Volume 1
Issue 1
November, 2013
The Extra Mass By Alex Brown November 4, 2013 For years people wondered why atoms had such a high mass. The mass of elements were at least double the element’s atomic number. People couldn’t figure out where all this mass was coming from. In 1932, this mystery was solved. The discovery of the neutron answered the question of why atoms were so heavy. James Chadwick was the scientist that made this discovery. He conducted an experiment that proved there were uncharged particles in the nucleus of atoms. These particles were named neutrons. In his experiment, he shot alpha-particles at elements and paraffin wax. Paraffin wax is a substance that has protons on it. When he did this, he observed that what came out had a much smaller mass but still had a positive charge. He came to the conclusion that there must be neutral particles in the elements.
A diagram of Chadwick’s experiment.
Chadwick’s experiment answered a question that people had for years. There was something that was wrong with his conclusion. He believed that neutrons were connected to both protons and electrons. This was later proved wrong. We know now that neutrons are only a part of the nucleus of an atom and are not involved with the electrons. His discovery of the neutron was still a very important one. Because of his discovery, we know where the extra mass of the atoms comes from.
By Aziza Juma November, 2013
WHO WAS RUTHERFORD? English physicist Ernest Rutherford is credited with discovering of the nucleus in the early 1900s, he had a long career in atomic physics, during which he taught many other famous physicists.
WHAT DID RUTHERFORD DO? In one of his most well-known experiments which took place while he was a professor of University of Manchester, the physicist beamed alpha particles at a thin sheet of gold foil. He fired energetic particles to be deflected but most of the particles were not deflected at all. They passed through the foil and most were emerged undisturbed, a few of them bounced back directly. Rutherford’s experiment made him believe that most of the foil particles was made by empty space. The force that causes large deflection had to be large. The positively charged alpha particles were being repelled by a massive positively charged object. If the charge were collected on a sphere size of an atom, the repulsion would be far too weak. To explain the force experienced by the alpha particles would have to be collected in a much smaller. With this experiment, Rutherford discovered the nucleus. Rutherford’s ideas are supported by his gold foil experiment, as he went step by step and being so curious to discover the nucleus. The idea that he expected all the particles to be deflected was disproven, since most were not. IMPORTANT VOCABULARY WORDS Atom- Is the basic unit of matter consist of dense central nucleus. Nucleus- Is the very dense region consisting of protons at the center of the atom.
Andrea Rider
It wasn’t until the year 1919, scientist Ernest Rutherford, had discovered the proton, a positively charged particle within the atom's nucleus. Under Rutherford’s leadership, scientist James Chadwick found that the proton did not seem to be the only particle in the nucleus. He realized that the atomic number (number of protons in the nucleus, same as the positive charge of the atom) was less than the atomic mass (average mass of the atom). There had to be another particle besides the protons in the nucleus that were adding to the mass of the atom.1 In order to find this unknown particle, in 1932, Chadwick used scientists Frederic and Irene Joliot-Curie’s experiment in tracking particle radiation, but with his goal of looking for a neutral particle. This experiment is known as the “Polonium Alpha Source Experiment.” The source only emits alpha rays, and polonium (radioactive metal, atomic #84).2 As the experiment was tested the results showed that Beryllium, when interacted with alpha particles, produced a very energetic stream of radiation. When the stream of radiation hit the paraffin wax, protons were knocked loose which were easily detected. The alpha particles interacting with the Beryllium and Paraffin wax ended up being neutrons.3 The significance of this discovery changed the outlook of the atom and furthered discoveries in atomic physics. Chadwick’s discovery also helped create a nuclear chain reaction, which eventually led to the atomic bomb, and later to nuclear power production! Altogether, it has helped to further enhance studies, develop new understandings, and create a new stance in science.
By: Amra Softic
November, 2013
What are the tiny particles that make up everything in the universe? The atom of course! This is a very complex concept that we become introduced to in grammar school. But how was the atom born? Did a scientist wake up one day and declare the atom? Of course not! The atom evolved to what it is today. A step in this evolution was Ernest Rutherford. Rutherford was born in 1871, and created his idea of the atom in 1911. Rutherford did this using his famous gold foil experiment. He set up a gold foil with detectors all around. When he would shoot alpha particles at the gold foil he noticed that most would pass straight through the foil while others would bounce and hit a detector on the side. The reason for this was the positively charged nucleus. (see figure a) To supply the alpha particles he used an isotope of another element (it emitted alpha particles as it decayed). Why do you care about this? Through Rutherford’s research he was able to take a step towards findings an atoms atomic number, thus organizing the periodic table. This would help scientist assign and understand the different properties of an element.
