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450 BCE
About 450 BC - Empheodocles
The first observation made by humans on matter, was suggested by Emphedocles, a Greek scholar who suggested that the world was made up of 4 elements: Earth, Air, Water and Fire. He believed matter was made of different portions of each "element". For example, trees could've been considered 1 part air, 2 parts water and 3 parts earth. Emphedocles also proposed that air is a real form of matter and even being invisible, that didnt mean it doesn't exist around us. -
400 BCE
About 400 BC - Democritus
Democritus, another Greek philosopher proposed that all matter was made up of little particles that could never be broken down. After the greek word "invisible", Democritus called the particles, atoms and concluded that they would be in constant motion. That meant that these different atoms would be a step up from the 4 "elements" because humans knew what the elements were made of. Even if this was important historical info, Democritus's theory was not accepted by many people. -
350 BCE
About 350 BC - Aristotle
Aristotle, another historical Greek philosopher and polymath, greatly believed in the four element theory, made by Emphedocles, even though Democritus's theories on atoms and his model were made decades before. He was the reason the four element rule lasted for so long before it was changed, which took thousands of years because of his popular writings on the topic. -
Period: 500 to
500-1600
Chemical symbols were used to show what we call elements and compounds now. Humans were experimenting, trying to see if different types of cheap metal could grow into gold like plants. It was a real theory a lot of people believed, but it was obviously not true. Many important lab tools were invented that are used regularly now, like the beaker, stirring rod, and filter papers. Remarkably, people still thought there were four elements. -
1660 - Robert Boyle
A new definition of the word element was changed as of the year 1660, by an English scientist who did not believe in the four element theory, and he was one of the earliest scientists to believe air was not an element, but a mixture. He called elements a pure substance that could not be chemically broken down, and they were the most basic materials of the planet. -
Late 1700s
In the early 1700s, people still thought that air was an element. Scientists would be studying it, the first being Joseph Priestley. It wasn't until several years later until it was not considered an element, but a mixture of gases that contained oxygen. -
Period: to
1800s - A change to John Dalton's model
John Dalton did not include positive and negative electric charges in his theory. A new model was made, which can explain different atoms having different charges. Michael Faraday found atoms can gain electric charges during 1831. By electric charges, atoms could form Ions, charged atoms.
The new version of Dalton's theory:
- Matter contains a positive or negative charge.
- Opposite charges attract and like charges repel.
- Molecules are created by electrical attractions between atoms. -
1808 - John Dalton
An English chemist, John Dalton published his theory about the differences of elements, and his explanation of non-elements. By 1808, people understood all matter to be made of elements. John Dalton's theory concluded:
- Everything is matter, matter is made of tiny atoms which are particles that are so small we can't see them.
- Each atom has its own unique mass.
- Molecules are formed when at least 2 elements combine to make a compound.
- We cannot create, delete or subdivide atoms. -
1904 - J.J Thompson
J.J Thompson added electrons to the model, particles with a negative charge. Then he later added what we now call protons. The new model was called the "raisin bun". Nagaoka made the model look like a big positive sphere, surrounded by electrons in a ring. The model includes:
- Atoms contain electrons
- Electrons have a small mass and negative charge
- The remaining part of the atom is a positive charged ball
- Electrons are inside the sphere so the resulting atoms are neutral or uncharged. -
1911 - Ernest Rutherford
Ernest Rutherford intended to test Thompson and Nagaoka's model. He made a discovery and made a model called the nuclear model. For testing, he pointed radiation called alpha particles at some thin, gold foil and he expected them all to go through the paper, but some of them bounced back. The model includes:
- An atom contains a dense, tiny, positive nucleus that contains protons. They were deflected
- Around the nucleus are quick moving negatively charged electrons, which went through the foil -
1913 - Niels Bohr
Niels Bohr, a Danish scientist, proposed a theory for the hydrogen atom, that the electrons has a certain prescribed orbit around the nucleus. This was the reason he won a Nobel Prize because of the model he introduced. The Bohr atom is a nucleus surrounded by different orbits of electrons corresponding to their state of energy. -
1932 - James Chadwick
The neutron wasn't discovered until a British physicist, James Chadwick discovered the neutron in 1935. He discovered this by going through the process of striking a small particle with a bigger one, and used "scattering data" to calculate the mass of the neutron, which was a neutral particle. He was an inspiration for the U.S to start research efforts on the atomic bomb.
