P4 Atomic structure

Models of the atom
1. Developing the atom
  1. After discovering the electron in 1897, J J Thomson proposed that the atom looked like a plum pudding.
    1. Definition of Plum Pudding model = The scientific idea that an atom is a sphere of positive charge, with negatively charged electrons in it
    2. To explain the two types of static electricity, he suggested that the atom consisted of positive 'dough' with a lot of negative electrons stuck in it. This was consistent with the evidence available at the time:
      1. solids cannot be squashed, therefore the atoms which make them up must be solid throughout
      2. rubbing two solids together often results in static charge so there must be something (electrons) on the outsides of atoms which can be transferred as atoms collide
2. Rutherford and the nucleus
  1. In 1905, Ernest Rutherford did an experiment to test the plum pudding model. His two students, Hans Geiger and Ernest Marsden, directed a beam of alpha particles at a very thin gold leaf suspended in a vacuum.
    1. It was thought that the alpha particles could pass straight through the thin foil, or possibly puncture it. If the plum pudding model had been correct then all of the fast, highly charged alpha particles would have whizzed straight through undeflected. The scientists were very surprised when other things happened:
      1. most of the alpha particles did pass straight through the foil
        the fact that most alpha particles went straight through the foil is evidence for the atom being mostly empty space
      2. a small number of alpha particles were deflected by large angles (> 4°) as they passed through the foil
        a small number of alpha particles being deflected at large angles suggested that there is a concentration of positive charge in the atom - like charges repel, so the positive alpha particles were being repelled by positive charges
      3. a very small number of alpha particles came straight back off the foil
        the very small number of alpha particles coming straight back suggested that the positive charge and mass are concentrated in a tiny volume in the atom (the nucleus) - the tiny number doing this means the chance of being on that exact collision course was very small, and so the 'target' being aimed at had to be equally tiny
  2. Rutherford had discovered the nuclear atom, a small, positively-charged nucleus surrounded by empty space and then a layer of electrons to form the outside of the atom.
    1. The discovery of the make-up of the nucleus (protons and neutrons) came much later, and was not made by Rutherford. The nucleus was calculated to be about 1/10,000th the size of the atom.
  3. The discovery of the make-up of the nucleus (protons and neutrons) came much later, and was not made by Rutherford. The nucleus was calculated to be about 1/10,000th the size of the atom.
3. Further developments to the atomic model
  1. In 1913, Niels Bohr revised Rutherford's model by suggesting that the electrons orbited the nucleus in different energy levels or at specific distances from the nucleus.
    1. By doing this, he was able to explain that since particular chemicals burn with certain-coloured flames, the pattern of energy released by electrons in the chemical reaction must be the same for every single atom of that element.
Atoms, Isotopes and ions
1. Structure Of the atom
  1. Atoms are very small, they have a radius of around 1 × 10-10 metres.
  2. The modern view of the atom is of a nucleus containing protons and neutrons with smaller electrons orbiting outside the nucleus.
  3. Relative: charge Relative mass Proton +1 1 Neutron 0 1 Electron -1 Close to 0 (1/2,000)
    1. Protons and neutrons are the heaviest particles in an atom and as a result they make up most of the mass of the atom. The mass of electrons is often not considered to be significant.
  4. In a neutral atom, the number of electrons is always the same as the number of protons.
2. Atoms and Isotopes
  1. an element's mass numbers can vary, which means that it can have different numbers of neutrons.
    1. So although chlorine has a mass number of 35 which means it has 18 neutrons, it can also have a mass number of 37, which means it has 20 neutrons.
      1. The different types of chlorine are called isotopes.
        Isotopes = Atoms of the same element with the same number of protons and electrons but different number of neutrons
3. Ions
  1. Atoms are normally Neutral charge. They usually have the same number of protons as electrons making the atom a neutral charge overall
    1. Atoms however can lose or gain electrons due to collisions or other interactions. When they do they form other particles called ions
      1. If an atom loses one or more electrons, it becomes a positively charged ion
      2. If an atom gains one or more electron, it becomes a negatively charged ion
Radioactive decay
1. Nuclear radiation
  1. An unstable nucleus can decay by emitting an alpha particle, a beta particle, a gamma ray or in some cases a single neutron
    1. Alpha Particle
      1. If the nucleus has too few neutrons, it will emit a two protons and two neutrons called an alpha particle.
      2. An alpha particle is also a Helium-4 nucleus
      3. Alpha decay causes the mass number of the nucleus to decrease by four and the atomic number of the nucleus to decrease by two.
    2. Beta Particle
      1. If the nucleus has too many neutrons, a neutron will turn into a proton and emit a fast-moving electron. This electron is called a beta (β) particle – this process is known as beta radiation.
