2.5 Atomic & Nuclear Physics

2.5 Atomic & Nuclear Physics 3 Credits - Internal

Rutherford’s Model of the Atom Planetary Model Ernest Rutherford (1871 – 1937)

Rutherford’s Model of the Atom Protons have a positive charge which is equal to the negative charge of the electron. The neutron has no charge. Protons and neutrons are nuclear particles called nucleons.

Rutherford’s Model of the Atom In a stable nucleus, nucleons are bound together by very strong balanced forces. Attraction: Nuclear Force Repulsion: Electric Force

Rutherford’s Model of the Atom Splitting a nucleus frees up a large amount of energy (nuclear Power) Adding or removing electrons within their orbits (ionisation) also involves energy but much less than nuclear energy. (chemical energy)

Composition of the Atom - Nuclides the number of nucleons in the nucleus Mass Number A A = Z + N Chemical Symbol Atomic Number Z the number of protons in the nucleus (determines the type of element) The neutron number Nis the number of neutrons

Composition of the Atom Mass Number A Examples of Nuclides: 4 20 Chemical Symbol He Ne 2 10 Atomic Number Z A nuclide is a symbolic way of showing mass number and atomic number A = Z + N

Periodic Table of the Elements

Complete the following data table isotopes 4He 14N 24Mg 32S 40Ar 38Ar 2 7 18 18 12 16 4 24 32 38 14 40 18 18 2 7 12 16 20 2 7 12 16 22 7 12 16 18 2 18 A A = Z + N Ch Z

atoms of the same element that have a different number of neutrons

Nuclei of three naturally occurring isotopes of the element hydrogen 1 2 3 H H H 1 1 1

Electrons are arranged in orbital shells (energy levels) The innermost shell can take up to 2 electrons. The next two shells can take up to 8 electrons each 23 Na 11 The centripetal force holding the electrons in circular orbit is the electric force

Electrons are arranged in orbital shells (energy levels)

Draw a diagram of the structure of the following atoms 24Mg 32S 14N 12 16 7

Radioactivity Some elements or isotopes are less stable than others and can spontaneously emit particle or wave radiations from their nuclei 4 240 236 + He Pu U 2 94 92 Nuclear Equation

Radioactivity Conservation of Mass Number (A) Conservation of Atomic Number (Z) 4 238 234 + + γ He U Th 2 92 90 heavy nucleus new element alpha particle gamma radiation

Radioactivity Complete the following Nuclear Reaction Equations 214 4 218 + He Alpha particle Po Pb 2 84 82 99 99 0 + Beta particle Tc Ru β 43 44 -1

Radioactivity Complete the following Nuclear Reaction Equations 6 1 4 9 1 + + He Li Alpha particle Be H 2 4 1 3 42 42 0 Beta particle + K Ca β 20 19 -1

3 Types of Radiations 4 Alpha particle α a high speed helium nucleus Beta particle β a high energy electron formed when a neutron splits into a proton and an electron Gamma wave γ a very short wavelength (high frequency) electromagnetic wave. He 2 0 β -1

Ionization and Radioactivity These radiations have the ability to ionise atoms (knock out electrons from their orbits) to produce ions To prevent radiation, shielding of varying thickness is used

Ionization and Radioactivity These radiations have the ability to ionise atoms (knock out electrons from their orbits) to produce ions Alpha particleα strong ionisers (heavy and slow) but can be stopped by paper Beta particle β less ionising, more penetrating (lighter, faster) can be stopped by metal foil Gamma particle γ least ionising but travel quickly. Dense materials such as concrete or lead can stop them To prevent radiation, shielding of varying thickness is used

Ionising Radiation Sources These radiations are charged particles that can cause the atoms they encounter to become charged

can travel a few cm in air, absorbed by paper particulate radiation 4 He High 2 up to 1 m in air, absorbed by aluminum sheet 0 particulate radiation Med e -1 hardly affected by air, partially absorbed by concrete, lead γ electromagnetic radiation Low

Half Life The half life of a radioactive element is the time it takes for half of the atoms in a sample to decay 0 – all nuclei are intact. Sample is most active 1 – after 1 half life (8 days), one half of the nuclei have decayed (16 g) leaving the other half intact. Radiation emitted is now half its initial level. 2 – after 2 half lives (16 days) three quarters of the nuclei have decayed (24g) leaving a quarter intact. e.g. Consider a 32g sample of iodine – 131 with a half life of 8 days.

