Models of the atom
Neils Bohr’s model of the hydrogen atom was developed by correcting the errors in Rutherford’s model. Ernest Rutherford’s atomic model was an scientific advance in terms of understanding the nucleus, however it did not explain the electrons very well, as a charged particle which is accelerating around the nucleus will emit electromagnetic radiation and will therefore lose energy, causing it to spiral into the nucleus making the atom collapse. This would happen in a very short time making the atom extremely short lived. This also would mean that the radiation emitted would have a spectrum of wavelengths as the electrons have varying distances from the nucleus, however when looking at the wavelengths of light given off by a hydrogen atom, only some colours are seen. Bohr assumed that electrons move around the nucleus due to the electrostatic force between the positively charged protons and negatively charged electrons, and so the energy of the electrons is dependant on the distance from the nucleus. Bohr’s most innovational suggestion was that electrons are quantised. This means that they have fixed orbits with discrete radii, and can only exist in the first, second, third etc. ‘shells’ or ‘energy levels’ but never in between them. The shells closest to the nucleus have the least amount of energy while the shells furthest from the nucleus have the most energy, because the energy of the electron depends on the size of it’s orbit. When an electron absorbs energy it is able to ‘jump’ up to the next energy level, and when it loses energy it will jump down an energy level.
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A photon is the name of a discrete unit of electromagnetic radiation, or light energy. When an electron jumps down an electron shell, it emits photons, and the energy of which is equal to the difference of energy between the two shells. The energy of a photon equals the frequency of the radiation multiplied by Planck’s constant, 6.626176 x 10-34 joule-seconds. This is given by the formula E=hf . The amount of energy is very small however, so is often measured in electron volts. This is the amount of kinetic energy gained by an electron with a charge of -1.6 x 10-19 C when it moves across an electric potential difference of one volt, so 1 eV= 1.6 x 10-19 J.
For example, red light with a frequency of 4 x 1014 Hz:
E = hf
E = 6.626176 x 10-34 x 4 x 1014
E = 2.6505 x 10-19 J
This is a very small so we convert to eV:
2.6505 x 10-19 / 1.6 x 10-19 = 1.657 eV
This means that if red light is emitted, the electron jumped down an energy level to one which has 1.657 eV less than the energy level it was in.
Quantisation of energy is the idea that the energy emitted by an atom is in discrete amounts, which proposed to explain the atomic line spectrum.
Atomic line spectra is the spectrum produced when an individual element is heated, and the light emitted is passed through a prism. For example, when hydrogen gas is energised in a discharge tube, it will glow red. When this light is viewed through a prism to separate it into its component wavelengths, only a few lines will be visible rather than a continuous spectrum of colours.
https://chem.libretexts.org/Textbook_Maps/General_Chemistry_Textbook_Maps/Map%3A_Chemistry%3A_The_Central_Science_(Brown_et_al.)/06._Electronic_Structure_of_Atoms/6.3%3A_Line_Spectra_and_the_Bohr_Model
Bohr’s model explains this by stating that when an electron drops down an energy level, it emits photons with an amount of energy equal to the difference in energy levels. Since the electron in a hydrogen atom has quantised energy levels, only photons of certain energies with be emitted. The most intense colour is red therefore a red glow is visible when hydrogen gas is heated, although it also emits violet, blue and green light.
This meant that the atom will only emit photons when electrons shift energy levels, and the energy emitted is quantised, so does not have a spectrum of wavelengths. This also meant that the atom does not spiral into the nucleus.
Ionisation is when electrons from the outer shell of an atom are lost, giving the atom a positive charge, or gained giving the atom a negative charge. A hydrogen atom that has had its electron removed is positively ionised, while a hydrogen atom with one or more additional electrons will be negatively ionised.
Ionisation energy is the energy required to remove one mole of electrons from one mole of atoms. The second ionisation energy is the energy required to remove another mole of electrons from a mole of atoms which already have a charge of +1. This can be repeated until all electrons have been removed from the atom to determine how many electrons the atom has and how they are arranged. Electrons from outer shells require less energy to be removed than electrons from shells closer to the nucleus because the electrostatic forces are weaker. Successive ionisation provides evidence for Bohr’s model.
