The Atom

1. The Atom

2. The Atomic Number = number of protons • ( = number of electrons in neutral atom) • The Mass Number = Number of protons + number of neutrons. • Isotope = atoms with same atomic number but a different mass – different number of neutrons.

3. Radioactivity • The stability of atom depends on how many protons and neutrons it has. • If the numbers of protons are approximately equal to the number of neutrons then the atom will be stable. Example – the lighter elements. • However – if the is a large difference between the number of protons and neutrons the atom is unstable. • As the atoms get heavier, the neutron:proton ratio increases. The neutrons help keep the nucleus from flying apart since the + protons repulse each other.

4. What is radioactivity? • The unstable nuclei start to release energy spontaneously – we call this decay. • The nuclei will keep doing this until the neutron:proton ratio lie in the band of stability. • Radioisotopes are isotopes that are radioactive. • Radioactive decay is unlike normal chemical reaction because it is the nucleus that changes and not the electrons.

5. The nature of Radiation • When a nucleus undergoes decay it emits small radioactive particles. • There are 3 types: alpha (α), beta (β)and gamma (γ).

6. Alpha and beta are particles. • Gamma is electromagnetic radiation. • Alpha 4 He 2+ 2 Beta are high energy electrons o e – -1

7. Each type of radiation has unique properties. • Alpha is the heaviest and least penetrative – it travels for a very short distance and can be stopped by paper. • Beta – very light will travel almost 0.6 cm and be stopped by Aluminium foil. • Gamma is the most penetrative – it is stopped by lead or thick concrete.

9. Nuclear Reactions • When a radioactive atom starts to decay the original nuclei will change. • We can balance the change by making sure the total mass on the LHS and RHS are the same and the total atomic number on the LHS and RHS are the same.

10. Nuclear Equations • Alpha decay • When the original atom releases an alpha particle we can figure out the name of the new atom by taking away the mass and atomic number of an alpha particle from the original atom. • Example • Polonium with a mass of 210 emits an alpha particles. • 210 P —> 4 α 2++ 206 Pb 84 282

11. Beta Decay • 90 Sr —> 0 e + 90 Y 38 -1 39 Gamma Decay Gamma particles have no mass and charge – there emission has no effect on the mass and atomic number of the radioisotope.

12. Making Radioisotopes • This can be done by bombarding the target nucleus with small particles: alpha, neutrons or protons. • Example • Neutron bombardment • 238 U + 1 n —> 239 U 92092

13. Alpha bombardment • 27 Al + 4 He 2+ —> 30 P + 1 n 132150 Proton Capture 14 N + 1P —> 15 O 7 18

14. Half Life ( t ½) • This is the time taken for a radioisotope to to half it’s initial activity. The time taken for half the radioactive atoms to disintegrate. • The rate of decay depends on the mass of the isotope. • However – the half life of a particular isotope will alwaysbe the same regards of the mass of the sample size – 1 g of Uranium will have the same half life as 100g of Uranium. • Temperature and pressure and particle size etc have no effect on radioactive decay.

15. n = number of half lives ( t ½) • After n half lives the fraction of the original sample left is ( 1/2) n • The length of half life various from one radioisotope to the next – some are very short, others very long. • The half life of an atom will always be the same if it is in element form or in a compound. • This is because radioactive decay is nuclear – unlike chemical reactions. The nucleus of an atom in an element and in a compound will be the same. Only their electron arrangement will be different.

16. Calculations using half life ( t ½) • Phosphorous has a half life of 14 days. • The sample has a mass of 80g. • Calculate the mass of the sample of the radioisotope after 56 days. • Step 1 – Work out how many t ½ have passed. • Time = 56 days • T ½ = 14 days • Therefore 4 t1/2 have passed.

17. Step 2 • Time ( days) Mass (g) 0 80 14 ( 1 t ½) 40 28 ( 2 t ½) 20 42 ( 3 t1/2) 10 56 ( 4 t1/2) 5 The remaining mass is 5 g.

18. If a radioisotope has a t ½ of 5.6 years. How long will it take 50g of the sample to decay leaving 12.5g of the original sample? • Step 1 • How many t ½ has the sample gone through to become 12.5 g • Time Mass • 0 50 • 1 t1/2 25 • 2 t1/2 12.5g

19. The sample has gone through 2 half lives to leave 12.5g. • Therefore 2 x 5.6 years = 11.2 years. • What is the half life of a sample if it’s radioactive count changes from 200 counts per minute to 50 counts per minute in 36 days? • Time Activity ( counts per min) • 0 200 • 1 t ½ 100 • 2 t1/2 50 • Therefore 2 half lives have passed in 36 days – t ½ = 18 days.

20. Uses of radioisotopes • Medical • Cobalt 60 is a gamma emitter and is used to treat deep rooted tumours. • 32 P is a beta emitter and is used to treat skin cancer. • Radioactive Iodine is used to monitor the Thyroid gland. It is taken orally or injected and the gland is scanned over a certain period – plotting the concentrations of the isotope in the body. The isotope can also be used to kill cancerous cells. • Gold 198 – a wire of the element is placed directly into the tumour – the isotope decays over a few days – dosing the tumour.

21. Industrial Uses of Radioisotopes • Co 60 is a gamma emitter and is used to examine castings and welds for imperfections. • Radioisotopes can be used to measure thickness of sheet material. Beta and gamma is used for this. 2 identical sources used – i as a reference and the other passes through the material. The 2 are compared and the distance can be measured. • Smoke alarms contain an alpha emitter – not very penetrative . 241 americium is used.

22. Scientific Research • Carbon dating • 14 C is produced in the atmosphere by the bombardment of N by neutrons. • 14 N + 1 n —> 14 C + 1 p 7 0 6 1 The 14 C is always being taken up by plants the same as 12 C so there is a known ratio for 14 C : 12C We measure the ratio in the substance to be dated and then using the half life of 14C we can age the object.

23. Example • Piece of wood contains 3.75 counts per minute. • New wood has an activity of 15 cpm, 14C has a t ½ of 5600 years. • t1/2 Counts 0 15 1 t ½ 7.5 2 t ½ 3.75 2 t ½ have passed therefore the wood is 2 x 5600 = 11,200 years old.

24. 3 H ( tritium) is used to date the age of vintage wines. ( t ½ = 12.4 years) • 32P is used to trace how plants use phosphate fertilisers. • Background Radiation • There will always be a safe level of radiation in the air called background radiation. • This comes from cosmic rays, igneous rocks, gamma rays.

25. Nuclear Fusion • This is when to light nuclei fuse to form a heavier nuclei. • 1H + 2 H —> 3 He 1 1 2 Nuclear fusion happens in the sun releasing vast amounts of energy. 2H + 2 H —> 3 He + 1 n 1 1 2 0

26. Because the T and P in the centre stars are so high further fusion reaction take place producing even heavier nuclei. All natural elements where produced in nuclear fusion in the stars! • The Hydrogen Bomb uses nuclear fusion. Very high T allow the small nuclei to join and not repulse one another.

27. Nuclear Fission • This is when heavy nuclei split into lighter nuclei. • The energy can be harnessed – this is the type of reaction used in nuclear power stations, generate electricity, submarines etc. • 235 U + 1n —> 90 Sr + 144 Xe + 2 1n 92 0 38 54 0 The 2 neutrons produced will bombard more 235 Uranium starting a chain reaction.

28. The reaction can be controlled by adding a non fissionable material to the reactor that will absorb the neutrons – preventing further bombardment of 235 U. • The Atomic Bomb was a sample of 235U above critical size and so leading to a nuclear explosion. • 239 Plutonium is used in fast breeder reactors.