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In this lesson, we will explore the structure of an atom, including the roles of protons, neutrons, and electrons. We will also examine the concept of isotopes and the evidence supporting our understanding of atomic structure. Additionally, we will discuss the different types of radiation and their effects.

jhardwick
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  1. 2. Draw a diagram of an atom Do now 1. Can you write the title Radioactivity in your books?

  2. Radioactivity

  3. Today’s lesson • describe the structure of an atom in terms of protons, neutrons and electrons and use symbols to describe particular nuclei • understand the terms atomic (proton) number, mass (nucleon) number and isotope • What is the evidence?

  4. The atom orbiting electrons Nucleus (protons and neutrons)

  5. Nuclide notation Atomic mass (mass number) = number of protons and neutrons 7 Li 3 Atomic number (proton number) = number of protons

  6. 7 6 Li Li 3 3 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)

  7. 7 6 Li Li 3 3 Isotopes For example, Lithium atoms occur in two forms, Lithium-6 and Lithium-7 3 neutrons 4 neutrons

  8. Relative atomic mass On average, lithium atoms have a mass of 6.941 (relative to Carbon 12) 6.941 Li 3

  9. 1 2 3 H H H 1 1 1 Isotopes of Hydrogen The three isotopes of Hydrogen even have their own names! Hola! Mi nombre es tritium y yo soy de Madrid! They call me deuterium Hi! I’m hydrogen

  10. Questions!

  11. Particles in the modern model

  12. Atomic structure – key words

  13. How do we know the structure of the atom?

  14. 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

  15. 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

  16. Types of radiation New nucleus 1) Alpha () – an atom decays into a new atom and emits an alpha particle (2 protons and 2 neutrons – the nucleus of a helium atom) Unstable nucleus New nucleus Alpha particle 2) Beta () – an atom decays into a new atom by changing a neutron into a proton and electron. The fast moving, high energy electron is called a beta particle. Beta particle Unstable nucleus 3) Gamma – after  or  decay surplus energy is sometimes emitted. This is called gamma radiation and has a very high frequency with short wavelength. The atom is not changed. Unstable nucleus New nucleus Gamma radiation

  17. 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

  18. The apparatus 2 1 5 3 4

  19. 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.

  20. 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.

  21. How the results can be explained atom nucleus (highly enlarged) • 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.

  22. 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.

  23. Rutherford did the calculations! That’s 100 000 times smaller than the size of an atom(about 10-10 metres).

  24. Stadium as atom If the nucleus of an atom was a ping-pong ball, the atom would be the size of a football stadium (and mostly full of nothing)! Nucleus (ping-pong ball

  25. 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

  26. Unstable nuclei Some nuclei are unstable, for example Uranium 235 Hi! I’m uranium-235 and I’m unstable. I really need to lose some particles from my nucleus to become more stable.

  27. Unstable nuclei To become stable, an unstable nuclei emits a particle Weeeeeeeeeeeeee!

  28. Unstable nuclei We say the atom has decayed Weeeeeeeeeeeeee!

  29. Unstable nuclei The decay of an unstable nucleus is random. We know it’s going to happen, but we can’t say when! It cannot be affected by temperature/pressure etc. Weeeeeeeeeeeeee!

  30. Becquerels (Bq) • The amount of radioactivity given out by a substance is measured in Becquerels. One becquerel is one particle emitted per second.

  31. 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

  32. Background radiation There are small amounts radioactive particles around us all the time. This is called background radioactivity. The amount varies depending on location.

  33. Background radiation Background radiation comes from • Cosmic rays from space • Radioactive rocks in the ground • Nuclear tests • Nuclear bombs • Nuclear accidents

  34. Radiation Safety

  35. Radiation Safety • Run away! Mr Porter

  36. Radiation Safety • Run away! • In other words keep the distance between you and a radioactive source as big as possible! Mr Porter

  37. Radiation Safety • Don’t waste time!

  38. Radiation Safety • Don’t waste time! • In other words limit the time you are exposed to radiation.

  39. Radiation Safety • If you can’t run away, hide behind something!

  40. Radiation Safety • If you can’t run away, hide behind something! • Put a barrier between you and the radiation source that can absorb the radioactive particles

  41. Let’s try some questions. Let’s try some questions.

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