Genius right? Not quite. Although Rutherford discovered the nucleus and that it was positively charged he didn’t think about neutrons. Neutrons help the nucleus of an atom stick together because it provides space between all the protons, which repel each other (because they have the same charge). They also account for much of the atoms mass, adding to the atomic number. Although Rutherford got the train going he didn’t check all the passengers’ tickets.
Schrödinger was the main scientist responsible for the electron cloud model. Heisenberg, however, contributed his principle of uncertainty (which states that at the miniscule level of quantum mechanics it is impossible to precisely measure a particle’s exact locations) before Schrödinger’s model. Schrödinger proposed his model in 1926. His theory was that the electrons behaved as waves (Schrödinger used Louis de Broglie’s equations), and his wave equations allowed for the electron to occupy a three-dimensional region of space. Schrödinger used three coordinates, the principal (n), angular (l), and magnetic (m) numbers. This model differed greatly from Bohr’s model which was a two- dimensional model with fixed coordinates. (the Bohr model was the accepted scientific model from 1922 until around 1930). Schrödinger’s equation does not give an exact position for the electron but rather a probability of where the electron will be. The equation uses orbitals (a volume of
Atomic Model
Electron Cloud By: Ben Bourland November, 2013 The electron cloud model was proposed by two scientists, Werner Heisenberg and Erwin Schrödinger. The electron cloud model was developed in 1952. It differed from the previously model, proposed by Niels Bohr (the orbital model), in that instead of the electrons following a fixed orbital their exact position is nearly impossible to know, however, there are certain areas (orbitals) where the electrons are likely to be found.
space around the nucleus that contains the electron 90% of the time) as the most probable place to find the electron. Schrödinger’s equation: One of the problems with Schrödinger’s model is that it is almost entirely theoretical. There are relatively few, if any, experiments to measure the location of electrons. Schrödinger’s model is based on mathematics. And while it is effective to a certain degree, there is no tangible way to test his theory. And since it is dealing quantum mechanics it is pretty much impossible to accurately measure a particle’s exact location. Another point to consider is that there might still be more to the atom that we haven’t discovered yet. Every previous model of the atom has been refined and changed as the scientific community has learned more about the atom. While the electron cloud might be the most recent model, as well as the scientifically accepted model, I personally believe that is much we don’t know about the atom and there may never be a complete model for the atom. In closing, the electron cloud model is a definite improvement from the previous models because it allows for a three-dimensional model of the atom. Despite its flaws, Schrödinger’s model is still the best and most accurate model that we have for the atom.
By Dylan Mangano, November 2013 Bohr made his atomic model in 1913 after he proposed electrons have certain classical motions. Electrons in atoms orbit the nucleus. The electrons can only orbit stably, without radiating, in certain orbits (called by Bohr the "stationary orbits"): at a certain discrete set of distances from the nucleus. These orbits are associated with definite energies and are also called energy shells or energy levels. In these orbits, the electron's acceleration does not result in radiation and energy loss as required by classical electromagnetics. Electrons can only gain and lose energy by jumping from one allowed orbit to another, absorbing or emitting electromagnetic radiation with a frequency ν determined by the energy difference of the levels according to the Planck relation Neil’s Bohr applies quantum theory to Rutherford's atomic structure by assuming that electrons travel in stationary orbits defined by their momentum. “This led to the calculation of possible energy levels for these orbits and the postulation that the emission of light occurs when an electron moves into a lower energy orbit.” (www.rsc.org) In other words Bohr found out that electrons in certain energy levels gave off different light using the experiment to your right. Bohr was the scientist that found out that electrons orbit the atom. Before Bohr, people believed that electrons could not orbit the atom because the gravity from the nucleus would pull the electrons into it and could explode but there are not random explosions all over the place so they believed the electrons were in the atom. Bohr proved that the electrons did in fact orbit the atom by doing the experiment in the figure above. As said before different light is given off for each level the electrons are in. Of course there are flaws with his logic. Different light could have been given off by different amounts of energy or different reasons all together. This became the look of what scientists believed an atom looked like after Bohr’s discovery. Because of Bohr’s contribution to the look of the atom they named an element after him on the periodic table called Bohrium. Its atomic number is 107. If you were to find the electron configuration for Bohr’s atom it would look like this, 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f14 6d5. This may look confusing but basically it shows how many electrons are in each level of orbit around the atom. Niels Bohr used what he knew about the electromagnetic spectrum when he found the different levels of light. Each one is a different electromagnetic wave. Modern atoms look similar to Bohr’s - made a significant contribution to science.
