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Evolution of the Atom

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Rachel Todd

on 12 April 2013

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Transcript of Evolution of the Atom

Evolution at Its Finest: Tracing the History of the Atomic Model By: Rachel Todd Democritus 400 B.C.E. Model at the Time 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. Experiment 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 John Dalton 1803 J.J. Thomson 1897 Model at the Time Experiment New Model 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. Thomson proposed a 'plum pudding' model after combining the evidence h had found with the evidence that Millikan proposed. 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 pudding. 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. Robert Millikan Ernest Rutherford 1911 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. Rutherford fired positively-charged alpha particles at a 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 as they passed through Rutherford was a student of Thomson who didn't completely agree with his teacher. He did begin 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. New 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. James Chadwick 1932 New Model Erwin Schrodinger 1926 Model at the Time Model at the Time Dalton began his experimentation after___. The law of conservation of mass had already been developed at this time by Antoine Lavoisier. This law states that mass cannot be created nor destroyed. The law of definite composition had also been developed by Joseph Proust. This law stated that a compound is made up of the same elements in the same ratio by mass regardless of the source or size of the compound. Simply put, stuff is stuff. Experiment Dalton conducted many experiments with gases. These included experimentation with the constitution of mixed gases, the pressure of vapors at various temperatures, evaporation, and thermal expansion with gases. While trying to explain his law of partial pressures and working with nitrogen oxides, Dalton realized that oxygen combined with nitrogen in a 1:1.7 ratio and a 1:3.4 ratio. He then published his new law, the law of multiple proportions. This law states that when different compounds are formed by the same two elements, different masses of one element combine with a fixed mass of the other in small whole number ratios. This law laid the foundation of his own atomic theory. He continued to conduct many chemical reactions and made careful observations. Once he had collected a substantial amount of data to support his law of multiple proportions, Dalton came to the conclusion that the three laws concerning mass and compounds (law of conservation of mass, law of definite composition, and the law of multiple proportions) simply could not be supported by experimental data unless matter was thought of as being made up of tiny, indivisible particles that Democritus had dubbed atoms.' New Model Dalton conducted many experiments that contributed toward his atomic theory. Dalton's atomic theory states that matter is made up of tiny, indestructible particles called atoms. He believed that these atoms of the same element were identical. In other words, gold is gold down to the last atom. Also, his theory states that different elements can be told apart by their atomic weights and that atoms of elements are what combine to form chemical compounds. However, in these combinations, atoms are never destroyed--instead, they are grouped differently. Like Democritus' model, Dalton's model can be thought of as the BB model. 1909 Model at the Time Model at the Time Model at the Time Model at the Time Model at the Time Model at the Time Model at the Time Experiment New Model Millikan is most famous for his oil-drop experiment. This
experiment was conducted by trying to balance the force of gravity
with the buoyant and upward forces that are exerted on a drop of oil
between two metal plates inside a closed cylinder. These two plates were
used to create a uniform electric field. Holes were cut into the side of the
cylinder to place a microscope in the side along with an 'atomizer' and a source
of ionizing radiation. The atomizer was a spraying device located toward the top of the cylinder that released oil droplets into the cylinder and onto the top, positively-charged plate. This top plate had a small hole drilled into it to allow droplets of oil to fall into the space between the two charged plates (the electric field). The source of ionizing radiation would beam out x-rays. These x-rays would knock electrons from the particles in the air. The freed electrons would then stick to the oil drops, giving them a negative charge. By altering the intensity of the electric field, Millikan controlled the rate of the droplet's fall. -28 By determining and calculating the drag on the drop,
the true weight of the drop, the upthrust on the drop,
the voltage of the drop, and the force of gravity on the drop, Millikan determined that the charge of the electron was 1.602 x 10 coulombs, which Millikan equated with a -1. Also, the mass of an electron was determined. Millikan calculated the mass of an electron to equal 9.1 x 10 g, which is 1/1840 the mass of a hydrogen atom (the lightest known element). Due to the extremely small size of the electron, Millikan realized that there must be other 'stuff' that held most of the mass of the atom that was positively charged. This opened a door for the scientific community to hit their lab benches and attempt to
crack the code of the atom. -19 Experiment Experiment New Model Chadwick began his experimentation with the belief that atoms were made up of two types of subatomic particles--positively-charged protons and negatively-charged electrons. The protons were thought of as being condensed together at the center of the atom, making up a positive nucleus. After finding problems with how the atomic mass compared with the atomic number in the current model of the atom, Chadwick set out to find where the missing mass of the atom was hiding. After analyzing the data he had collected from all of the trials of his experiment with polonium, Chadwick determined that the particles released from the Beryllium after the alpha particles hit the opposing size were uncharged. He also determined that these uncharged particles had nearly the same mass as a proton. This new particle Chadwick called the neutron. He
