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400 BCE
Democritus
He hypothesized that a point would be reached at which matter wouldn’t be able to be cut into smaller pieces anymore. He called the pieces of matter “atomos,” which means uncuttable in Greek. This created the basis of the atomic theory, by creating the structural element of the atom for the future to construct upon and develop. He believed that atoms were even/uniform, indestructible, incompressible, solid, and that they moved in infinite numbers through empty space until they were stopped. -
Period: 400 BCE to
Evolution of the Atomic Theory
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Law of Conservation of Mass
Perhaps one of the most well known laws there are, the Law of Conservation of Mass states that mass is neither created nor destroyed. This law was proven when Lavoisier placed a candle inside a sealed glass jar and when he melted the candle, he saw that the mass remained the same. This law is important because it helped scientists in the future realize that substances did not just appear as a result of a reaction, but instead the reactants just became a substance of equal mass. -
Law of Definite Proportions
Stated by Jeremias Benjamin Richter and also called Proust’s Law or the Law of Constant Composition, it states that every chemical compound has fixed and constant proportions of its component elements. It was important because it gives detail about the proportion of elements in a compound not concerning itself with the weight of it’s source. -
Dalton’s Atomic Theory
All matter is made of tiny particles called atoms. Atoms of given elements are identical in size, mass, and other properties. Atoms for different elements are different in mass, size, and other properties. Atoms cannot be divided, created, or destroyed. Compounds are formed from a combination of two or more different atoms. A chemical reaction is a rearrangement of atoms. This has advanced our knowledge of the atom by setting the basis for the laws that would be changed. -
Law of Multiple Proportions
This law was important because it confirmed Dalton’s theory that atoms are indivisible atoms, and it laid the basis for chemical formulas created for compounds. This law says, “When two elements combine with each other to form two or more compounds, the ratios of the masses of one element that combines with the fixed mass of the other are simple whole numbers.” -
John Dalton
John Dalton’s Atomic Theory states: 1. All matter is made of very small particles called atoms; 2. Atoms of given elements are identical in size, mass, and other properties; 3. Atoms for different elements are different in mass, size, and other properties; 4. Atoms cannot be subdivided, created, or destroyed; 5. Compounds are formed from a combination of two or more different atoms; 6. A chemical reaction is a rearrangement of atoms. This would later contribute to the periodic table. -
Dmitri Mendeleev
Dmitri Mendeleev created what would become the basis for the modern-day periodic table (a table that would classify all the elements), and he would arrange them into categories according to their atomic weight. He also created the periodic law, which stated that certain properties of elements would occur periodically when arranged by atomic number. -
Henri Becquerel
Henri Becquerel was the man who discovered radioactivity, seemingly by accident. He discovered spontaneous radioactivity when he saw uranium salts emit a radiation that could be registered on a photographic plate. This radiation sets a whole basis on all theories from radiation. He hypothesized that uranium would have absorbed light and then reemitted it as x-rays. -
Cathode Ray Tube Experiment
Invention of the electron when using a Crookes for Thomson to see if by bending the rays with a magnet, you could separate the charge from the rays. He found out that when the rays entered the slit in the cylinders, the electrometer there measured enormous amounts of negative charge. He did this experiment by using 2 electric plates in the middle of the tube, with an anode and a cathode. This discovery was huge because it allowed future scientists to change their depiction of the atomic model. -
J.J. Thomson
J.J. Thomson found that atoms were made up of even smaller components, and with his cathode ray tube he found out about the existence of electrons, which were negatively charged particles. He proposed the plum-pudding model, which was the representation of negatively charged particles in a positively charged pudding. His work led to the invention of the mass spectrograph, which would find out the colors of visible light based on the photons bouncing off metal. -
Marie Curie
Marie Curie was a very important scientist that discovered Polonium (500x the radiation given off by any other element) and radium (900x the radiation given off by Polonium) and she deduced that radioactivity doesn’t depend on atoms being originated into molecules, but it actually is originated from the atoms itself. -
Plum Pudding Model