      2. A beta particle has a relative mass of zero, so its mass number is zero, and as the beta particle is an electron it is written as 'e' as the element number and '-1' as the atomic number (bottom number) .Beta decay causes the atomic number of the nucleus to increase by one and the mass number remains the same.
      3. Electrons are not normally expected to be found in the nucleus but neutrons can split into a positive proton (same mass but positive charge). An electron is then ejected at high speed and carries away a lot of energy.
    3. Gamma Ray
      1. After emitting an alpha or beta particle, the nucleus will often still be too ‘hot’ and will lose energy in a similar way to how a hot gas cools down. A hot gas cools by emitting infrared radiation which is an electromagnetic wave.
        High energy particles will emit energy as they drop to lower energy levels. Since energy levels in the nucleus are much higher than those in the gas, the nucleus will cool down by emitting a more energetic electromagnetic wave called a gamma ray.
      2. Gamma ray emission causes no change in the number of particles in the nucleus meaning both the atomic number and mass number remain the same.
    4. Neutron emission
      1. It is possible for a neutron to be emitted by radioactive decay . This can occur naturally. Neutron emission causes the mass number of the nucleus to decrease by one and the atomic number remains the same.
  2. Properties of nuclear radiations
    1. The different types of radiation are often compared in terms of their penetrating power. ionising power and how far they can travel in air
      1. Alpha Particle - Symbol is an a. has a high ionising power and has a very low penetrating power only going though skin and paper and it's range in air is only 5cm which is very low. Beta Particle - Symbol is B. It has a low ionising power and can travel 1m in air and it's penetrating power means it can only go though 3mm aluminium foil. Gamma Ray- Symbol is an y it penetrating power is high going though concrete and lead but it's ionsing power is very low
    2. All types of radioactive decay can be detected by a Geiger-Muller tube, or G-M tube.
2. Radioactive decay
  1. An atom’s nucleus can only be stable if it has a certain amount of neutrons for the amount of protons it has.
    1. Elements with fewer protons, such as the ones near the top of the periodic table, are stable if they have the same number of neutrons and protons.
    2. However number of protons increases the number of neutrons are needed to keep the nucleus stable
    3. Nuclei with too many or few neutrons do exist naturally but are unstable and will decay by emitting radiation
3. Half life
  1. Half-life is the time it takes for half of the unstable nuclei in a sample to decay or for the activity of the sample to halve or for the count rate to halve. Count-rate is the number of decays recorded each second by a detector, such as the Geiger-Muller tube.
  2. Radioactive decay is a random process. A block of radioactive material will contain many trillions of nuclei and not all nuclei are likely to decay at the same time so it is impossible to tell when a particular nucleus will decay. It is not possible to say a particular nucleus will decay next but given there are tons it is possible to say a certain number will decay at a certain time. Scientists use statistical methods to tell when half the unstable nuclei in a sample will have decayed this is called half-life
  3. The illustration below shows how a radioactive sample is decaying over time. From the start of timing it takes two days for the count to halve from 80 down to 40. It takes another two days for the count rate to halve again, this time from 40 to 20. This process continues but does not drop down to zero completely
  4. It is possible to state how much sample remains or count should become after a length of time. This can be a fraction, decimal or ratio: a fraction - a 1/2 of a 1/2 of a 1/2 of a 1/2, which is *1/2 *1/2*1/2*1/2= 1/16
4. Nuclear equations
  1. A nucleaus changes into a new element by emitting alpha or beta particles. These changes are described using nuclear equations.
    1. Alpha decay (two protons and two neutrons) changes the mass number of the element by -4 and the atomic number by -2. An alpha particle is the same as a helium-4 nucleus.
    2. Beta decay changes the atomic number by +1 (the nucleus gains a proton) but the mass number remains unchanged (it gains a proton but loses a neutron by ejecting an electron, so a beta particle is an electron)
    3. Gamma is a pure energy and will not change the structure of the nucleus in any way.
Uses and dangers of radiation
1. Irradiation
  1. Exposing objects to beams of radiation is called irradiation. The term applies to all types of radiation including radiation from the nuclei of atoms
  2. Irradiation from radioactive decay can damage living cells. This can be put to good use as well as being a hazard
  3. Irradiation for sterilisation
    1. Irradiation can be used to preserve fruit sold in supermarkets by exposing the fruit to radioactive source. The gamma rays emitted by not change the fruit in any significant way. The process of irradiation does not cause the irradiated object to become radioactive
  4. Advantages and disadvantages of Irradiation
    1. Advantages
      1. Sterilisation can be done without high temperature
      2. It can be used to kill bacteria on things that would have melt
    2. Disadvantages
      1. It may not kill all bacteria on an object
      2. It can be very harmful- standing in the environment where objects are being treated by irradiation could expose people's cells to damage and mutation

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P4 Atomic structure

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