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370 BCE
Democritus theory
He took a stone and cut it until he reasoned that if you continued to cut the stone into smaller and smaller pieces, at some point you would reach a piece so tiny that it could no longer be divided.
To that small piece called it atom, and for him an atom can be divisible and unique because the atom of a stone is not the same as the fur. This theory attempted to explain the whole physical world in terms of a small number of ideas. -
Dalton
He said that matter is made up of tiny particles called "atoms," and that atoms of one element are the same as all of the other atoms of that element, while atoms of different elements were different. He then elaborated to say that a "compound" forms when atoms of different elements combine together, and a chemical reaction occurs when these atoms are rearranged, but the atoms are not affected themselves. Dalton called his atom the "Billiard Ball" model. -
Thomson
He use a cathode ray gun, to pass an electric current through magnetized plates, and found positive & negative charges that change the direction of the electrical current. Because the charges dictated the direction that the current would move, he understood that the atom had to be made up of positive and negative charges. So he called his model "Plum Pudding" model. Because he had the idea that the sphere was positive and the negative charge (electrone) were floating within. -
Rutherford
He conducted the famous "Gold Foil Experiment." In it the particles were directed through a thin slit in a lead screen towards a thin gold foil. Most of the particles passed through as expected, but some were deflected off of the foil, and some even bounced backwards. So he concluded that atoms have to be made up of mostly empty space, there had to be something causing the deflection that is the center now known as the nucleus and is very small compared to the total size of the atom. -
Bohr
He explain why the electrons orbiting around the atom do not lose energy and proposed that the electrons orbit in distinct orbits, but electrons could jump between these orbitals by either gaining energy or by losing energy. He also proposed that the size of the atom was smaller than previously thought, and explained that the electrons orbited the nucleus in concentric circles. His model became widely known as the "Bohr Model." -
Schrodinger
He developed a mathematical formula to explain de Broglie's work. From this new understanding, we came to view the atom as electrons moving in waves around the nucleus. The orbits would then be different sizes depending on the electrons' wavelengths and the electrons would be found in clouds of space around the nucleus. You can calculate the probability of finding it within the given space. Their model is known as the "Quantum Mechanical Model."
