Unit 2 Atomic Theory

1. Unit 2Atomic 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 12 30 12 30 13 40 12 30 25 70

19. 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

20. 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

21. 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

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

23. Mass of Individual Atoms • Atomic mass: • The weighted average mass of the isotopes of an element • Example: Chlorine • Mixture of 75.00% chlorine-35 and 25.00% chlorine-37 Atomic mass = (0.7500)*35.00 + (0.2500)*37.00 = 35.50 amu

24. Try this one • 3 isotopes of neon: • Ne-20 (90.92%) • Ne-21 (0.25%) • Ne-22 (8.83%) What is the atomic mass of Ne? (add .00 to masses) (20.00)*(0.9092) + (21.00)*(0.0025) + (22.00)*(0.0883) = 20.17 amu

25. There are two isotopes of magnesium in a container: Mg-24 and Mg-25. The mass of 121 atoms of magnesium is 2988 amu. How many of each isotope are present? X = 37  37 Mg-24 atoms Y = 84  84 Mg-25 atoms

26. A container has 10,000. carbon atoms have a mass of 120,110. amu. There are 2 carbon isotopes: C-12 and C-13. How many of each isotope? C-12 = 9890. C-13 = 110. The atomic weight of lithium is 6.072 amu. There are two isotopes:Li-6 and Li-7. the mass of 151 atoms is 917 amu? How many of each isotope? Li-6 = 140. Li-7 = 11 The mass of one nickel is 5.13 grams, the mass of one dime is 2.73 grams, the mass of 1 envelope is 4.38 grams. Eight coins are put into an envelope. The mass of the envelope and coins is 38.22 grams. How many dimes and nickels are in the envelope? Dimes = 3 Nickels = 5

27. A container with a mass of 4.13 grams has a total of 25 dimes and quarters. The mass of a dime is 2.73 grams, and a quarter is 7.26 grams. If the total mass of the container and coins is 149.39 grams, how many dimes and how many quarters are there? • Dimes = 8 Q = 17

29. c wl f

30. Energy of light is: - directly related to the frequency of the light - inversely related to the wavelength. - As frequency , energy - As wavelength , energy Energy has units of Joules (J) E = hf h = Planck’s constant = 6.626 x 10-34 J. s E f h

31. What is the energy and wavelength of radiation whose frequency is 6.775 x 1018 s-1? 3.00 x108 m/s = 4.44 x10-11 m 6.775 x1018 s-1 E = hf = (6.626 x10-34 Js) (6.775 x1018 s-1) = 4.489 x10-15 J

32. What is the wavelength of light that has a frequency of 2.27 x1017 s-1? • What is the frequency of light that has a wavelength of 9.13 x10-10 m? • What is the wavelength of light with a frequency of 4.27 x1018 s-1? • Radiation has an energy of 1.23 x10-16 J. What is the frequency and wavelength of the radiation?

33. wavelength = 1.32 x10-9 m • frequency = 3.29 x1017 s-1 • wavelength = 7.03 x10-11 m • frequency = 1.86 x1017 s-1 • wavelength = 1.62 x10-9 m

34. 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

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

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

37. Gamma Decay • energy is released from the nucleus • the nucleus itself does not change, but almost always accompanies alpha and beta decay

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

39. 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

40. 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

41. Fission – when an atom splits into two or more smaller atoms in a nuclear reaction. • 1 n + U  Kr + Ba + 3 n + energy • This is what takes place in a nuclear reactor or an atomic bomb. 92 36 141 56 235 92

42. 2 1 3 1 4 2 Fusion – when several smaller atoms combine to make a larger atom H + H  He + 1n + energy • This is how all of the elements were created in nature.