Nuclear Physics

Atomic structure A Lithium atom protons neutrons electrons An atom consists of a small central nucleus composed of protons and neutrons surrounded by electrons. An atom will always have the same number of electrons as protons.

Atomic and mass number The atomic number (Z) of an atom is equal to the number of protons in its nucleus. The mass number (A) of an atom is equal to the number of protons plus neutrons in its nucleus. protons = 3 neutrons = 4 electrons = 3 This Lithium atom has: atomic number = 3 mass number = 7

Properties of protons, neutrons and electrons Position in the atom Relative mass Relative electric charge PROTON NEUTRON ELECTRON nucleus 1 + 1 nucleus 1 outside nucleus 0.005 - 1

Ions An atom becomes an ion when it loses or gains one or more electrons. Lithium atom Positive Lithium ion Negative Lithium ion protons neutrons electrons

The three isotopes of hydrogen The atoms of an element always have the same number of protons. Isotopes are atoms of the same element with different numbers of neutrons. The three isotopes of hydrogen neutrons hydrogen 1 hydrogen 2 (deuterium) hydrogen 3 (tritium) Note: The number after ‘hydrogen’ is the mass number of the isotope.

C Nuclear notation 14 6 carbon 14 An isotope of carbon consists of 6 protons and 8 neutrons. This can be written as: carbon 14 Number of protons PLUS neutrons (Mass number) OR: C 14 6 Chemical symbol Number of protons (Atomic number)

Question 1 An isotope of uranium (chemical symbol U) consists of 92 protons and 143 neutrons. Give the two different ways of notating this isotope. The mass number of the Uranium isotope: = 92 + 143 = 235 235 U uranium 235 AND 92

Question 2 N Co Au Pu (a) (b) (c) (d) Determine the number of protons and neutrons in the isotopes notated below: N 13 7 (a) Co 60 27 (b) protons = 7 p = 27 neutrons = 6 n = 33 Au 197 79 (c) Pu 239 94 (d) p = 79 p = 94 n = 118 n = 145 Note: Apart from the smallest atoms, most nuclei have more neutrons than protons.

Alpha decay Alpha particles consist of two protons plus two neutrons. They are emitted by some of the isotopes of the heaviest elements.

Example: The decay of Uranium 238 92 Th 234 90 α 4 2 + Uranium 238 decays to Thorium 234 plus an alpha particle. Notes: 1. The mass and atomic numbers must balance on each side of the equation: (238 = 234 + 4 AND 92 = 90 +2) 2. The alpha particle can also be notated as: He 4 2

Question Show the equation for Plutonium 239 (Pu) decaying by alpha emission to Uranium (atomic number 92). Pu 239 94 U 235 92 α 4 2 +

Beta decay Beta particles consist of high speed electrons. They are emitted by isotopes that have too many neutrons. One of these neutrons decays into a proton and an electron. The proton remains in the nucleus but the electron is emitted as the beta particle.

Example: The decay of Carbon 14 6 N 14 7 β- -1 + Carbon 14 decays to Nitrogen 14 plus a beta particle. Notes: 1. The beta particle, being negatively charged, has an effective atomic number of minus one. 2. The beta particle can also be notated as: e -1

Question Show the equation for Sodium 25 (Na), atomic number 11, decaying by beta emission to Magnesium (Mg). Na 25 11 Mg 12 β- -1 +

Gamma decay Gamma decay is the emission of electromagnetic radiation from an unstable nucleus Gamma radiation often occurs after a nucleus has emitted an alpha or beta particle. Example: Cobalt 90 Co 90 27 Co 90 27 γ + Cobalt 90 with excess ENERGY decays to Cobalt 90 with less ENERGY plus gamma radiation.