then went back to Rutherford's model of the atom and made an adjustment--he depicted the nucleus of the atom, which
held 99.99999% of atom's mass, to contain both
protons and neutrons. Chadwick is famous for his experiment in which he bombarded Beryllium with alpha particles. He began with the source of the alpha particles. These positively-charged particles then passed through Beryllium, which is known as a 'light element.' The beryllium then ejected particles (that Chadwick later determined to be neutrons) that continued on to hit a piece of paraffin wax. The paraffin wax ejected protons out the other side, and these particles then entered a proton chamber that detected and counted the positively-charged particles as they entered the chamber. Experiment New Model Max Planck early 1900s Experiment New Model Einstein began his work with Chadwick's planetary model in mind. Chadwick's model included a nucleus made of neutrons (which has a neutral or no charge) and protons (which has a positive charge) with electrons (which have negative charges) surrounding the outside of the nucleus. Einstein was also familiar with Plank's work and resulting theories. Planck's work included the idea of a quantum (which is accompanied by Planck's constant), waves and their characteristics. Einstein had also had success with his theory of relativity before he delved into the atomic world. However, he had found a problem with the wave model of light and the photoelectric effect that had been observed by so many other scientists. Louis de Broglie Model at the Time Thomson began with Dalton's model of the atom, also known as the BB model. Atoms were thought of as indestructible spheres of matter. As Millikan began his oil drop experiment (among others), the atom was popularly depicted as plum pudding. Positive mass (the pudding) took up most of the space, but negatively-charged electrons (the plums) were stuck throughout the positive mass so that the total charge of the atom was neutral. Thanks to the 'fix' that James Chadwick did to Rutherford's planetary model,
the atom was now depicted as a nucleus surrounded by negatively charged
electrons. The nucleus was thought of as being composed of neutrons (particles that had no/neutral charge) and protons (particles that had a positive charge), both of which were nearly the same mass-wise. Scientists working around Chadwick and Planck's time had been doing work with the electromagnetic spectrum as well as visible light, a part of the EM spectrum. Much was learned around this time about waves and their characteristics. They observed elements that emitted visible light when they were heated. Waves were described using their wavelengths, frequencies, and amplitudes. Wavelength is defined as the shortest distance between two like points of a wave. Frequency is the number of wavelengths that pass a certain point in space during a given time period. These two properties (frequency and wavelength) have an inverse relationship. As on increases, the other decreases. Amplitude is the height of the wave and is measured from the origin to the crest (highest point of a wave) and is a measure of the energy contained in the wave. At the time light was thought of as a wave. However, the wave model of light did not explain why objects, when heated, only gave off certain frequencies (colors) of light
at a given temperature. Albert Einstein early 1900s Planck is not known for a specific experiment he conducted
to contribute to our current understanding of the atom. In fact,
most if not all of the experiments Planck conducted were
conducted during his years at the University of Munich. Instead of conducting experiments, Planck went on a hunt for a reason to explain the emission of various wavelengths by heated objects. Though he did reach a conclusion that explained these phenomena, it was one that was radical and wasn't easily excepted in the scientific community. His theory stated that matter can only lose or gain energy in small, specific amounts of energy that he called quanta. In other words, a quantum is the smallest amount of energy that an atom can gain or lose. He also established a mathematical relationship between the energy of a quantum and the frequency of the radiation that is emitted when an energy change occurs.
In this mathematical relationship, Planck used Planck's constant, which is 6.626 x 10 J s. Experiment Shrodinger, much like Democritus, conducted 'mind experiments.' However, he also used math equations. One of these that he used in his 'experiments' employed deBroglie's equation. This he combined with the known characteristics of waves. He described the electron using the ideas within quantum mechanics instead of Newtonian mechanics. Shrodinger developed an equation that helped determine where the electron had the highest probability of being. It does attempt to predict quantized energy levels of electrons as well. Schrodinger's model uses what are known as wave functions to predict the probability of various electrons. His model is based on the wave characteristics of electrons. Unlike many of the other previous models of the atom, Schrodinger didn't try to tell where exactly each electron is. Instead, he developed an equation to determine the probability of finding an electron in a given region of space at any time. This model only tells us where the electron might be. Another alteration to the atomic model that Schrodinger did was make it three-dimensional. Schrodinger's equation could be used to calculate the probability of finding an electron(s) in all three axes--x, y, and z. Schrodinger used three quantum numbers to help describe the possible location of the electrons--n, l, and m. These quantum numbers describe the size, shape, and orientation in space of the orbitals in an atom. . -34 The model of the atom didn't change very much by the end of Planck's hunt. The nucleus was still the middle of the atom and was still made up of neutrons and protons. His two formulas were Energy of a Quantum=hv. H is Planck's constant, and v represents requency Electrons still floated around the nucleus. According to his new ideas, an atom can only emit or absorb energy in multiples of hv's (1hv, 2hv, 3hv, etc.). In an easier understanding, a person can only build a wall will whole blocks--not 3/4 of a block or 1/2 a block. The formula states mathematically that the energy of radiation and the frequency of the radiation is directly related. As one increases, the other increases as well. In an attempt to choose a different take on the idea of