By Edson Bonilla, November 2013 Many scientists have tried to propose different models of how an atom actually looks. Although many of them were wrong, they are still quite interesting to look at. One of those models in particular was the “Plum Pudding” model. Discovered by Joseph John (J.J.) Thomson on 1904, this model was based of the discovery of electrons by Thomson as well. When Thomson discovered the electron in 1897, his proposed model was the plum pudding because he thought atoms did not have a nucleus. Since he knew electrons are negatively charged, he assumed that the electrons were surrounded in a “soup” or “cloud” of positive charge that balanced out the electrons’ negative charges - similar to plum pudding. The plums represent the negative charges (electrons) that are inside the pudding (positive charge). The experiment that Thomson used in order to discover his plum pudding model was the cathode rays and Crooks tube experiment. Thomson created a Crooks tube with an electrometer on one side and made sure it wouldn’t be in the way of the cathode rays. Thomson could then trace the path the cathode rays took by looking at the flashing patch it created when it hit the surface of the tube. The electrometer only registered a charge when Thomson deflected the ray with a magnet, thus concluding that the negative charge and rays were the same. The significance of this discovery was huge. It was the first time electrons had been introduced to the world. All of the modern and future models after Thomson’s wouldn’t be what they are if it wasn’t for Thomson’s discovery of the electron. Thomson’s work and impact on our society today may not be as big as it once was, but without Thomson’s work, it would have taken longer for electrons to be discovered, ultimately delaying the final atomic model.
By Erin Atchison, Nov 2013 The electron cloud model, sometime referred to as quantum mechanical model of the atom, was created by Erwin Schrödinger in 1926. This model took some ideas from Neils Bohr’s atomic model and expanded on them. In Bohr’s model, he defines the electrons as being on fixed orbitals that are stationary around the nucleus. Schrödinger took this model further by stating that the electrons do not have an exact path, but rather that you can only predict the odds of the location of the electron at a given time. Such as the picture to the left, it shows an example of the electron cloud, Schrödinger is describing. Although for Schrödinger’s model there is no real experiment that can be done to prove it, unlike many other scientists such as Chadwick and Rutherford, who had created an experiment that provides evidence that their models are correct. Schrödinger mainly proved his theory through mathematical equations and theorems. This leads to skepticism of his work because there is no real way to prove his theory or model for that matter. To increase the validity of or improve Schrödinger’s model, one would only have to create an experiment. More evidence backing up Schrödinger’s model, the better. The experiment could take pointers from the double-slit experiment, dubbed the ‘most beautiful experiment’ ever in a 2002 poll of Physics World readers. Another experiment to try includes dyeing electrons and recording the locations of the dyed electrons at certain times. The examples are endless, but all that matters is making Schrödinger’s model better and more valid.