Changing elements alpha DOWN by 2 DOWN by 4 beta UP by 1 NO CHANGE Both alpha and beta decay cause the an isotope to change atomic number and therefore element. Alpha decay also causes a change in mass number. Decay type Atomic number Mass number alpha DOWN by 2 DOWN by 4 beta UP by 1 NO CHANGE gamma

Complete the decay equations below: Fe 59 26 Co 27 β- -1 + Ra 224 88 Rn 220 86 α 4 2 N 16 7 O 8 (a) (c) (b)

Write equations showing how Lead 202 could decay into Gold Write equations showing how Lead 202 could decay into Gold. (This cannot happen in reality!) Element Sym Z Platinum Pt 78 Gold Au 79 Mercury Hg 80 Thallium Tl 81 Lead Pb 82 Bismuth Bi 83 Pb 202 82 Hg 198 80 α 4 2 + Hg 198 80 Pt 194 78 α 4 2 + Pt 194 78 Au 79 β- -1 + There are other correct solutions

Background radiation Background radiation is ionising radiation from space (cosmic rays), devices such as X-ray tubes, and from radioactive isotopes in the environment (for example radon gas from rocks in the ground). Most of this radiation occurs naturally but a small amount is due to nuclear weapon testing and nuclear power stations.

Background radiation pie-chart

Nuclear reactions ANSWERS In text questions: 92p, 143n 23890Th = 90p + 138n; 22488Ra = 88p + 136n 4019K = 19p + 21n; 4020Ca = 20p +20n Summary questions: (a) 6p + 6n (b) 27p + 33n (c) 92p + 143n (a) 92p + 146n (b) 90p + 144n (c) 91p + 143n

The Plum Pudding Atomic Model Before about 1910 many scientists believed that an atom consisted of: Positively charged matter spread out like a pudding embedded by negatively charged electrons (like plums in a pudding). The ‘Plum Pudding’ Model

Rutherford’s Atomic Model In 1909 Ernest Rutherford suggested that an atom consists of a a tiny positively charged nucleus surrounded by negatively charged electrons. Lord Rutherford 1871 - 1937

Geiger & Marsden’s alpha particle scattering experiment In 1909 Hans Geiger and Ernest Marsden performed an experiment using alpha particles to determine which of the two models was the better in describing the structure of an atom. Geiger and Marsden

The apparatus 2 5 4 3 1

What was observed alpha source thin metal foil Virtually all of the alpha particles went straight through the metal foil. A few alpha particles were deflected through a small angle. About 1 in 10 000 were deflected backwards.

How the results can be explained nucleus (highly enlarged) atom Deflections occur because there is a force between the charged nucleus and the positively charged alpha particles. Most of the alpha particles do not go near enough to the nucleus to be deflected. Backwards deflections occur when the alpha particles make near head on collisions with the positively charged nucleus.

How their results supported Rutherford’s atomic model The relatively small number of deflections indicates that most of the atom is empty space with only a very small nucleus. The backward deflections can only occur if the nucleus is positively charged and contains most of the atom’s mass. The ‘plum pudding’ model would not produce backward deflections.

Choose appropriate words to fill in the gaps below: According to __________ an atom consists of a tiny, ___________ charged __________ surrounded by a cloud of ________ electrons. The nucleus also contains most of the ______ of an atom. This model was supported by the ______ particle scattering experiment in 1909. In this experiment most alpha particles passed ________ through a thin metal foil with only about 1 in 10000 being deflected _________. Rutherford positively nucleus negative mass alpha straight backwards WORD SELECTION: Rutherford mass backwards negative straight positively alpha nucleus

Nuclear fission Nuclear fission is the splitting of an atomic nucleus. Nuclear fission can be used as an energy source in a nuclear reactor. There are two fissionable substances in common use in nuclear reactors, uranium 235 and plutonium 239.

