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The atom orbiting electrons Nucleus (protons and neutrons) Nuclide notation Nucleon number (A) = number of protons and neutrons Neutron number (N) = A - Z 7 Li 3 Proton number (Z) = number of protons Isotopes It is possible for the nuclei of the same element to have different numbers of neutrons in the nucleus (but it must have the same number of protons) For example, Lithium atoms occur in two forms, Lithium-6 and Lithium-7 3 neutrons 4 neutrons 7 6 3 3 Li Li How do we know the structure of the atom? The famous Geiger-Marsden Alpha scattering experiment In 1909, Geiger and Marsden were studying how alpha particles are scattered by a thin gold foil. Thin gold foil Alpha source Geiger-Marsden As expected, most alpha particles were detected at very small scattering angles Thin gold foil Alpha particles Small-angle scattering Geiger-Marsden To their great surprise, they found that some alpha particles (1 in 20 000) had very large scattering angles Thin gold foil Alpha particles Large-angle scattering Small-angle scattering Explaining Geiger and Marsdens’ results The results suggested that the positive (repulsive) charge must be concentrated at the centre of the atom. Most alpha particles do not pass close to this so pass undisturbed, only alpha particles passing very close to this small nucleus get repelled backwards (the nucleus must also be very massive for this to happen). nucleus Rutherford did the calculations! Rutherford (their supervisor) calculated theoretically the number of alpha particles that should be scattered at different angles. He found agreement with the experimental results if he assumed the atomic nucleus was confined to a diameter of about 10-15 metres. That’s 100 000 times smaller than the size of an atom (about 10-10 metres). Limitations of this model? • According to the theory of electromagnetism, an accelerating charge (and the orbiting electrons ARE accelerating centripetally) should radiate energy and thus spiral into the nucleus. Evidence for atomic energy levels When a gas is heated to a high temperature, or if an electric current is passed through the gas, it begins to glow. Light emitted cathode Low pressure gas anode electric current Emission spectrum If we look at the light emitted (using a spectroscope) we see a series of sharp lines of different colours. This is called an emission spectrum. Absorption Spectrum Similarly, if light is shone through a cold gas, there are sharp dark lines in exactly the same place the bright lines appeared in the emission spectrum. Light source gas Some wavelengths missing! Why? Scientists had known about these lines since the 19th century, and they had been used to identify elements (including helium in the sun), but scientists could not explain them. Niels Bohr In 1913, a Danish physicist called Niels Bohr realised that the secret of atomic structure lay in its discreteness, that energy could only be absorbed or emitted at certain values. At school they called me “Bohr the Bore”! The Bohr Model Bohr realised that the electrons could only be at specific energy levels (or states) around the atom. We say that the energy of the electron (and thus the atom) can exist in a number of states n=1, n=2, n=3 etc. (Similar to the “shells” or electron orbitals that chemists talk about!) n=1 n=2 n=3 The Bohr Model The energy level diagram of the hydrogen atom according to the Bohr model Energy eV 0 High energy n levels are very close to each other n=5 n=4 n=3 n=2 Electron can’t have less energy than this -13.6 n = 1 (the ground state) The Bohr Model An electron in a higher state than the ground state is called an excited electron. Energy eV 0 High energy n levels are very close to each other n=5 n=4 n=3 electron n=2 -13.6 n = 1 (the ground state) Atomic transitions If a hydrogen atom is in an excited state, it can make a transition to a lower state. Thus an atom in state n = 2 can go to n = 1 (an electron jumps from orbit n = 2 to n = 1) Energy eV 0 n=5 n=4 Wheeee! n=3 electron n=2 -13.6 n = 1 (the ground state) Atomic transitions Every time an atom (electron in the atom) makes a transition, a single photon of light is emitted. Energy eV 0 n=5 n=4 n=3 electron n=2 -13.6 n = 1 (the ground state) Atomic transitions The energy of the photon is equal to the difference in energy (ΔE) between the two states. It is equal to hf. ΔE = hf Energy eV 0 n=5 n=4 n=3 electron n=2 ΔE = hf -13.6 n = 1 (the ground state) Emission Spectrum of Hydrogen The emission and absorption spectrum of hydrogen is thus predicted to contain a line spectrum at very specific wavelengths, a fact verified by experiment. Which is the emission spectrum and which is the absorption spectrum? Pattern of lines Since the higher states are closer to one another, the wavelengths of the photons emitted tend to be close too. There is a “crowding” of wavelengths at the low wavelength part of the spectrum Energy eV 0 n=5 n=4 n=3 n=2 Spectrum produced -13.6 n = 1 (the ground state) How do you excite an atom? 1. Heating to a high temperature 2. Bombarding with electrons 3. Having photons fall on the atom I’m excited! Limitations of the Bohr Model 1. Can only treat atoms or ions with one electron 2. Does not predict the intensities of the spectral lines 3. Inconsistent with the uncertainty principle (see later!) 