Modern Atomic Theory Chapter 3

1. Modern Atomic TheoryChapter 3

2. The atom…Draw this!

3. Modern Atomic Theory States: Atoms are composed of protons, neutrons and electrons. ***amu = “atomic mass unit”

4. An atom’s identity comes from the number of protons in the nucleus. this number is called the atomic number. The atomic number can be found on the periodic table. It is usually located in the upper corner. Atomic number? Carbon Chlorine Phosphorus Aluminum 6 17 15 13

5. If an atom is neutral (has no charge), the number of electrons will be the same as the number of protons. Positive charge (protons) = negative charge (electrons) Ex. Helium has an atomic number of 2. So it has 2 protons And, if neutral, has 2 electrons. So if an atom is neutral, you can use the atomic number to find the number of electrons as well.

6. How many electrons are in each of the following neutral atom? Number of electrons? Fluorine Boron Neon Oxygen Carbon Argon 9 5 10 8 6 18

7. Mass Number Mass number = number of protons + number of neutrons To find the number of neutrons, you use the mass number of the element. The mass number can be found by rounding the average atomic mass of an element to a whole number. Ex. Hydrogen has an average atomic mass of 1.00794 so its mass number would be 1.

8. Finding the number of neutrons.. Number of neutrons = mass number – atomic number Example: The mass number of sodium is 23. Sodium has 11 protons… so it has 12 neutrons (23-11).

9. Determining the Number of Neutrons # Neutrons = Mass Number – Number of Protons Mass # (6) - # protons (3) = # neutrons (3) Number of Neutrons? Aluminum Gallium Silicon 27 - 13 = 14 70 - 31 = 39 28 – 14 = 14

10. More examples... Protons Electrons Neutrons 1 0 Hydrogen 1 20 20 20 Calcium Chlorine 17 17 18

11. Na +1 Ions Ba+2 O-2 Cl-1 Atoms can lose and gain electrons. An ion is an atom that has lost or gained electrons to take on a net electrical charge. Charge of ion = # of protons - # electrons Example: Mg+2 always has 12 protons but because it has a charge of +2 it only has 10 electrons (12-10 = 2).

12. More Examples Ion Protons Electrons K+1 19 18 38 36 Sr +2 Br-1 35 36 S-2 16 18

13. Daily Grade Assignment: Be sure to put your name on it! For each question, write down the name of the element, the symbol of the element, and then the specific information that is being asked for about the element: For these elements, tell me the atomic number: Hydrogen 2) Helium 3) Fluorine 4) Lead 5) Silver For these elements, tell me the number of electrons: 6) Nickel 7) Nitrogen 8) Oxygen 9) Sodium 10) Calcium For these elements, tell me the number of neutrons: 11) Potassium 12) Phosphorus 13) Iodine 14) Tin 15) Carbon For these elements, tell me the number of protons, neutrons, and electrons: 16) Lithium 17) Barium 18) Magnesium 19) Chlorine 20) Iron

14. Isotopes (Hyperlink) In nature, elements can be found with different numbers of neutrons. An isotope is an atom that has the same number of protons but has different numbers of neutrons. Because of the differing number of neutrons, isotopes differ in mass. Example: All chlorine atoms have 17 protons-but some chlorine nuclei have 18 neutrons while others have 20 neutrons.

15. Protons Neutrons Electrons 30 26 Iron-56 26 35Cl 17 18 17 Examples 17 36 Cu+2-65 29 27

16. Calculating Average Atomic Mass • The average atomic mass of an element takes into account all of an element’s isotopes. Remember… because isotopes differ in #’s of neutrons, their masses will differ • Avg. Atomic Mass = the sum of each elements fractional abundance multiplied by its mass

17. Examples: Mass Abundance Avg. Atomic Mass Carbon Isotope 1 Isotope 2 Chlorine Isotope 1 Isotope 2 Silicon Isotope 1 Isotope 2 Isotope 3 12 98.89% 13.003 1.11% 34.969 75.53% 36.966 24.47 27.977 92.21% 28.976 4.70% 29.974 3.09%