THE PHOTOELECTRIC EFFECT
The photoelectric effect is when a material absorbs electromagnetic energy and emits electrons. Previous physicists determined that light was a wave since it will experience diffraction, interference,reflection and refraction. Albert Einstein however suggested that it could also act as a particle due to this effect.
The photoelectric effect was first discovered by Heinrich Hertz in 1887, by using a spark gap generator. He found that the length of the spark generated was longer if he shone ultra violet light onto the metal electrodes. It was later experimented with using two pure metal samples at opposite ends of a vacuum, which were connected to a microammeter. One of the metal plates was then illuminated with different frequencies of light, and it was found that a current would run between the two plates. This happened because the photons from the light would knock electrons off the metal plate, causing it to become positively charged. Since both plates would be connected, the other plate would also be positive which would attract the electrons to it, therefore creating a current through the wires. The current produced would be quite small, but it was found that light of shorter wavelengths would knock off more electrons than light of shorter wavelengths, which is why ultraviolet light had a more noticeable effect than ight of other colours. When light of different intensity (but the same frequency) was used it was found that although more electrons would be emitted using a bright light compared to a dim light, the electrons would have the same amount of energy as those from a dimmer light. The photoelectric cannot be explained by the wave theory of light since the energy isn’t affected by the intensity of the light, because the energy did not have to accumulate on the surface before electrons were emitted. The particle theory of light however can explain the phenomenon.
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The energy of a photon is given by the equation E = hf, where h is Planck’s constant and f is the frequency. Since the frequency of the wave is determined by f = v / λ where v is the same for all light as the speed of light is 3 x 108 m s-1 , then the greater the wavelength of light, the lower its frequency and therefore the less energy the photons will have. This means that ultraviolet light, which has a short wavelength will have high energy photons while red light, which has a longer wavelength, will have lower energy photons.
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The maximum kinetic energy is the energy required for the electrons to move from one plate to the other. To measure the amount of the maximum kinetic energy of the electrons in the circuit, an opposing voltage is used to stop the current. Because the intensity of the light had no impact on the energy of the emitted electron, it could be implied that the light was acting as a particle which would transfer its energy to an electron, which would escape from the metal plate with the same energy as the photon had minus the energy used to escape from its metal: Ek=hf-Ф, where Φ is the work function in joules. If the frequency of the light is too low then no electrons will be emitted from the surface. The minimum frequency for the photoelectric current to occur is known as the threshold frequency, and is different for different substances. Rarely does an element have a threshold frequency below ultraviolet. The work function which is the minimum energy required to eject an electron from the surface, or in other words the threshold frequency. This is a constant and has different values for different metals. When this equation is graphed it looks like:
http://dev.physicslab.org/img/17a0d8fb-7882-4256-9531-9883e39d1557.gif
Photovoltaic panels, more commonly known as solar panels, are used to generate electricity from sunlight. They work by using ultraviolet rays from the sun to knock electrons from metal to create an electric current.
The most obvious benefit of using solar power rather than burning fossil fuels is the sustainability. Sunlight is an entirely renewable source, the depletion of which will never be an issue. It is also entirely free and available to almost every part of the globe. Solar panels are able to convert sunlight into electricity continually while the sun is on it. Fossil fuels on the other hand have to be sourced before being processed into energy, and supplies are rapidly decreasing. It is estimated that coal will run out in 250 years, while oil only has 50 years. The sun, on the other hand has 5 billion years left.
Another drastic issue with the burning of fossil fuels is the effect on the environment. The production of energy from burning of fossil fuels, and the use of the machinery to obtain the fuels, results in a total 21.3 billion tons of carbon dioxide released each year. This the leading cause of climate change, which is extremely dangerous to the environment and the future of our planet. Emissions from fossil fuels can also be harmful to human health, such as asthma sufferers being exposed to pollution, and even carbon monoxide which is a highly toxic chemical. Solar panels produce electricity with no emissions or waste products. Solar panels could be placed almost anywhere, but most commonly on the roofs of buildings. This does not take up nearly as much room as fossil fuel power plants, which means they are not invasive to the environment.
On a larger scale, solar power plants where panels are placed on an area of desert can disturb their environment. However this is miniscule compared to the environmental impacts of fossil fuels, and there are many places across the world such as the Sahara Desert, and Western Australia in which a small area will produce large amounts of energy.