Jeremiah Ziaty November, 2013
Physicist Ernest Rutherford is recognized as having discovered the nucleus in the early 1900s while a professor at the English University of Manchester. Rutherford had worked with atomic physics for a long time. He and his assistants, Hans Geiger and Ernest Marsden, conducted experiments around 1909 that were published in 1911 and known as the Gold Foil Experiment. They fired alpha particles at a thin sheet of gold foil. During the experiment, most of the particles passed through the gold foil onto the screen behind it, but some bounced back at the researchers or changed their direction. Rutherford was testing the Plum Pudding theory that concluded the particles would go straight through the foil with only a small change in trajectory. Rutherford determined that the particles that made up the gold foil must consist mostly of empty space since most of the alpha particles passed through. However, some regions of the gold must have been too dense to allow the alpha particles through. This experiment was like shooting a gun at a pile of snow with the bullets going straight through and then a few bullets hitting something hard and getting deflected. Rutherford called the heavy part of the atom the nucleus. He concluded that it was the nucleus that held most of the atom's mass. Rutherford also conducted the experiment on lead, aluminum and iron. The experiment was conducted a completely dark room to allow the researchers to see the atoms as well as having the screen behind the foil covered with Rutherford used the term nucleus in relationship to the atom where nucleus had been used before to describe the head of a comet or a kernel. Rutherford’s work created the field of “nuclear physics.” Since Rutherford conducted his tests the process of shooting atoms at a solid to determine it’s density has become a popular way of testing density. The process is now known as Rutherford Back Splattering. If Rutherford had not discovered the atom it would not have been possible to determine atomic weight and create the periodic table. Also if Rutherford had not made the discovery of the nucleus later scientists would have been able to split the atom.
Top Illustration: Results expected from Plum Pudding Theory
Bottom Illustration: Actual Gold Foil Results
By Katarina Luongvan November 2013 Who was James Chadwick? He was born in Cheshire, England, on the 20th October, 1891. James Chadwick had worked under Rutherford as an assistant director of the lab at Cambridge. His research focused mainly on radioactivity. Chadwick proved the existence of the neutron. With Rutherford’s discovery of the proton in 1919, others questioned the particles in the nucleus. Chadwick working side by side with Rutherford, they studied the atomic disintegration which showed the atomic mass and atomic number would not end up the same. Rutherford thought there was a particle with mass but no charge, the neutron was paired by protons and electrons, no evidence was shown. Chadwick was curious, and kept the experiment in the back of his head while doing other projects. Chadwick repeated their experiments but with the goal of looking for a neutral particle, one with the same mass as a proton with no charge. Walter Bothe and Herbert Becker shot alpha rays at beryllium, and thought the neutral radiation were high energy gamma rays. Chadwick had experimented similar, and had observed high-speed protons coming out from the paraffin. The beryllium rays were not gamma rays because the rays coming from the beryllium had more energy, which he recognized the beryllium rays as neutrons. Wanting to do his own experiment, Chadwick bombarded boron with alpha particles which emitted neutral rays, and placed a hydrogen target in the path of the rays. Once the rays hit the hydrogen target, protons flew out, and he measured the velocity of the protons. He used the laws of conservation of momentum energy, and was able to calculate the mass of the neutral particle. What if not all the protons flew out? What if another element was used? There was no end. There will always be someone to improve or discover something new. Discovering the neutron was one of the most significant discoveries, and remains an essential component of the structure of matter and the development of the Standard Model for particle physics.
By: Karmen Kaskie November, 2013 Did you know that the neutron was not discovered until 1932 when James Chadwick used scattered data to calculate the mass of this neutral particle? Before the 1930s, the neutron was thought to be made up of both electrons and protons. Nowadays, we know the neutron does not have electrons inside, but instead that it has electrons in “rings” around the neutron. James Chadwick named it the neutron in a paper published in an issue of a magazine named Nature on February 17, 1932. In 1935, James Chadwick received the Nobel Prize in physics for finding the neutron.
Theorized: Ernest Rutherford (1920) Discovered: James Chadwick (1932) Cool facts: In 1933 someone realized that the neutron might be able to cause something called a “nuclear chain reaction” and later nuclear bomb was discovered. In 1932, Irene Joliot-Curie and her husband, Frederic Joliot-Curie tested an alpha source and was amazed at what they thought was happening, but used an incorrect formula to determine what the nucleus was made up of… Chadwick realized that, then did his own tests and eventually figured out the correct theory. It’s funny that Irene Joliot-Curie and Frederic Joliot-Curie were so close to getting the noble prize... but it’s okay since they won the Noble Prize in chemistry in 1935 (the same year as James Chadwick) for their discovery of artificial radioactivity.