4. Does not predict the observed splitting of the spectral lines Forces in the nucleus The Coulomb Force • The repulsive force between protons in the nucleus + + The Strong Force The nucleons (protons and neutrons) in the nucleus are bound together by the strong nuclear force • acts over short distance (10-15 m) • acts only between adjacent particles in the nucleus • is carried by gluons Unstable nuclei Some nuclei are unstable, for example Uranium 235 (it’s to do with the relative numbers of protons and neutrons) Hi! I’m uranium-235 and I’m unstable. I really need to lose some particles from my nucleus to become more stable. Unstable nuclei To become stable, an unstable nuclei emits a particle Weeeeeeeeeeeeee! Unstable nuclei We say the atom has decayed Weeeeeeeeeeeeee! Unstable nuclei The decay of an unstable nucleus is random. We know it’s going to happen, but we can’t say when! It is spontaneous. It cannot be affected by temperature/pressure etc. Weeeeeeeeeeeeee! Becquerels (Bq) • The amount of radioactivity given out by a substance is measured in Becquerels. One becquerel is one particle emitted per second. Detection • Particles can be detected by photographic film • Particles can also be detected (and counted) by a Geiger-Müller tube (GM tube) connected to a counter Alpha particles • • • • • 2 protons and 2 neutrons joined together The same as the nucleus of a helium atom Stopped by paper or a few cm of air Highly ionising Deflected by electric and strong magnetic fields 2+ 4 2 He Alpha Decay Atomic mass goes down by 4 235 231 4 92 90 2 U 2+ He + Th Atomic number goes down by 2 Beta particles • • • • Fast moving electrons Stopped by about 3 mm of aluminium Weakly ionising Deflected by electric and magnetic fields 0 e -1 Beta decay • In the nucleus a neutron changes into an electron (the beta particle which is ejected) and a proton (which stays in the nucleus) • During beta decay the mass number stays the same but the proton number goes up by 1. 231 Th 90 antineutrino Pa + -1e + עe 231 91 0 0 0 Gamma rays • • • • High frequency electromagnetic radiation Stopped by several cm of lead Very weakly ionising NOT affected by electric or magnetic fields Gamma rays Associated with alpha decay 235 231 92 90 U Th + α ½ - life • This is the time it takes half the nuclei to decay Number of nuclei undecayed time half-life (t½) ½ - life • This is the time it takes half the nuclei to decay Number of nuclei undecayed A graph of the count rate against time will be the same shape time half-life (t½) Different ½ - lives • Different isotopes have different half-lives • The ½-life could be a few milliseconds or 5000 million years! Number of nuclei undecayed time half-life (t½) Nuclear reactions 14 7 4 2+ 17 2 8 1 N + He O + p 1 Unified mass unit (u) • Defined as 1/12 of the mass of an atom of Carbon-12 u = 1.6605402 x 10-27 kg Energy mass equivalence • E = mc2 • E = 1.6605402 x 10-27 x (2.9979 x 108)2 • E = 1.4923946316 x 10-10 J • Remembering 1 eV = 1.602177 x 10-19 J • 1 u = 931.5 MeV Mass defect For helium, the mass of the nucleus = 4.00156 u But, the mass of two protons and two nuetrons = 4.0320 u!!!! Where is the missing mass? Mass defect The missing mass (mass defect) has been stored as energy in the nucleus. It is called the binding energy of the nucleus. It can be found from E = mc2 Mass defect calculation • Find the mass defect of the nucleus of gold, 196.97 - Au Mass defect calculation • The mass of this isotope is 196.97u • Since it has 79 electrons its nuclear mass is 196.97u – 79x0.000549u = 196.924u • This nucleus has 79 protons and 118 neutrons, individually these have a mass of 79x1.0007276u + 118x1.008665u = 198.080u • The difference in mass (mass defect) is therefore 1.156u Mass defect calculation • The difference in mass (mass defect) is therefore 1.156u • This “missing mass” is stored as energy in the nucleus (binding energy). • 1u is equivalent to 931.5 MeV Binding energy This is the work required to completely separate the nucleons of the nucleus. Binding energy per nucleon This is the work required to completely separate the nucleons of the nucleus divided by the number of nucleons. It is a measure of how stable the nucleus is. The binding energy curve Nuclear Fission Uranium Uranium 235 has a large unstable nucleus. Capture A lone neutron hitting the nucleus can be captured by the nucleus, forming Uranium 236. Capture A lone neutron hitting the nucleus can be captured by the nucleus, forming Uranium 236. Fission The Uranium 236 is very unstable and splits into two smaller nuclei (this is called nuclear fission) Free neutrons As well as the two smaller nuclei (called daughter nuclei), three neutrons are released (with lots of kinetic energy) Fission These free neutrons can strike more uranium nuclei, causing them to split. Chain Reaction If there is enough uranium (critical mass) a chain reaction occurs. Huge amounts of energy are released very quickly. Bang! This can result in a nuclear explosion!YouTube nuclear bomb 4 Nuclear fusion – Star power! The binding energy curve