Modern Atomic Theory

1. Modern Atomic Theory

2. Modern Atomic Theory States: Atoms are composed of protons, neutrons and electrons.

3. 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 right hand corner.

4. If an atom is neutral (has no charge), the number of electrons will be the same as the number of protons. Positive charge = negative charge 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.

5. Mass Number To find the number of neutrons, you use the mass number of the element. Mass number = number of protons + number of neutrons 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.

6. 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).

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

8. 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).

9. More examples: Ion Protons Electrons K+1 18 19 38 36 Sr +2 35 36 Br-1 S-2 16 18

10. 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.

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

12. Calculating Average Atomic Mass • The average atomic mass of an element takes into account all of an elements 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

13. 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

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

16. 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.

17. 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

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

19. Beta • 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

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

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

22. 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.

23. 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!!!

24. 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? • After 4797 years, how much of the original 0.250 g of radium-226 remains if the half-life is 1599 years?

25. Fusion vs. Fission Fission Fusion

26. 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 108 K is required to induce fusion!!!!

27. 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.