J.J. Thomson discovered the electron in 1897, and as a follow-up to this work, proposed the plum pudding model of the atom in 1904 which would include these newly discovered electrons. The inspiration for the name came from the way the negatively charged electrons acted as plums that were surrounded by a pudding, which was the positively charged part of an atom. Sometimes this area was interpreted as a positively charged cloud that the electrons were free to fly around in a rotating pattern, unable to escape the positively charged grip of the cloud. The discovery of electrons was due to Thomson’s experiments with Cathode rays using Cathode ray tubes. At the time, physicists were aware of the existence of Cathode rays, but could not agree on what they were. The two theories were that the Cathode rays were composed of either electrically charged atoms, or it was some form of electromagnetic radiation. Thomson cleared up this debate in 1897 when he measured the mass of the rays, which proved they were made of a particle. But this particle was around 1,800 times lighter than the lightest atom, hydrogen, proving that these were not atoms, but a new particle altogether. For this work, Thomson received the Nobel Prize in 1906. In 1909, the plum pudding model was disproven after Ernest Rutherford’s gold foil experiment was conducted by Hans Geiger and Ernest Marsden. Rutherford surmised from this result, that the atom contained a very small nucleus with a very high positive charge instead of being a large area which contained electrons. This was a major step in forming the modern atomic model we know today. Article by Mark Bodiroga, November 2013
Once upon a time, in times before our current understanding of the atom, lived a man who loved experimenting. This man, named JJ Thomson, discovered electrons through cathode ray experiments. Filling glass pipes with low-density gas, Thomson began firing electrical currents throughout them. The results surprised Thomson. He found that the ratio of the mass of the cathode ray to the electrical charge of the gas was always the same –and that it was huge! Thomson found that the charge of the mass ratio was so large that the particles either carried a huge charge, or were a thousand times smaller than a hydrogen ion (He then decided that the second idea made more sense.) This must have meant that there must have been negative subatomic particles in the gas. Thomson then gave these negative particles a name -electrons. Excited about this new discovery, Thomson created a The atom in his model was composed of a positive model to go with it. He called it the sphere –or the cookie- with negative electrons plum-pudding model. (Let’s be honest randomly spaced throughout –A.K.A. the though…it looks more like a chocolate chocolate chips. chip cookie!)
*Atom- the general term used to describe pieces of matter *Subatomic particles- Subatomic particles are the particles composing nucleons and atoms *Particles- Subatomic particles is the particles composing nucleons and atoms *Electrons- Subatomic particles is the particles composing nucleons and atoms
By Melissa Pham, November 2013 In 1932 Sir James Chadwick discovered the neutron. In his experiment, he shot alpha particles into beryllium, letting the radiation to then go through a screen of paraffin wax. When the radiation hit the hydrogen gas of the wax, the atoms then went through a detecting chamber. The end result was the release of neutral particles Chadwick named: neutrons. Chadwick’s discovery had a profound impact on science; his discovery was used to examine different material structured, determine water content in soil, treat tumors and it allowed even the heaviest of elements to be fissured leading to the development of nuclear power. The discovery also gave way to better understanding of isotopes, atomic mass, and the structures of the atom. Neutrons completed the relationship between atomic number and atomic mass by adding to the weight of the protons already present on the periodic table. When paired with ions, you get the innermost structures of matter. Being neutral, the neutron had little to do with electron configurations, but neutrons on their own can create neutron stars; some even have the same mass as the sun! Chadwick’s discovery of the neutron contributed to many improvements and new discoveries in science. The neutron can be applied in nearly all spectrums of life and human development. It was the missing puzzle piece in the atomic structure.
