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400 BCE
Democritus
He was a Greek philosopher who theorized that everything in life is made up of atoms: the basic building blocks of matter. He explained that atoms are unchangeable, indestructible, and have always existed. -
300 BCE
Aristotle
He explained that all matter is composed of four basic elements. Fire, earth, air, and water. -
Robert Boyle
He is known for “Boyle’s Law”. The law is the discovery of how the volume of a gas decreases as pressure increases, and vice versa. When air is added to an enclosed space, the gas molecules inside get compressed and packed closer together. -
Lavoisier
He is known for his discovery of the law of conservation of mass. Total mass of products and reactants in a chemical reaction will forever be the same. -
The Alchemists
They broke down the chemical compositions into four basic elements: Fire, Earth, Wind, and Water. They laid down the modern foundation for what would become the Periodic Table later on. -
John Dalton
He created the atomic theory. He explained that all matter is made of atoms: the building blocks of life. Atoms can neither be created nor destroyed. All atoms within an element are identical, differing elements have atoms differing in size and mass. He also created the Solid Sphere of "Billiard Ball" Model to represent an atom. -
Plato
He used Empedocles to support his theory. He believed these five fundamental solids must be the source of all atoms of matter. At the time, he referenced his knowledge from the Greek parodic table which included earth, water, air, fire, and finally quintessence. -
Dmitri Mendeleev
He wrote down every symbol for the chemical elements and put them in order according to their atomic weights. Doing this process, he created the periodic table. -
J.J. Thomson
His experiments using cathode ray tubes proved that all atoms contain negatively charged subatomic particles: electrons. The cathode ray tube was a vacuum-sealed tube where electrons flow from the cathode to the anode and then finally pass through to create a beam. He created the "Plum Pudding" model to support his ideas. -
The Curies
They discovered the two very radioactive elements polonium and radium. Both elements occur naturally in uranium minerals. They worked with radioactivity. -
Photoelectric effect
This proved that light has particle like activity. It confirmed that the atomic structure of an element is related to the electromagnetic radiation from the Photoelectric effect. The wavelength of the effect determines how many electrons are released. This information determines the energy of photons. -
Albert Einstein
He mathematically proved the existence of atoms. His work revolutionized all sciences through statistics and probability. -
Planck’s Quantum Theory of Light
It discovered that the minimum amount of energy could be gained or lost by an atom. -
Robert Millikan
He conducted an analysis known as the oil drop experiment. The experiment helped determine the precise magnitude of the electron’s charge. Small electrically charged drops of oil were suspended between two metal plates. The plates were then subjected to a downward force of gravity and upward attraction of an electrical field. Obtaining information such as this, helped towards obtaining greater understanding of the atom. -
Ernest Rutherford
He discovered that atoms have a tiny, charged nucleus that is surrounded by a large empty space within the atom, and have small electrons circling around the atom’s nucleus. He shot alpha particles at gold foil to see if it would go through the particles of the foil. The alpha particles ended up being scattered. His theory was proven by the result of the experiment. This model would be known as the Gold foil experiment. -
Neils Bohr
He created a model that was able to give a visual representation of an atom, and how to identify its properties. The atom has a small positively charged nucleus that is surrounded by orbiting electrons. The orbiting electrons travel around in separate orbits. By seeing the number of electrons, Bohr concluded that the number of outer orbiting electrons would help determine the properties of an element. He created the Solar system model to support his beliefs. -
Henry G. J. Mosely
He created Mosely’s law. The discovery of atomic numbers. He concluded that the atomic number of the atom is the number of protons within the nucleus. -
Schrodinger Equation
Erwin Schrodinger came up with an equation that could mathematically get the precise energy levels of electrons in atoms. He also was the creator of the electron cloud model that helped support his beliefs. -
Werner Heisenberg
He contributed to the atomic theory by formulating quantum mechanics. He explained that there is no possible way to know exactly what position or momentum particles will be in -
James Chadwick
He discovered the neutrons. Particles that are in the nucleus with no charge.
