is a pure substance that cannot be chemically broken down into simpler substances. If we look back at Democritus' Atomic Theory, this means that an element is made up of only one type of atom. -
Oxygen, Hydrogen and Water
Oxygen was first isolated by Joseph Priestly in the 1700s. It was later discovered to be an element by Antoine Lavoisier while he was experimenting with oxygen. Finally, Henry Cavendish found that water isn't really an element after he mixed oxygen with one of his discoveries: flammable gas that was lighter than air. (Later realized to be hydrogen.) Water was produced from the mixture. -
Dalton's Theory
John Dalton came up with a theory to explain elements and how they differ from each other. Parts of his theory can be recognized today as the Particle Theory, such as the part of Dalton's Theory when he states that all matter is made up of atoms (particles) with their own masses. -
Negatively and Positively Charged Particles
In 1831, the idea that negatively and positively charged particles exist was introduced. Micheal Faraday realized that an electric current could run through atoms, turning them into charged atoms, which he called "ions". This could also affect the atoms chemically. Because of these discoveries, changes were made to Dalton's Theory: matter must contain positive and negative charges, opposite charges attract and like charges repel, and so on. -
Electrons and Protons
In 1904, scientist J. J. Thomson began experimenting with light, negatively charged particles, known as neutrons, and heavy, positively charged particles, later known as protons. The earlier theories were updated once again: atoms contain protons and neutrons, an electron is a particle with a negative charge, the rest of the atom is positive. Another scientist, H. Nagaoka, had the same ideas of positively and negatively charged atoms, however he modelled his ideas differently. -
The Nuculear Model
In 1911, Ernest Rutherford experimented with Thomson's and Nagaoka's models by aiming small, positivlely charged particles (alpha particles) at a thin sheet of gold foil. Contrary to his prediction that all the particles would pass through the foil, a few particles bounced back from the foil. To explain this phenomonen, Rutherford came up with the nuclear model: an atom contains a positive core, the nucleus, which is surrounded by space containing fast moving electrons.