Chain reaction The fission of a nucleus of Uranium 235 can be initiated by a neutron. When this nucleus splits further neutrons are produced. These neutrons in turn can cause more nuclei to split. An avalanche effect, called a ‘chain reaction’ can then occur. A chain reaction neutron

Nuclear fission reactor 5 2 6 1 3 & 4 7

Nuclear reactor parts 1. Fuel rods These contain U235 or Pu239. They become very hot due to nuclear fission. 2. Control rods Made of boron, when placed in-between the fuel rods these absorb neutrons and so reduce the rate of fission. Their depth is adjusted to maintain a constant rate of fission. 3. Moderator This surrounds the fuel rods and slows neutrons down to make further fission more likely. The moderator can be water or graphite. 4. Coolant This transfers the heat energy of the fuel rods to the heat exchanger. Coolant be water, carbon dioxide gas or liquid sodium. 5. Heat exchanger Here water is converted into high pressure steam using the heat energy of the coolant. 6. Reactor core This is a thick steal vessel designed to withstand the very high pressure and temperature in the core. 7. Concrete shield This absorbs the radiation coming from the nuclear reactor.

Choose appropriate words to fill in the gaps below: Nuclear fission is the _________ up of the nucleus of an atom into two smaller nuclei. ________ and neutrons are also usually emitted. Nuclear ________ use Uranium _____ or Plutonium _____to produce energy by nuclear ________. A controlled chain reaction is maintained by the use of _______ rods which absorb some of the _________ produced. An _______ bomb is the consequence of an uncontrolled chain reaction. splitting energy reactors 235 239 fission control neutrons atomic WORD SELECTION: reactors energy 239 atomic splitting neutrons 235 fission control

Hydrogen nuclei undergo fusion in stars to make helium nuclei Nuclear fusion Nuclear fusion is the joining of two atomic nuclei to form a larger one. Hydrogen nuclei undergo fusion in stars to make helium nuclei

Energy from fusion Nuclear fusion is the process by which energy is released in the Sun and other stars. It is also the energy source of the hydrogen bomb.

Nuclear fusion reactors Scientists are currently working to make nuclear fusion reactors. The fuel for fusion reactors is the isotope hydrogen 2 (deuterium) which is found in sea water. An experimental fusion reactor in Seatle USA

Virtual Physics Laboratory Simulations NOTE: Links work only in school Alpha Scattering.exe Geiger Muller Tube Nuclear Stability With stability curve Nuclear Reactions Fission & Fusion with binding energy

Online Simulations Atoms, ions & isotopes (GCSE) - Powerpoint presentation by KT Build an atom - eChalk Atomic Structure Quiz - by KT - Microsoft WORD Hidden Pairs Game on Atomic Structure - by KT - Microsoft WORD Fifty-Fifty Game on What particles are positive - by KT - Microsoft WORD Rutherford Scattering - PhET - How did Rutherford figure out the structure of the atomic nucleus without looking at it? Simulate the famous experiment in which he disproved the Plum Pudding model of the atom by observing alpha particles bouncing off atoms and determining that they must have a small core. Rutherford Scattering Experiment Thomson Model of evenly distributed charge and Nuclear Model - Michael Fowler Types of Radiation - S-Cool section on types of radiations including an animation of absorption and a couple of decay equations to fill in on screen. Alpha Decay - PhET - Watch alpha particles escape from a Polonium nucleus, causing radioactive alpha decay. See how random decay times relate to the half life. Beta Decay - PhET - Watch beta decay occur for a collection of nuclei or for an individual nucleus Decay series - Fendt Andy Darvill's Radioactivity Pages Understanding Radiation - National Radiological Protection Board - Useful starting point to get at useful areas of the site. Nuclear Fission - PhET - Start a chain reaction, or introduce non-radioactive isotopes to prevent one. Control energy production in a nuclear reactor! Nuclear Fission - Powerpoint presentation by Richard Miller of 5SJW (2005) Nuclear Fission - Powerpoint presentation that includes a link to the 'mousetrap' demonstration Power Station Animation - eChalk Managing a Nuclear Power Plant Simulation - by Henrik Eriksson BBC AQA GCSE Bitesize Revision: Atoms, isotopes & radioactivity - Core Science Structure of an atom Isotopes Evidence for atomic structure Alpha, beta & gamma radiation Detecting radiation Radioactive decay equations Natural sources of background radiation Artificial radiation Nuclear fission Nuclear fusion