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Chapter 4 Atomic Theory

Chapter 4 Atomic Theory. Matter. All matter is made of atoms Alone as elements Au, Na, O, He In combination of elements as compounds H 2 O, NaCl , LiO 2. Early Theories of Matter. Democritus (460-370 B.C.) proposed & believed that Matter was not infinitely divisible

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Chapter 4 Atomic Theory

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  1. Chapter 4Atomic Theory

  2. Matter • All matter is made of atoms • Alone as elements • Au, Na, O, He • In combination of elements as compounds • H2O, NaCl, LiO2

  3. Early Theories of Matter • Democritus (460-370 B.C.) proposed & believed that • Matter was not infinitely divisible • Made up of tiny particles called atomos • Atoms could not be created, destroyed, or further divided

  4. John Dalton, 19th Century • Dalton revised Democritus's ideas based upon the results of scientific research he conducted • Dalton’s atomic theory • Not totally correct

  5. Dalton’s First Atomic Theory: • Elements are made up of small indivisible particles called atoms • Atoms of the same element are identical, different elements are different - (Same size, mass and chemical properties) • Atoms are not created or destroyed in a chemical reaction • A compound always has the same relative numbers and kinds of atoms

  6. What is an atom? • An atom is the smallest particle of an element that retains the properties of the element

  7. Atomic Models • Thomson: Plum Pudding Model • Rutherford: Electron Cloud Model

  8. Rutherford: The Nuclear Atom • His model consisted of the following ideas: • an atom consists mostly of empty space through which electrons move • electrons are held within the atom by their attraction to the positively charged nucleus • small, dense, positive charged nucleus

  9. Bohr Model • Based on Rutherford’s model • Adds idea of “quantized” energy levels

  10. Inside an atom: subatomic particles • These particles have mass and charge • Nucleus holds protons & neutrons • Protons: + charge • Neutrons: Ø charge • Outside of nucleus is the electron cloud • Electron: - charge

  11. Parts of the Atom

  12. Atomic Mass Unit (amu) • Small mass #’s are not easy to work with, so the atomic mass unit (amu) was developed • The mass of 1 amu is nearly equal to the mass of one proton or neutron

  13. Reading the Periodic Table

  14. Atomic Number • The number of protons determine the element (ALWAYS!!!) • Number of protons are unique to each element • Examples: • Carbon (C) has 6 protons • Atomic number is 6 • Copper has 29 protons • Atomic number is 29

  15. Atomic Number • In uncharged atoms, atomic number is also the number of electrons • Why? • If an atom is charged, then it is an ion • Uncharged atom: Atomic number = # of protons = # of electrons Charge = # protons - # electrons

  16. Mass Number • To find the Mass number # protons + # neutrons = mass number • To find # neutrons mass number – proton (or atomic number) = neutrons • Mass numbers are always WHOLE #’s!!

  17. Symbols for Atoms X= symbol of element A= mass number Z= number of protons X or X • Can also be written “element - A” • i.e. carbon - 12 A A Z

  18. Mg-25 Zn atomic # proton neutron electron mass # 70 30 30 12 12 30 13 40 30 12 70 25

  19. Turn to your partner and summarize… • How does the number of protons affect the identity of the atom? • How do the number of protons and electrons relate to each other in a neutral atom? • What is an ion?

  20. Isotopes and Mass Number C C carbon-12 carbon-13 Isotopes are atoms with the same number of protons but different number of neutrons 12 6 13 6

  21. Isotopes and Mass Number • Example: 3 types of Potassium All 3 types contain 19 protons and __ electrons # of Protons # of Neutrons Mass Number 19 20 19 21 19 22 19 39 40 41

  22. What’s the difference between mass number and average atomic mass(weight)? C carbon-12 But if you look on the periodic table, the number states 12.01… Atomic Mass (atomic weight) – a weighted average of the masses of all of the isotopes of that element. It is not the same as the mass number. 12 6

  23. What’s the difference between mass number and average atomic mass? • Mass number- specifically about one isotope; the number of protons + neutrons • Average atomic mass- includes the masses of all the different isotopes for that atom

  24. Mass of Individual Atoms • Average atomic mass: • The weighted average mass of the isotopes of an element • Example: Chlorine • Mixture of 75% chlorine-35 and 25% chlorine-37 Atomic mass = (0.75)*35.0 + (0.25)*37.0 = 35.5 amu

  25. Try this one • 3 isotopes of neon: • Ne-20 (90.92%) • Ne-21 (0.25%) • Ne-22 (8.83%) What is the average atomic mass of Ne? (20)*(0.9092) + (21)*(0.0025) + (22)*(0.0883) = 20.18 amu

  26. Radioactivity – when the nucleus of an atom is unstable causing it to decompose into another nucleus • There are three types of radioactive decay: • Alpha Decay • Beta Decay • Gamma Decay

  27. Shielding: Alphas, Betas, Gammas and Neutrons

  28. Alpha Decay • An alpha particle (α ) is produced • An alpha particle is just a helium nucleus, He+2 4 2

  29. Beta Decay • An e- is kicked out of the nucleus (a neutron breaks up into a proton and e-), e 0 -1

  30. Gamma Decay • Only ENERGYis released from the nucleus • the nucleus itself does not change, but almost always accompanies alpha and beta decay

  31. Alpha Decay • 23090Th  42He + 22688Ra • 22288Ra  • Beta Decay • 23490Th  0-1e + 23491Pa • 13153I  • Gamma Decay • 23892U  42He + 23490Th + energy 42He + 21886Rn 0-1e + 13154Xe

  32. What are the products of Po-218 after it undergoes alpha decay followed by a beta decay, followed by beta decay followed by alpha decay? 21884Po  42He + 21482Pb 21482Pb  0-1e + 21483Bi 21483Bi  0-1e + 21484Po 21484Po  42He + 21082Pb

  33. Where Does the Radiation Come From? • The radiation you receive can be either Natural or Man-made

  34. Turn to your partner and summarize… • List the 3 types of radioactive decay and the particle that is released in each • Which type of radioactive decay is most penetrating?

  35. Half-Life (t1/2) - time required for one half of the original sample of nuclei to decay. • The half-life of Ra-223 is 12 days. If you start with 100.0 grams of Ra-223, how much will be left after 36 days? • 100.0 g  50.00 g  25.00 g  12.50 g • The half life of Ra-225 is 15 minutes. If you have 10.0 grams now, how much did you start with 60 minutes ago? • 10.0 g  20.0 g  40.0 g  80.0 g  160.g

  36. Fission – splitting a nucleus into two or more smaller nuclei • 1 n + U  Kr + Ba + 3 n + energy • This is what takes place in a nuclear reactor or an atomic bomb. 141 56 92 36 235 92

  37. Fission • chain reaction - self-propagating reaction

  38. 4 2 2 1 3 1 Fusion – combining of two nuclei to form one nucleus of larger mass H + H  He + 1n + energy • This is how all of the elements were created in natureand occurs naturally in stars.

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