Half Life Result is an exponential decay curve 0 – all nulei are intact. Sample is most active (32g) 1 – after 1 half life (8 days), one half of the nuclei have decayed (16 g) leaving the other half intact. Radiation emitted is now half its initial level. 2 – after 2 half lives (16 days) three quarters of the nuclei have decayed (24g) leaving a quarter intact. 8 days

Half Life exponential decay curve of a radioactive substance What is the half life of this substance? What fraction is left after 8 days? How long does it take for 75% of this substance to decay? What is the probability that an atom will decay in 2 days? 2 days 1/16 4 days 1/2

Half Life exponential decay curve of a Radon-220 What is the half life of Radon-220? What fraction is left after about 100 seconds? How long does it take for 800 g of this sample to decay to 50 g ? How much longer will it take this 50 g sample to decay to 12.5 g? 52 seconds 1/4 4 half lives – 208 s 2 more half lives – 104 s

Half Life The half life of a radioactive element is the time it takes for half of the atoms in a sample to decay

Starter -Half Life exponential decay curve of Carbon - 14 What is the half life of Carbon-14? What fraction is left after 28,650 years? How long does it take for 800 g of this sample to decay to 200 g ? What is the probability an atom will decay in 17,190 years? 5730 years 1/32 2 half lives – 11,460 years 7/8

Starter Complete the following Nuclear Reaction Equations and state what type of decay is occurring 222 4 226 + He Alpha Decay Ra Rn 2 88 86 89 89 0 + Beta Decay Y Tc e 38 39 -1

Starter – Match Terms with Descriptions The total charge before and after a nuclear reaction remains constant The number of particles emitted per second An electron emitted by a radioactive nucleus The time taken for half the nuclei in a sample to decay An intense electromagnetic wave emitted by a radioactive nucleus A helium nucleus emitted by a radioactive atom The process of nuclei spontaneously breaking up and emitting particle or wave radiations Radioactive Decay Parent nucleus Alpha α particle Half Life Activity Conservation of charge Beta βparticle Gamma γ radiation

Starter -Half Life Living plants continually absorb carbon dioxide from the air that contains a small proportion of the Carbon-14 isotope. Their radioactivity is constant whilst they are alive but declines once they die. Material derived from dead plants can therefore be dated from their activity. a.) The Dead Sea Scrolls were dated at 100 BC. What would be the activity A measured from the parchment? 2100 years -> 11.5 particles per second b.) The charcoal from Stonehenge had an activity A of 9.4 particles per second. How old is Stonehenge? 4000 years c.) Carbon-14 emits beta particles to produce nitrogen. Determine the atomic and mass numbers for Nitrogen. A N Z Z = 6 + 1 = 7 A = 14

Starter -Half Life The half-life of Plutonium-238 is 87.7 years. A sample contains 0.34 grams of Pu-234. Calculate the mass of Pu-238 that would have existed 263.1 years before the measurement was taken. 263.1/87.7 = 3 half lives One half life ago the sample would contain 0.34 x 2 = 0.68 g of Pu-234 Two half lives ago the sample would contain 0.68 x 2 = 1.36 g of Pu-234 Three half lives ago the sample would contain 1.36 x 2 = 2.72 g of Pu-234 When will the sample of Pu-238 fall below 0.01 grams ? 2 3 4 5 6 0.17 0.085 0.0425 0.02125 0.010625 0.0053125

Starter -Half Life The half-life of Plutonium-238 is 87.7 years. A sample contains 0.34 grams of Pu-234. When will the sample of Pu-238 fall below 0.01 grams ? 2 3 4 5 6 0.17 0.085 0.0425 0.02125 0.010625 0.0053125 Between 5 and 6 half lives (430.8 < t < 526.2 years) Due to the shape of the exponential curve we see that it is closer to 5 half lives so a good estimate would be 450 years.

Nuclear Fission The process of splitting an atomic nucleus. Can be achieved by bombarding a nucleus with a high speed particle (usually neutrons) e.g. an alpha particle collides with a nitrogen nucleus to produce an oxygen atom & hydrogen atom 1 4 17 14 + + He O H N 8 7 2 1

Nuclear Fission e.g. a chain reaction where the neutrons produced in the first fission produce further fissions 92 1 141 1 235 + + + 3 n Ba Kr n U 56 92 0 0 36

Nuclear Fusion In a fusion reaction the reactants are two relatively small nuclei which fuse together to form a single, heavier, product nucleus. 17.59 Mega Electronvolts of energy is produced in this reaction 3 4 2 1 + + H He H n 2 1 1 0

Nuclear Fusion Complete the following fusion reactions 3 4 2 1 + + He He H p 1 1 2 2 6 4 2 + 2 He H Li 2 1 3

Compare The splitting of a large atom into two or more smaller ones The fusing of two or more lighter atoms into a larger one. does not normally occur in nature. occurs in stars, such as the sun. Critical mass of the substance and high-speed neutrons are required. High density, high temperature environment is required energy released by fusion is three to four times greater than the energy released by fission. a million times greater than that released in chemical reactions; Fission produces many highly radioactive particles. Few radioactive particles are produced by fusion reaction Extremely high energy is required to bring two or more protons close enough that nuclear forces overcome their electrostatic repulsion. Takes little energy to split two atoms in a fission reaction.

How Nuclear Reactors Work The principle behind generating electricity in a nuclear reactor is relatively simple. Atoms in the nuclear fuel undergo a chain reaction, and these reactions generate heat. The heat is used to turn water into steam. The steam turns turbines. When the turbines rotate, the power generators produce the electricity that is eventually fed into your home.

2.5 Atomic & Nuclear Physics