Some would argue that the cost of solar power outweighs their benefits, however this is not entirely true. Solar panels run by themselves, they require very little maintenance so the running costs are almost zero. Installation and manufacture costs are reasonably high however it is likely for the price to decrease in the future. The burning of fossil fuels requires mining and transportation, the costs of which are additional to the running of power plants. Environmental and health damage from fossil fuels have been said to cost 500 billion dollars. Currently the production of energy from fossil fuels receives government subsidies, therefore if we were to invest in the development of affordable solar power it would be cheaper in the long run. Solar panels installed on the roofs of homes have been known to produce enough energy that excess is returned and sold to the electrical companies, which will eventually cover the cost of the panels and be profitable to the homeowner.
There are a few drawbacks to solar power however. Solar panels do not have the energy density comparable to fossil fuels, which means that a car will not be able to run on its own solar panels. However, energy produced by solar panels at a plant would be able to be used to power an electric car. Another problem with photovoltaic panels is that electricity generation is limited at night and on cloudy days, and in winter when days are short.
Overall, the benefits of photovoltaic panels far outweigh those of fossil fuels. If fossil fuel power generation was replaced with solar power then there would be an enormous decrease in the amount of greenhouse gases in the environment.
NUCLEAR REACTIONS
To separate the protons and neutrons in a nucleus, a vast amount of energy is required. This is because the nuclear forces between nucleons is incredibly strong as they need to overcome the electrostatic forces. Protons have a positive charge while neutrons have no charge, therefore nucleons have coulombic repulsion between them. To overcome this to form a nucleus, a strong interaction force attracts the nucleons together. This force has a very short range, which means that the smaller an atom is, the more stable it is (except below a certain size) because the nucleons are all close to each other, whereas the nucleons around the outsides of larger nuclei are further apart from each other, making the atom unstable. Nuclear binding energy is the energy required to overcome the strong interaction force and separate the nucleons.
The mass of a nucleus is always less than the sum of the masses of its’ nucleons. This difference in mass is known as the mass deficit, which can be used to calculate the binding energy using ∆E=Δmc2. This is because energy is required to separate nucleons therefore free nucleons must have more energy than those in a nucleus, and they lose energy when forming a nucleus. Einstein’s relationship between energy mass states that if a nucleon has more energy, it has more mass, therefore the mass ‘lost’ when the nucleus is formed is released as energy.
For example, the mass defect of Nickel 62, which is the most stable nuclei:
Mass of Ni nucleus = 102.80892 x 10⁻²⁷ kg
Mass of neutron = 1.67493 x 10⁻²⁷ kg Mass of proton = 1.67262 x 10⁻²⁷ kg
Nickel 62 has 34 x neutrons and 28 x protons
Total mass of free nucleons = 34 x 1.67493 x 10⁻²⁷ + 28 x 1.67262 x 10⁻²⁷
= 103.78098 x 10⁻²⁷ kg
Δm = 9.7206 x 10⁻²⁸ kg
E = mc²
= 9.7206 x 10⁻²⁸ x (3.00 x 10⁸)²
= 8.74854 x 10⁻¹¹ J
= 546.78 MeV
This is the binding energy of a nickel atom. When this is divided by the total number of nucleons it gives the binding energy per nucleon which is 8.819 MeV.
In a nuclear reaction the total amount of mass and energy is conserved; energy is released and the mass gets smaller. The energy is released at the speed of light, 3 x 108 m s-1 , and because this is a large number it means that a small amount of mass can release a very large amount of energy.
In a fission reaction, a large unstable nucleus splits into smaller, more stable nuclei. Fast moving free neutrons, and a large amount of energy are also released. In nuclear power plants the fission reaction is induced, which means that a neutron is collided with a large nucleus such as Uranium-235, which will then become Uranium-236. The interactive forces between nucleons are no longer able to hold the whole nucleus together causing it to split. There are different possibilities of which the smaller nuclei will be, but fission reactions of uranium also release two or three neutrons. These are moving very fast and will cause further fission reactions if they collide with other large nuclei, causing a chain reaction. The energy released in a fission reaction can accumulate rapidly if the chain reaction is not controlled, as each reaction releases a lot of energy. This is how nuclear explosions happen.