The plum pudding model depicts electrons as the negatively charged particles in a sea of positive charge (protons were not yet discovered). The Pudding Model is an important step in the Atomic Theory because it shows that the atom was a non-inert, divisible mass, and it incorporated the existence of the electron. Other experiments were based off of this model as a basis. -
Oil Drop Experiment
Performed by Robert A. Millikan and Harvey Fletcher, the oil drop experiment was done to measure the fundamental unit of electric charge (electron). The charge that they discovered was 1.5924(17) x 10^-19 Coulombs. They found this out by measuring the electric field strength that was needed to stop continuous oil drops from falling. This is important to the atomic theory because finding the charge of an electron is important because they established the quantization of electric charge. -
Robert Millikan
Robert Millikan designed and fine-tuned the experiments which confirmed a lot of the theories proposed on the atomic theory during his time. His oil drop experiment confirmed the existence of the electron, which was said to be the infinitesimally miniscule particle that conveyed electricity. Robert Millikan successfully determined the precise magnitude of the electron’s charge, and the electron’s fundamental charge were simple multiples of a single integer. -
Ernest Rutherford
Ernest Rutherford told us that the atom consisted mainly of empty space, with almost all of the atom’s mass located in the small nucleus in the center. He said that the nucleus was a dense core that was positively charged, and that electrons orbited around the center like planets around the sun. He also discovered/named the alpha particle (helium nuclei) and beta particles, as well as alpha rays and beta rays, which set the laws of radioactive decay. -
Gold Foil Experiment
Rutherford’s Gold Foil Experiment showed that the atom is made up mostly of empty space with a tiny nucleus, and they proved this by bombarding alpha particles (positively charged nuclei) through thin sheets of gold foil. Proving this allowed him to make the nuclear model of the atom which was then able to be modified with quantums and other theories for the future of the atomic theory. -
Rutherford Model
The Rutherford Model depicted negatively charged particles surrounding and orbiting the nucleus on set circular paths Rutherford named orbitals. The electrons orbit on these paths at high speeds. The model also showed the atom as containing a highly dense, positively charged core called a nucleus, where all the mass was concentrated. This model showed that the plum pudding model was incorrect and reestablished the atom model that we call the planetary model. -
Niels Bohr
Niels Bohr was the man who proposed a theory for the hydrogen atom, which had the basis set on quantum theory where only physical quantities only take discrete values. This basically means that electrons have specified distances between the nucleus and themselves at life, which were the “quantified” distances. If electrons jump to a lower energy orbit, the energy is sent out as radiation. He created the Bohr model where he represented his theory of orbitals and quantums. -
Bohr Planetary Model
The Bohr’s planetary model showed that electrons can be moved into different orbits around the nucleus by adding energy, and when the energy is removed the electrons fall back down emitting a corresponding amount of energy. This was important because it set the basis for the quantum theory, and it showed how we can determine the element’s properties by the number of electrons in the valence shell. It also describes most of the ideas in the Atomic Theory accurately, with only some errors. -
Erwin Schrodinger
Erwin Schrodinger theorized that particles of matter have a dual nature and electrons within atoms can be explained as if you were treating them mathematically as matter waves. This is called wave-particle duality, and he formulated the equation which accurately calculated the energy levels of electrons in atoms. -
Quantum Mechanical Model
This model is a modification of the Bohr Model by Erwin Schrodinger, and it showed us the likelihood of finding electrons in a certain position by using mathematical equations. This was shown to describe the behavior of electrons in a hydrogen atom, and it was important because it specified the probability of finding an electron in the 3D space around the nucleus based on his equations. -
James Chadwick
Chadwick proved the existence of neutrons within the nucleus, and proposed they were particles that did not have any electrical charge. James Chadwick made the Chadwick Atomic Model, which showed us the protons and neutrons bound together in the nucleus, with electrons orbiting the nucleus from a specified quantum away. This in turn said that the mass of the nucleus consisted of neutrons and protons. -
Murray Gell-Mann
Murray Gell-Mann observed that the subatomic particles are composite, which means that they were composed of even smaller particles called quarks. He did this using the 8-fold way using gasses to experiment upon. Murray Gell-Mann also found gluons, which are what hold quarks together in the subatomic particle.