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400 BCE
Democritus
Democritus thought that atoms were solid, hard, incompressible, and indestructible. He also believed that they moved in infinite numbers through empty space until they were stopped. Also, the differences in atomic shape and size determined the various properties of matter. -
387 BCE
Plato
Plato introduced the atomic theory. That is when atoms broke down mathematically into triangles. Then the elements had the following shape: fire (tetrahedron), air (octahedron), water (icosahedron), earth (cube). -
334 BCE
Aristotle
Aristotle did not believe in the atomic theory. He believed that all the materials on Earth were made of the four elements, earth, fire, water, and air instead of atoms. He also believed all substances were made of small amounts of these four elements of matter. -
500
The Alchemists
Alchemists were the first to use the word element in reference to chemicals and they also examined matter and its form in a chemical reaction. Those things provided proof of atoms and therefore set a foundation of the contemporary science. -
Lavoisier
Antoine Lavoisier's work in defining the law of conservation of mass would help to shape atomic theory. His discovery had an influence on the atomic theory because it defined that matter was composed of atoms that were not destroyed or created during chemical reactions. -
John Dalton
Dalton's theory was based on the law of conservation of mass and the law of constant composition. The 4 things were
1- Elements consist of indivisible small particles (atoms).
2- All atoms of the same element are identical; different elements have different types of atom.
3- Atoms can neither be created nor destroyed.
4- ‘Compound elements’ are formed when atoms of different elements join in simple ratios to form ‘compound atoms -
Newland Law of Octavates
The English chemist J.A.R. Newlands discovered that if the chemical elements are arranged by increasing atomic weight, those with similar physical and chemical properties happen after each interval of seven elements. -
Mendeleev's Pd. Table
Mendeleev created a periodic table of all the elements that were known at the time. The rows of the table (which are called periods) each had eight elements that increased in atomic mass from left to right. The columns of the table (which are called groups) contained elements with similar properties. -
Photoelectric Effect
The photoelectric effect was discovered in 1887 by Heinrich Rudolf Hertz. Hertz observed that, when ultraviolet light shines on two metal electrodes with a voltage applied to them, the light changes the voltage at which causes sparking to happen. -
Discovery of Radioactivity
When Henri Becquerel was using fluorescent minerals to study the properties of x-rays he exposed potassium uranyl sulfate to sunlight. Next he placed it on photographic plates wrapped in black paper. Then he develop his photographic plates and the images were strong and clear, proving that the uranium emitted radiation without an external source of energy such as the sun. That is how Becquerel had discovered radioactivity. -
Discovery of the Electron
J.J. Thomson discovered the electron in 1897 with a cathode ray, tube. He showed that cathode rays were negatively charged. He also studied positively charged particles in neon gas. -
Plum Pudding Model
J.J Thomson made the Plum Pudding Model which is a model of atomic structure. He discovered that atoms are composite objects made with positive and negative charge. And the negatively charged electrons in the atom were very small compared to the whole atom. -
Planck's Quantum Theory of Light
In Planck's quantum theory, different atoms and molecules can emit or absorb energy in discrete quantities only. The smallest amount of energy that can be emitted or absorbed in the form of electromagnetic radiation (known as quantum). -
Robert Millikan
Robert Millikan discovered the elementary charge of an electron from the oil-drop experiment. He charged droplets of oil between two electrodes and balanced the gravitational force with the upward forces, which he found within one percent of the current accepted value -
Rutherford's Gold Foil Experiment
Ernest Rutherford's experiment is the gold foil experiment in which a beam of alpha particles was aimed at a piece of gold foil. Most of the alpha particles passed through the foil, but a few were scattered backward. That showed that most of the atom is empty space which surrounds a tiny nucleus. -
Bohr's Planetary Model
The Bohr Model is a structural model of an atom and was proposed by physicist Niels Bohr in 1913. In the model, the electrons travel around the nucleus of an atom in distinct circular orbits( also known as shells).The model is also referred to as the planetary model of an atom. -
Mosley's Atomic Numbers
Mosley observed and measured the X-ray spectra of various chemical elements. To do that he used diffraction in crystals and he discovered a systematic relation between wave length and atomic number. -
Discovery of the Protron
The proton was discovered by Ernest Rutherford. His research resulted in the discovery of protons from a nuclear reaction which led to the first 'splitting' of the atom. He named his discovery “protons” based on the Greek word “protos” which means first. -
Charge of the Electron
J.J. Thomson discovered that the charge of a single electron is referred to as the unit electrical charge. It carries a negative charge (that is equal but opposite) to the positive charge on a proton or hole. The amount of electrical charge is usually not measured on a single electron because that amount is too small. -
Schrodinger Equation
The Schrödinger equation is a linear equation that shows the wave function of a quantum-mechanical system. It describes the form of the probability waves (or wave functions) that make the motion of small particles. It also shows how these waves are altered by external influences. -
Heisenberg Uncertainty Principle
Made by Werner Heisenberg in 1927, the uncertainty principle states that we cant know the position and speed of a particle, (photon or electron) with perfect accuracy. -
Discovery of the Neutron
James Chadwick observed that there could be a particle with mass but no charge. He called it a neutron, and imagined it as a paired proton and electron. His experiments were successful and he was able to determine that the neutron did exist and that its mass was about 0.1% more than the proton's.