The energy released in a fission reaction is mainly the kinetic energy of the products. The fast movement produces heat, which is used to generate electricity. The total mass of the products is less than the mass of the reactant nuclei. This is because the more binding energy the less mass because of E = mc²
In a fusion reaction, two or more fast travelling light nuclei collide to form one single heavier nuclei, which can also release a free neutron and energy. The reactants used for this are usually isotopes of hydrogen, and the product formed being helium or hydrogen isotope. Fusion releases energy because the heavier nuclei which are formed have a lower mass than the total mass of the products, because the nucleons are bonded tightly. Decrease in mass is released as energy.
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Both fission and fusion have products with increased binding energy and are therefore more stable than the reactants. Splitting heavy nuclei in a fission reaction increases binding energy by making the nucleus smaller so that the nucleons around the edges are more affected by the strong interaction force, while combining light nuclei in a fusion reaction increases binding energy by reducing the mass of the nucleus. More nucleons can also mean more strong force per nucleon. Increasing the binding means that the mass will decrease, therefore energy is released. Increased binding energy also means that the nuclei are more stable.
http://www.a-levelphysicstutor.com/images/nuclear/binding-e-graph.jpg
Fusion reactions produce three to for times more energy per kilogram of fuel than fission reactions, because the atoms are smaller therefore there are more of them within the same mass.
Fission reactors require control rods to absorb neutrons to prevent nuclear meltdown. Fusion does not have a chain reaction and cannot cause a meltdown or explosion. If an incident happened in which the plant was not running properly, fusion reactions would just stop since they are so difficult to maintain, while fission reactions would cause a chain reaction and quickly accumulate extremely dangerous amounts of energy.
The fuels used for fission reactions can be obtained from water molecules and are practically limitless, and hydrogen isotopes are naturally occurring and easy to extract. the uranium which is used in fission power plants is more difficult to obtain, because while it is naturally occurring it has to be mined, and it has to go through purification processes. Uranium-235 is not a common isotope and is non-renewable, so there is limited supply
Fusion can only occur at temperatures around 100 million degrees celsius, and although this is achievable it is difficult to control. Technology has not advanced so that the reactions will produce more energy than is being used to initiate the reaction but it is still being researched. Researchers are at the stage of trying to produce just enough energy to make up for the energy used initially. Fission reactions take little energy to initiate as only one neutron needs to be moving at high speed to produce large amounts of energy. In fusion every atom needs to be moving extremely fast.Therefore fission reactions are currently much more cost effective than fusion however this could change in the future.
Fusion does not produce any radioactive or dangerous products, so in the unlikely event of an accident or leak there would be no threat to humans or the environment. Fission on the other hand produces radioactive products which can take from thousands to millions of years to decay, which release toxic radiation. Fission power plants can and have had drastic accidents where thousands of people have died, and which will remain dangerous for hundreds of years afterwards.
Nuclear power would be a suitable alternative to fossil fuel power in New Zealand. This is because of the huge amount of carbon dioxide released by fossil fuels, which contributes to pollution and global warming. Nuclear power does not release any greenhouse gases, so would have a hugely positive impact on our environment. The radioactive waste produced by fission reactions is contained in concrete containers underground and are unlikely to ever escape into the soil or atmosphere, unlike fossil fuel power which releases tons of toxic chemicals into the air each year. A report by NASA states that “nuclear power prevented an average of 64 gigatonnes of CO2-equivalent (GtCO2-eq) net GHG emissions globally between 1971-2009. This is about 15 times more emissions than it caused.”
Although nuclear power plants can have catastrophic accidents, there has been plenty of research done to prevent these from happening. One example being able to make the plant safe for natural disasters such as earthquakes. Fossil fuel power plants can also have explosions, and leaks such as oil spills. According to NASA research “natural gas burning emits less fatal pollutants and GHGs than coal burning, it is far deadlier than nuclear power, causing about 40 times more deaths per unit electric energy produced.”
The fuel used in fission reactions has to be mined and has a limited supply, however it is more accessible than many fossil fuels, and is much less likely to run out soon. Fossil fuel energy requires thousands of times more fuel than nuclear plants to produce the same amount of energy.
Fusion power does not have any of the main issues of fission power, so a nuclear fusion power plant would be ideal for New Zealand. However, an investment in nuclear fission power plants in New Zealand is still beneficial for our future.