Nejla Memic November, 2013
The Father of Nuclear Physics The first structure of the atom was called the “Plum Pudding Model” and was discovered by J.J. Thompson in 1904. According to this model the atom consisted of negatively charged electrons in a positively charged cloud of matter. The second, more accurate, structure of the atom was discovered in 1911 by Ernest Rutherford, J.J. Thompson’s former student. Rutherford performed an experiment called the “Gold Foil Experiment.” In the experiment positively charged alpha particles were shot at a piece of gold foil that was 8.6*10^-6 cm thick. According to Thompson’s model all of the particles should have shot directly through. A fluorescent screen made of zinc sulfide was put behind the foil, so that the alpha particles could appear on. The experiment had to be performed in complete darkness in order for Rutherford to see the light the alpha particles would produce. A microscope was placed above the screen to observe when and where the alpha particles hit the screen. Most particles shot through the foil but, a handful of the alpha particles were deflected at various angles. Due to this observation Rutherford concluded that the alpha particles had to be hitting a small dense center. The dense center was called the nucleus and contained neutrons and electrons. This explained why the particles were being deflected. Based on the information, he gathered his conclusions about the nucleus and electron, these conclusions are correct because the experiment was performed multiple times by multiple people. Rutherford had the help of Hans Geiger and Ernest Mardsen his associates. When observing the experiment in the dark, the experimenters made sure to sit in a dark room for one hour before doing the experiment to make sure their eyes would adjust correctly. Things were kept constant and accurate. Rutherford’s model consisted of a positively charged nucleus in the center, and was surrounded by negatively charged electrons. Rutherford didn’t discover the neutrons and didn’t really address where the electrons were placed in the atom. The model that Rutherford discovered helped scientists after him to make new discoveries. His monumental discovery helps us, 100 years later, get a better understanding of what an atom is, and how it works
Newton Nguyen November, 2013 In 1913, Niels Bohr proposed what was called the “Planetary” model, or Bohr’s model. In the model, electrons are found in set energy levels called “orbits” while in the center of the model the nucleus containing a small, positively charged nucleus. This model consisted of these principles which were 1) Electrons assume only certain orbits around the nucleus. These orbits are stable and called "stationary" orbits. 2) Each orbit has an energy associated with it. For example the orbit closest to the nucleus has energy E1, the next closest E2 and so on. 3) Light is emitted when an electron jumps from a higher orbit to a lower orbit and absorbed when it jumps from a lower to higher orbit. 4) The energy and frequency of light emitted or absorbed is given by the difference between the two orbit energies
However, some of these principles were assumed because it violated the principles of classical mechanics such as electrons travel around in a circular orbit. He tested the hydrogen atom and was able to explain the stability of the atom and the emission spectrum of the element, hydrogen. He also explained how electrons could jump from one orbit to another only by emitting or absorbing energy as shown in the image above. However, this model only worked for hydrogen so the final atomic model at this point in time was not developed yet. Although this was an incomplete model, the Bohr model describes many of the accepted features in modern times today and usually is shown in basic science classes. In modern times teachers show the model of electrons on orbits with a nucleus with protons and neutrons in the center. These “basics” of the atom is shown and therefore teaches the essentials of the atom without the higher level math that comes with it. This would be perfect for middle school students or elementary students starting to learn about the atom. Overall, the “Planetary” model shows the simplicity of the model without overcomplicating the learning and eases students into what the atom is.
Tumsa Musa November, 2013 The planetary atomic model is associated with the year 1913, when Niels Bork finished his work on developing model consists of four main principles that added on to previous models.
Electrons assume only certain orbits around the nucleus. These orbits are stable and called "stationary" orbits. Each orbit has an energy associated with it. For example the orbit closest to the nucleus has an energy E1, the next closest E2 and so on. Light is emitted when an electron jumps from a higher orbit to a lower orbit and absorbed when it jumps from a lower to higher orbit. The energy and frequency of light emitted or absorbed is given by the difference between the two orbit energies, e.g.,
From these four principles he was able to deduce these principles from analyzing the short comings from Rutherford’s “pudding model” and connecting it to his own experiments with hydrogen electron emissions. Essentially by exiting hydrogen atoms he was able to analyze the energy they gave off as light and postulated that this was because electrons where jumping up and falling down a level in orbit. Using the formulas: E(light) = Ef – Ei ;n = E(light)/h ;h= Planck's constant = 6.627x10 -34 Js where "f" and "i" represent final and initial orbits. Bohr was able to deduce what energy these energy levels were at and explains more about the atom than any other model before. Unfortunately this model proved useful only for Hydrogen atom and even then it broke the uncertainty principle. So while it was a significant step and helped define orbits and energy level it was not by any means perfect. One huge problem was that electrons orbiting around the nucleus have acceleration and therefore radiates and loses energy. The model also fails with multi-electron atoms and can’t predict what happens with duplets or triplets. Schrödinger eventually helped improve the Bohr model and push it into the Quantum world!
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