18. Examples: Mass Abundance Avg. Atomic Mass Carbon Isotope 1 Isotope 2 12 98.89% 13.003 1.11% 12.011 (12)(0.9889) + (13.003)(0.0111) = 12.011

19. Examples: Mass Abundance Avg. Atomic Mass Carbon Isotope 1 Isotope 2 Chlorine Isotope 1 Isotope 2 Silicon Isotope 1 Isotope 2 Isotope 3 12 98.89% 13.003 1.11% 34.969 75.53% 36.966 24.47 27.977 92.21% 28.976 4.70% 29.974 3.09% 12.011 35.45 28.09

20. Nuclear Reactions • Reactions in which changes occur in the nucleus of an atom and result in a change of composition in the nucleus

21. Emission of one of the three types of radiation: Alpha Beta Gamma Radioactivity

22. Electric repulsion between protons Strong nuclear force which overcomes the repulsion between protons in the nucleus and keeps it together. ***The presence of neutrons adds to the net attractive force in the nucleus. In the nucleus, two forces exist: If the strong nuclear force is not sufficient to overcome the repulsion between protons, the nucleus begins to fall apart. This is what causes an element to be radioactive.

23. Elements 1-20 are stable because they have almost equal #’s of protons and neutrons. Beyond 20 protons, nuclei need more neutrons than protons to stabilize. When the atomic # exceeds 83, no # of neutrons is sufficient to hold the nucleus together. All nuclei w/ atomic #’s greater than 83 are radioactive. Stable Nuclei

24. Alpha Decay • Stream of high energy alpha particles • Consists of 2 protons and 2 neutrons • Not very penetrating-can be blocked by paper or clothing

25. Alpha Decay • Ex: Write the nuclear equation for the alpha decay for U. • U  He + Th • He = an alpha (α) particle • Practice!!...get out yo paper! http://www.chemteam.info/Radioactivity/Writing-Alpha-Beta.html 238 92 238 92 4 2 234 90 4 2

26. Beta Decay • Stream of high speed electrons • Electrons are produced when a neutron splits into a proton and an electron (beta particle) - proton remains in nucleus, while electron is emitted as a beta particle. • 100 times more penetrating than alpha

27. Beta Decay • Neutron splits into protons + electron • So, add a proton to your original atom (changes the ID of the atom), but your mass number remains the same because you are swapping a neutron for a proton • Ex: Write the nuclear equation for the beta decay for Pa. • Pa  U + e 234 A wee bit more practice… 91 234 234 0 -1 91 92

28. Gamma Radiation • Very energetic form of light-does not consist of particles • Penetrates deeply into solid material-stopped by heavy shielding, such as concrete or lead.

29. Half-life • No two radioactive isotopes decay at the same rate. • Half-life (t1/2) is the time required for half the atoms of a radioactive nuclide to decay. • More stable nuclides decay slower and have longer half-lives.

30. Application- Radioactive Dating The half-life of potassium-40 is 1.3 x 109 years. A volcanic rock contains 1/8 of the amount of potassium-40 found in newly formed rocks. When was the rock formed? • First determine the number of half-lives that have passed: 1/8 = (1/2) x (1/2) x (1/2) Therefore, three half-lives have passed so… 3 x (1.3 x 109) = 3.9 x 109 years have passed since the rock was formed!!!

31. Example Problem: • Phosphorous-32 has a half-life of 14.3 days. How many milligrams of phosphorous-32 remain after 57.2 days if you start w/ 4.0 mg? 2. After 4797 years, how much of the original 0.250 g of radium-226 remains if the half-life is 1599 years?

32. Fusion vs. Fission Fission Fusion

33. Fusion • Nuclear fusion occurs when low-mass nuclei combine to form a heavier, more stable nucleus. • Fusion releases more energy per gram than nuclear fission. • In our sun, hydrogen nuclei combine to form a helium nucleus. • A temperature of 108K is required to induce fusion!!!!

34. Fission • Fission occurs when a very heavy nucleus splits into more stable nuclei. • Fission can occur spontaneously or when nuclei are bombarded by particles. • Nuclear power plants utilize fission to produce electricity.