Atomic Structure I. Picture of an Atom Nucleus Electron Cloud II. Subatomic particles Particle Symbol Charge Relative Mass (amu) protons p + +1 1.0073 neutrons n 0 1.0087 electrons e - -1 0.00054858 Compare charges and relative mass. An amu or atomic mass unit is a convenient relative mass unit, because a proton and neutron each have a mass of about 1 amu. 1 amu = 1.6606 x 10-24 g. III. How p +, n, and e fit together in atoms & ions A. p + & n are bound together in the nucleus, located in the center of the atom. Note: The p + number = the atomic number or. What is in the nucleus of the most common form of the lithium (Li) atom? Key proton neutron Note: 7 Li means that the sum of the p + and n in the nucleus = 7. 7 Li nucleus 1
B. Analogy for size of nucleus relative to the whole atom: The whole atom is the size of a major league baseball park. The nucleus would be like a marble sitting out past second base. This means: 1. Nucleus: very small & dense. Does something re. the nucleus bother you? 2. Most of the atom s space is occupied by e, which have very little mass. C. Electrons (e ) are found in orbitals located outside of the nucleus. A Li atom has 3 e. D. The Bohr model (planetary?) can be represented as: (Note: This drawing is not to scale.) 1. The circles that the e are located on are called orbits. 2. Electrons (e ) in orbits farther from the nucleus are less tightly bound 3. Energies of e in the different orbitals were determined by observing light emission from atoms. (Like neon lights.) e - e - e - = the nucleus Based on your previous studies, what holds the e near the nucleus? E. Symbolism: example for carbon with 6 neutrons, 1. E = elemental symbol, 2. A = the mass number (sum of the number of protons + neutrons.) 3. Z = the atomic number (the number of protons.) F. Isotopes 1. Atoms that have the same number of protons (ie the same atomic number), but a different number of neutrons (ie different mass number. 2. Neutrons are thought to act as a kind of glue that holds the protons and neutrons together. Otherwise the protons would repel each other. 3. Many atoms have isotopes, some of which are more stable than others. 2
4. Example: isotopes of carbon: 12 C, 13 C, 14 C 12 C is the most abundant isotope. 14 C is radioactive. (Used in determining age of old objects.) 12 C 13 C 14 C p + n e IV. Looking at the elemental symbols on the periodic table. A. For example, look at carbon on the Periodic Table: B. The atomic number is 6. The atomic weight is 12.011amu and is the weight of the average C atom on earth. Remember: The atomic mass unit (amu) is a convenient unit, defined relative to a 12 C atom. One atom of 12 C is defined to weigh 12.0000 amu. C. Instruments like the mass spectrometer allow chemists to make accurate determinations of the weight and abundance of the different isotopic forms of an element. D. Carbon isotopic mass & abundance data: isotope abundance (%) mass (amu) 12 C 98.89 12.00000 source: CRC Handbook, 59 th ed. 13 C 1.11 13.00335 E. Qualitatively: The weight of the average atom should be quite close to 12 (since most of the C atoms are 12 C), but a little bit above 12 (because there are some 13 C atoms which weigh more than 12 amu.) 6 C 12.011 F. Quantitatively: Calculation of the average weight 12 C component: 12.00000 amu 98.89/100 = 11.8668 amu 13 C component: 13.00335 amu 1.11/100 = 0.1443372 amu + 3
