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Evolution of Atomic Theory

by

Rachel Todd

on 12 April 2013

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Transcript of Evolution of Atomic Theory

Evolution of the Model of the Atom By:Rachel Todd Democritus 400 BC Model at the Time John Dalton 1803 English chemist, meteorologist and physicist J. J. Thomson 1897 (1856-1940) Ernest Rutherford 1911 Niels Bohr 1913 Danish physicist Erwin Schrodinger 1926 James Chadwick 1932 (1891-1974) Contribution: Werner Heisenberg 1932 (1901-1976) Contribution: German theoretical physicist New Model Dalton's depictions of atoms and molecules using his own symbols Thomson proposed a 'plum pudding' model. He added to Dalton's model and ideas that the atom is an unbreakable sphere, saying that there was positive and negative charge filling the sphere. This model of the atom showed that most of an atom is positive and that this positive matter is distributed throughout the sphere. The ray’s attraction to the positively-charged plate led Thomson to believe that a cathode ray must consist of negatively charged particles. These negative charges he called electrons and are thought of as the plums in the plum puuding. There are enough negative 'plums' distributed throughout the sphere to neutralize the charge of the particle (since atoms are neutral). This model did not include a nucleus, protons, or neutrons. Also, the atom has no empty space according to Thomson. Thompson's Model Because some alpha particles (which were positively charged) curved away, Rutherford reasoned that some part of the atom was positive since like charges repel. This part of the atom he called the proton. To explain the particles that passed straight through the gold foil atoms, Rutherford reasoned that most of the atom is nothing--empty space. Since only a few bounced nearly straight back, Rutherford reasoned that they must have hit a larger, more massive particle(s) that was big enough to send the alpha particles flying back. This massive particle would be in the center of the atom in Rutherford's planetary model of the atom. This model placed all the protons in the nucleus and all the electrons orbiting around the nucleus like planets around the sun. The mass of atom in this model is very condensed, and all of mass in the center is orbited by negatively-charged electrons. used the Bohr atom model and took it one step further using mathematical equations to describe the likelihood of finding an electron in a certain position quantum mechanical model
the quantum mechanical model does not define the exact path of an electron, but rather, predicts the odds of the location of the electron
can be portrayed as a nucleus surrounded by an electron cloud. Where the cloud is most dense, the probability of finding the electron is greatest, and conversely, the electron is less likely to be in a less dense area of the cloud. Introduced the concept of sub-energy levels. bombarded beryllium atoms with alpha particles, producing an unknown radiation Chadwick interpreted the radiation as being composed of particles with a neutral electrical charge and the approximate mass of a proton

he had discovered the neutron Chadwick's Atomic Model he calculated the behavior of electrons and other subatomic particles

helped clarify the modern view of the atom because scientists can compare atoms by their movements of electrons and how many electrons an atom contains

clarified that surrounding the outside of an atomic nucleus is an electron cloud, which is a name given to the electrons that are widely spreading and moving around included quantum mechanics, the branch of mechanics, based on quantum theory, used for interpreting the behavior of elementary particles and atoms Heisenberg's Atomic Model Original Model People and philosophers of Democritus's time believed that everything was made up of four elements- fire (Heraclitus), air (Anaximenses), earth (Empedocles), and water (Thales). Everything could also be broken down infinitesimally--there was no limit to how small something could broken down to. It was also a common belief that empty space simply couldn't exist. However, like Democritus, these other philosophers had no experimental data to base their ideas and theories off of. Democritus was not a scientist--he didn't conduct experiments. Rather, he conjured up ideas in his mind. He thought through experiments instead of actually conducting them. One version of the story tells of a time when Democritus was in prison. His sister brought him a freshly-baked loaf of bread. When Democritus began smelling the bread from all the way across the cell, he began pondering once again what stuff was made of. How was it possible that he could smell something that was so far away? After much thought, he reasoned that everything was made up of tiny things he called atomos, or atoms. In another version of the story, he asked [himself], "If I use a very sharp knife, can I cut a stone in half indefinitely?" Democritus reasoned that no, there came a point that matter could not be divided into two any more--this he called the atom, a Greek term meaning indivisible. Democritus worked with his mentor Leucippus to expand their atomic theory so that it helped explain the way the world functioned and existed. Louis de Broglie 1932 Max Planck 1932 Albert Einstein Robert Millikan Max Planck Though varying in size, these indivisible, solid, homogeneous, indestructible particles he thought were all spherical and always moving. In other words, everything was made up of little round BBs. Between these atoms, the philosopher believed there was empty space, an idea that contradicted any other theory of his time. Because Democritus didn't produce evidence to support his idea and was unable to answer many questions about his idea, the more popular idea of fire/air/earth/water was believed for the next 2,000 years. New Model Experiment father of the electron uncovered the relationship between the energy and the frequency of radiation. Furthermore, Planck had successfully developed the idea of the quanta, which he defined as a discrete unit of energy, Original Model Experiment At the time, the model of the atom was much like a BB. An atom was thought of as being a tiny, solid sphere. J.J. Thomson is known for his cathode-ray tube experiment. In this experiment, he used a tube filled with a gas at very low pressure (the cathode ray tube). Two metal plates attached to a high-voltage source were placed at one end of the tube. A screen that produced a bright light when the ray hits it was placed at the other end. The cathode (the negatively charged plate) emitted an invisible ray when the power source was powered on. This ray was drawn towards the anode (the positively charged plate) because opposites attract. There was a hole at the center of the anode through which this invisible ray would continue through. After traveling the length of the tube, the ray would hit the coated screen, creating a bright light at the point of impact. Thomson went on to apply an external electric field at a certain point in the tube. Two oppositely charged, metal plates were placed at the top and bottom of the outside of the tube. Now, the beam would pass through the hole in the anode, through the electric field, and then hit the screen. When the ray was emitted again, it was attracted to the positively-charged plate and created a fluorescent light that was not in the middle of the screen. Instead, it was on the same part that correlated with the placement of the positively charged plate. In other words, if the positive plate was put on the bottom of the tube, the ray hit the bottom of the screen, and vice versa. New Model Dalton realized that certain gases could only be combined in certain proportions. This was also the case even when two compounds were comprised of two common elements or groups of elements. Dalton's model of the atom was how Democritus viewed the atom. It didn't include protons, neutrons, or electrons. His model can be thought of as a sugar cookie model. If you cut it up, it’s dough throughout. Dalton stated that the Law of Conservation of Matter and the Law of Definite Composition could only be explained by assuming that matter is composed of atoms that can’t be broken. In other words, atoms can be described as whole numbers. This is where his new law, the Law of Multiple Proportions, comes into play. Rutherford tested Thomson's hypothesis by devising his "gold foil" experiment. Rutherford reasoned that if Thomson's model was correct, the mass of the atom was spread out throughout the atom. Also, if he was to shoot high-velocity alpha particles at an atom, there would be very little to deflect the alpha particles. He decided to test this with a thin film of gold atoms. Most of the alpha particles went right through the gold foil, but a few of the particles rebounded almost directly backwards. Others even curved away from the gold foil. Rutherford began his experimentation with the mindset that the atom was like chocolate chip cookie dough. There is no empty space in atoms. The mass of the atom is positive and equally distributed throughout, and negative little particles were scattered throughout this mass so that the total electron charge of the atom is zero. In other words, he started with Thomson's plum pudding model. Original Model Experiment New Model Experiment
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