light to help explain the photoelectric effect, Einstein proposed the idea of light having a dual nature. It acted as both a particle and a wave. If an experiment was conducted based on the wave properties of light, the light would act as a wave. However, if an experiment was conducted based on the particle properties of light, the light would act as a particle. These particles, or bundles of light, Einstein called photons. These photons are mass-less and carry only a quantum of energy. Using this information, Einstein proposed the formula relating the energy of a photon with the frequency. This mathematical equation is: Energy of a Quantum=hv. H is again Planck's constant, and v is representing frequency. Along with the equation to solve for the energy of a photon, Einstein proposed the idea that there is a threshold value of energy that a photon must have before it can cause the ejection of a photo-electron from the surface of a metal. Photons of light that were below the threshold value, no matter how many photons there were, would not cause the ejection of a photo-electron. Einstein's proposal of the dual nature of light helped scientists better understand the photo-electric effect. Also, the photon was a new idea that Einstein developed. Neils Bohr 1922 Experiment New Model When Bohr began his research, he had knowledge of the Planck's theory and equations, Einstein's energy of a photon, the idea of a quantum, wave characteristics, the planetary model of the atom, the atomic emission spectra, and the photo-electric effect. The atomic emissions spectrum is the set of frequencies of EM waves that is given off by an excited atom in order to return to a lower energy state. Bohr took Chadwick's planetary model and modified it to match his experimental data. Instead of electrons just orbiting randomly around the nucleus, Bohr placed them in quantized energy levels. This new model is known as the Rutherford-Bohr planetary model. The nucleus remained as a mixture of tightly-packed protons and neutrons. Electrons are still located outside of the nucleus--they're just in specific rings of travel. When energy is added, they jump from their ground state to a higher energy level. Various jumps cause various releases of energy and various colors of light or even other types waves that are in the electromagnetic spectrum. These jumps are called quantum leaps. However, Bohr's model didn't explain the spectrum of any element except hydrogen. 1923 Werner Heisenberg Planck's theory, the idea of a quantum, quantized energy levels, the atomic emission spectra, photons, the electromagnetic spectrum, wave characteristics, and Bohr's model of the atom were all ideas of popular knowledge when de Broglie began experimenting. Bohr's model states the an atom's nucleus contains protons and neutrons as well as practically all the mass of the atom, that most of the atom is empty space, and that electrons orbited around the nucleus in specific quantized energy levels. Experiment de Broglie reasoned that Bohr's model of the atom was very much based on the wave properties of electrons. With Einstein's dual-nature-of-light idea in mind, Bohr reasoned that particles could have wave-like properties as well. In order to test this, he used the double slit experiment. He started with a wave source. He then passed the wave through two slits and watched as they passed through the two slits and hit a screen behind it that showed where the wave/particles hit. Another explanation of his experiment can be likened to bowling photons through a wall with two slits in it so that they passed through and hit a screen behind. However, the photons seemed to pass through the barrier as if it was a wave instead of a particle. New Model de Broglie added to the theory of the atom by establishing his equation, the de Broglie equation. This equation predicts that all moving particles have wave characteristics. He knew that if electrons had wavelike motion and was restricted to circular orbits that were fixed around the nucleus, only certain wavelengths, freqnecies, and therefore energies would be possible. This is where his equation came in, which is: Lambda=h/mv. Or, wavelength= Planck's constant/mass x velocity. 1927 The atomic emission spectra, Planck's theory, quantum, the electromagnitic spectrum, wave characteristics, Bohr's model of the atom, the photoelectric effect, and the energy of a photon and the energy of a quantum were all known knowledge to Heisenberg. Experiment Bohr worked with the atomic emission spectrum of atoms of elements. He did so by studying light that was emitted by atoms that were blasted with energy. The main one he worked with was hydrogen. He proposed the idea that electrons were originally in their ground state in specific circular levels around the nucleus and that when they were excited they gained energy, jumped levels (quantum leap), and radiated energy. But, when they were in their ground state, they didn't. He named these levels with numbers (n). When the excited electron falls back down, the atom emits a photon of energy that was determined by the energy difference between the levels that were jumped. Heisenberg reasoned that matter and phenomenon weren't observable unless they were changed. In other words, something couldn't be observed in its actual state. For example, by placing a shirt in a position in which you can observe it, you change it. you can't see the shirt without blasting it with light--that's just how our eyes work. However, by blasting the shirt with light, don't we change it? The same idea, Heisenberg reasoned, applies with electrons. New Model With this radical idea in mind, Heisenberg reached the conclusion he called the Heisenberg Uncertainty Principle. This principle states that there is and always will be uncertainty when it comes to the position and momentum of an object. The more you know about the location of an object, the less you can possibly know about the object's momentum. Therefore, the more you know about the electron's momentum, the less we known about its actual location, and vice versa. The atomic emission spectra, Planck's theory, quantum, the electromagnetic spectrum, wave characteristics, Bohr's model of the atom, the photoelectric effect, Heisenberg's uncertainty principle, de Broglie's equation, the dual-nature of electrons, and the energy of a photon and the energy of a quantum were all known knowledge to Heisenberg.
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