average weight is 12.0111372 amu This is close to the 12.011 amu atomic weight value in Periodic Table. The % abundance is also called the natural abundance. Do you think the natural abundance values on earth are the same as those on other planets, meteors, asteroids, etc.? V. Quantum mechanics (views e as waves instead of particles) describes atomic behavior better than Bohr model. A. Bohr model only works well for H atoms. B. Using a specific mathematical approach (that requires very advanced math) gives a model with much better predictive capabilities. C. We will use results from quantum mechanics. Don t sweat the math. VI. Atomic orbitals (where e hang out) A. Principal quantum numbers: 1, 2, 3, etc.. Describes levels or shells around the nucleus (correlates to period numbers on periodic table) Shells Orbitals 1 (smallest shell) one 1s 2 one 2s, three 2p 3 one 3s, three 3p, five 3d 4 one 4s, three 4p, five 4d, seven 4f B. Orbital shapes: (see the Orbitron at http://winter.group.shef.ac.uk/orbitron/) 1. s orbitals are one lobed and spherical 2. p orbitals are 2 lobed and are roughly dumbbell shaped. 3. d and f orbitals have relatively complicated shapes. C. Energies You might consider that all of the orbitals of an atom (up to and beyond 7f) always exist, but only become interesting when they are occupied by e. Orbital occupancy by e - : 1. Lowest energy orbitals (those closest to nucleus) are occupied first. 4
1A 2. An orbital can only contain two e. 3. When orbitals of equal energy (ex.: 2p x, 2p y, 2p z ) are being filled, put one e in each orbital first, then add start adding additional e. D. We describe the orbital occupancy of an atom (or ion) by writing its electronic configuration. In this class you will do this by direct application of the Periodic Table. Periodic Table 1 H 1.00794 2A 3A 4A 5A 6A 7A 3 Li 6.941 11 Na 22.98977 19 K 39.0983 37 Rb 85.4678 55 Cs 132.9045 87 Fr (223) 4 Be 9.01218 < Atomic number < Elemental symbol < Atomic weight 12 Mg 24.305 3B 4B 5B 6B 7B <------------8B -----------> 1B 2B 20 Ca 40.07838 38 Sr 87.62 56 Ba 137.33 88 Ra 226.0254 21 Sc 44.9556 39 Y 88.9059 57 La 138.9055 89 Ac (227) 22 Ti 47.88 40 Zr 91.224 72 Hf 178.49 104 Rf (261) 23 V 50.9415 41 Nb 92.9064 73 Ta 180.9479 105 Ha (263) 24 Cr 51.994 42 Mo 95.94 74 W 183.85 106 Sg (263) 25 Mn 54.938 43 Tc (98) 75 Re 186.207 107 Ns (265) 26 Fe 55.847 44 Ru 101.07 76 Os 190.2 108 Hs (265) 27 Co 59.9332 45 Rh 102.9055 77 Ir 192.22 109 Mt (266) 28 Ni 58.6934 46 Pd 105.42 78 Pt 195.08 110 (269) 29 Cu 63.546 47 Ag 107.868 79 Au 196.966 111 (272) 30 Zn 65.39 48 Cd 112.41 80 Hg 200.59 112 (277) 5 B 10.811 13 Al 26.98154 31 Ga 69.723 49 In 114.82 81 Tl 204.383 6 C 12.011 14 Si 28.0855 32 Ge 72.61 50 Sn 118.710 82 Pb 207.2 7 N 14.0067 15 P 30.97376 33 As 74.9216 51 Sb 121.757 83 Bi 208.98 8 O 15.9994 16 S 32.066 34 Se 78.96 52 Te 127.60 84 Po (209) 9 F 18.99840 17 Cl 35.4527 35 Br 79.904 53 I 126.9045 85 At (210) 8A 2 He 4.00260 10 Ne 20.1797 18 Ar 39.948 36 Kr 93.80 54 Xe 131.29 86 Rn (222) 58 Ce 140.12 90 Th 232.0381 59 Pr 140.9077 91 Pa 231.0359 60 Nd 144.24 92 U 238.029 61 Pm (145) 93 Np 237.0482 62 Sm 150.36 94 Pu (244) 63 Eu 151.965 95 Am (243) 64 Gd 157.25 96 Cm (247) 65 Tb 158.9253 97 Bk (247) 66 Dy 162.50 98 Cf (251) 67 Ho 164.9303 99 Es (252) 68 Er 167.26 100 Fm (257) 69 Tm 168.9342 101 Md (258) 70 Yb 173.04 102 No (259) 71 Lu 174.967 103 Lr (260) E. Let s try a few. (Remember: Elemental identity is determined by p + number.) 1. How many electrons are there in a lithium (Li) atom, and in which orbitals are the electrons found? Draw an orbital filling diagram. Write the electronic configuration. 2. Write the electronic configuration for beryllium (Be). 5
3. For boron (B) 4. For carbon (C) 5. For nitrogen (N). 6. For oxygen (O). 7. For fluorine (F) 8. For neon (Ne) 9. For sodium (Na) 10. For magnesium (Mg) 11. For phosphorous (P) 12. For calcium (Ca) 13. For iron (Fe) 14. For selenium (Se) F. Important definition: The outermost s and p electrons are called valence electrons. Because they are outermost, they can be involved in sharing (covalent bond formation) or they may be lost or gained (in ion formation). Underline the valence electrons in the examples, above. 6
1A 1A H Periodic Table of the Elements: Orbital filling 2A 3B 4B 5B 6B 7B 8B 1B 2B 3A 4A 5A 6A 7A 8A He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Sc Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Tc Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Fr Ra Ac Blue = filling s orbitals Green = filling p orbitals Yellow = filling d orbitals 7