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Nuclear Changes Chapter 7. 7.1 What is Radioactivity? Large atoms are unstable. When the nucleus is crowded with protons and neutrons, it’s just ”too.

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Presentation on theme: "Nuclear Changes Chapter 7. 7.1 What is Radioactivity? Large atoms are unstable. When the nucleus is crowded with protons and neutrons, it’s just ”too."— Presentation transcript:

1 Nuclear Changes Chapter 7

2 7.1 What is Radioactivity? Large atoms are unstable. When the nucleus is crowded with protons and neutrons, it’s just ”too much.” The nucleus begins to emit (shoot out) particles and/or energy.

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5 Radioactivity Penetrating power of different forms of radiation:

6 Radioactivity Marie ( ) and Pierre Curie ( ) isolated polonium and radium from pitchblende both elements more radioactive than pure uranium discovered that the source of energy (radiation) were the atoms themselves nature of radioactivity was still unknown

7 Radioactivity Ernest Rutherford ( ) studied absorption of 'rays' emitted by uranium-containing minerals two types of rays:  - and  -rays  -rays are more penetrating than  -rays  - and  -rays are not rays at all (like X-rays or light) but streams of particles

8 Radioactivity  - and  -rays are streams of charged particles: How can you test if a particle is positively or negatively charged?

9 Radioactivity  - and  -rays are streams of charged particles: How about their mass? light particles are easier to deflect than heavy ones (pushing a freight train versus a bicycle!)

10 Radioactivity Ernest Rutherford ( )  -particles behave like electrons, (1 negative charge) - move very fast  -particles and have 4 times the mass of a hydrogen nucleus and twice the charge (2 positive charges)  -particle = Helium nucleus (2 protons, 2 neutrons)

11 Radioactivity  - and  -radiation are made up of particles,  -radiation is not!  -radiation is electromagnetic radiation (just like light and X-rays): no mass, no charge

12 Radioactivity Radioactive decay:  -decay U the atomic number counts the number of protons the mass number counts protons and neutrons

13 Radioactivity Radioactive decay:  -decay U  Th the atomic number decreases by 2 (loss of 2 protons) the mass number drops by 4 (loss of a total of 2 protons and 2 neutrons)

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15 Radioactivity Radioactive decay:  -decay Ra  Rn Rn  Po Cm  Pu

16 Radioactivity Radioactive decay:  -decay Proton Neutron a Neutron may split into a Proton plus an Electron Electron

17 Radioactivity Radioactive decay:  -decay Proton Neutron Electron the electron is ejected from the nucleus as  -radiation......leaving behind a nucleus with an extra proton

18 Radioactivity Radioactive decay:  -decay Bi  Po the atomic number increases by 1 amu (1 more proton) the mass number is unchanged (the electron mass in negligible)

19 Radioactivity Radioactive decay:  -decay C 6 14  N 7 14 H 1 3  He 2 3 Pb  Bi

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21 Nuclear vs Chemical Reaction Na NaOH + HCl  H 2 O + NaCl O H H Cl Na O H H Cl  *** Not a true representation of this reaction in solution Chemical Reaction Nuclear Reaction  212 Po  4  + 82 Pb *** Not a true representation of the nuclei

22 The Half-Life (t 1/2 ) of a Nuclear Reaction Half-life (t 1/2 ): The time it takes for half of the radioactive nuclei in a sample to decay. 48 radioactive particles at t=0 24 radioactive particles at t=1 (1 half life) 12 radioactive particles at t=1 (2 half life) 6 radioactive particles at t=1 (3 half life) # of radioactive nuclei

23 The Half-Life (t 1/2 ) of a Nuclear Reaction Half-life (t 1/2 ): The time it takes for half of the radioactive nuclei in a sample to decay. 48 radioactive particles at t=0 24 radioactive particles at t=1 (1 half life) 12 radioactive particles at t=2 (2 half lifes) 6 radioactive particles at t=3 (3 half lifes) # of radioactive nuclei Fraction of nuclei 48/48 = t 1/2 = 1 24 = 1 48 t 1/2 = 2 12 = 1 * 1 = t 1/2 = 3 6 = 1 * 1 * 1 =

24 The Half-Life (t 1/2 ) of a Nuclear Reaction Half-life (t 1/2 ): The time it takes for half of the radioactive nuclei in a sample to decay. 48 radioactive particles at t=0 24 radioactive particles at t=1 (1 half life) 12 radioactive particles at t=2 (2 half lifes) 6 radioactive particles at t=3 (3 half lifes) # of radioactive nuclei Fraction of nuclei 48/48 = t 1/2 = 1 24 = 1 48 t 1/2 = 2 12 = 1 * 1 = t 1/2 = 3 6 = 1 * 1 * 1 = General Formula Fraction remaining = 1 2 n where n is the # of half lifes

25 Let’s go over all that again!

26 Phenomenon of Radioactivity Some elements, such as uranium (U) and thorium (Th), are unstable: They decay spontaneously.

27 Uranium Nucleus spontaneously emits a particle from its nucleus called an alpha particle (2 protons + 2 neutrons).

28 Alpha Particle emits a particle from its nucleus called an alpha particle (2 protons + 2 neutrons).

29 Uranium - Thorium Decay U He + Th spontaneous decay “parent”“daughter product” alpha particle = 2 protons + 2 neutrons = positively charged ion of Helium Thorium: 90 protons neutrons

30 Beta Particle Emission But, Th is also unstable, and it emits a beta particle …

31 Th + Pa Thorium - Protactinium Decay beta particle beta particle = an electron discharged from the nucleus when a neutron splits into a proton and an electron Protactinium: 91 protons neutrons

32 Title beta particle = an electron discharged from the nucleus when a neutron splits into a proton and an electron

33 U Pb Series This process is called radioactive decay, and eventually uranium (parent) decays to lead (daughter product).

34 U Pb Series The rate at which this process occurs is measured in terms of the “half life”.

35 Half Life Half Life = Number of years for 1/2 of the original number of atoms to decay from U to Pb

36 Carbon-14 Dating

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