Stability Nuclear & Electronic (then ion formation/covalent bonding) Most elements are not stable in their atomic form. (Exceptions to that? ) They become stable by gaining or losing e! to form ions, or by sharing e! to form covalent bonds. Stability is usually not considered this early in a 100 level chem course, & not as a separate subject. We need to consider it in some detail because the existence of living things is absolutely dependent on their ability to acquire energy from unstable things. I. If something exists for a long time, we say it is stable. If something exists for a brief time & then changes, it was unstable. 1
This area can actually get pretty tricky, but in ways that are interesting regarding living things. Some materials appear to be quite stable energetically, when they are not. We need to understand stability in terms of both Equilibrium and Kinetics. 2
II. What makes nuclei stable? A. Ratio of n'p + (see band of stability graph) 1. ~1'1 for elements (d) with few p + ( p + 20) (Ca has 20 p + ) 2. Approaches 1.5'1 for as p + increases to 83 (Bi has 83 p + ) B. Size: Bismuth (Bi): heaviest element w/ indefinitely stable forms. All forms of all elements w/ p + > 83 are unstable. (They decay to more stable things radioactively.). C. Even vs. odd n and p + # s. Even # have more types of stable nuclei. 3
D. Example: isotopes (d) of carbon. ( 12 C is the most abundant isotope.) 12 C 13 C 14 C 14 C decays to: 14 N + p + n e! Write a chemical equation to show how 14 C decays into 14 N +. Remember principle of conservation of charge. 4
20 Chart of nuclide stability Indefinitely stable nuclei shown in green. 30 Al 28 Mg 29 Al 32 Si 31 Si 30 Si 15 26 Na 27 Mg 28 Al 29 Si neutron # 18 N 20 O 19 O 21 F 20 F 24 Ne 25 Na 26 Mg 27 Al 23 Ne 24 Na 25 Mg 26 Al 22 Ne 23 Na 24 Mg 25 Al 21 Ne 22 Na 23 Mg 24 Al 28 Si 27 Si 26 Si 25 Si 10 16 C 17 N 18 O 19 F 20 Ne 21 Na 15 C 16 N 17 O 18 F 19 Ne 20 Na 21 Mg 13 B 14 C 15 N 16 O 17 F 18 Ne 20 Mg 11 Be 12 B 13 C 14 N 15 O 17 Ne 8 He 9 Li 10 Be 11 B 12 C 13 N 14 O 5 8 Li 9 Be 10 B 11 C 12 N 13 O 6 He 7 Li 8 Be 9 B 10 C 5 He 6 Li 7 Be 8 B 3 H 4 He 5 Li 6 Be Nuclide refers to a specific proton/neutron combination present in an atom. 0 0 2 H 1 H 3 He 5 proton # 10 5
III. What makes atoms & ions be stable? Depends on e!. Are K atoms stable? http://www.youtube.com/watch?v=edo_ys9f2bi Focus: Lewis Octet Principle. (Some unstable atoms form ions.) A. For representative elements a full valence shell is stable. The valence e! are those in the outermost s & p orbitals. Do e! configurations of C, Ne, Mg, & Cl. 6
1. Circle the valence (d) electrons in the e! configuration of Ne. 2. More examples: Is the C atom above stable? Why or why not? Is the Ne atom above stable? Why or why not? Is the Cl atom above stable? Why or why not? What about the Cl! ion? 7
Is the Mg atom? Why or why not? What about the Mg 2+ ion? (valence shell e!?) How do Mg 2+ ions form? Mg o ý Mg 2+ + 2 e! (A Mg atom loses 2 e! ) Energy level diagram? How do Cl! ions form? Cl o + e! ý Cl! (A Cl atom gains one e! ) Is it easy to get Cl o? Does stability depend on environment? 8
B. For transition metals, lanthanides, & actinides it s trickier. Ex.: (You should know #1 & #2 from life.) 1. Gold (Au) is quite stable as an atom Au 0, & unstable as Au 1+ or Au 3+ 2. Iron (Fe) is moderately stable as Fe 0, Fe 2+, and Fe 3+. It can shift back & forth between these forms by altering the environment. 3. Manganese (Mn): relatively unstable as Mn 0 & fairly stable as Mn 2+. 9
C. Most of our emphasis is on the representative elements, those in Groups I-VIII IV. Before leaving this topic, what do the numbers below the elemental symbols on the periodic table mean? A. Ex.: look at carbon on the Periodic Table: 1. What does the 6 mean? 2. What about the 12.011? 6 C 12.011 10
B. 12.011 is for the weight (amu) of the average C atom on earth. C. Instruments (mass spectrometer) let chemists make accurate estimates of weight & abundance of the different isotopic forms of an element. Aside: The atomic mass unit (amu) is defined relative to a 12 C atom. One atom of 12 C is defined to weigh 12 amu. (How many s.f. in that?) 11
D. Carbon isotopic mass & abundance data: isotope abundance (%) mass (amu) source: CRC 12 C 98.89 12.00000 Handbook, 59 th ed. 13 C 1.11 13.00335 E. Qualitatively. The weight of the average atom should be quite close to 12. Most of the C atoms are 12 C. Also, it should be a little bit above 12, because the 13 C atoms weigh more than 12 amu. 12
F. Quantitatively. Calculate 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 + average 12.0111372 amu This (12.0111372) is close to the 12.011 amu atomic weight value in Periodic Table. 13
G. Closing thoughts: 1. Why/how does the atomic weight value given in the Periodic Table for 12 C have 5 s.f., and the value calculated above have only 4 s.f.? 2. What is limiting s.f. above, the % abundance measurement or the mass measurement? 3. The % abundance is also called the natural abundance. Are the natural abundance values on earth are the same as those on other planets, meteors, asteroids, etc.? 14
4. Sum masses of particles in a 12 C atom: p + 6 1.0073 = 6.0438 n 6 1.0087 = 6.0522 e! 6 0.0054858 = 0.00329148 + 12.09929148 amu 12.0993 is significantly more than 12.0000. What s happening here? The mass deficit!!! Nuclei have enough binding energy that... (You should be able to list examples of nuclear energy that are important to human existance.) 15
V. Development of the Periodic Table (Mendeleev) This relates how science works. A. Origin: Mendeleev organized the elements into a table based on chemical & physical properties & weight (p + had not yet been found!). 1. Chemical properties similar within a column: CH 4 NH 3 H 2 O HF SiH 4 PH 3 H 2 S HCl HBr 2. Increasing atomic weight (from LöR). So, F weighs more than O, O more than N, etc. 16
B. When he did this he saw gaps (see p. 18). Important: 1. He thought the gaps were elements existed that had not yet been discovered. 2. For the gap below silicon, he made very specific predictions (interpolation) of the properties of this undiscovered element. ( Eka-silicon ) 17
Periodic Table of the Elements Known in Mendeleev s Time H Li Be B C N O F Na Mg Al Si P S Cl K Ca Ti V Cr Mn Fe Co Ni Cu Zn As Se Br Sr Y Zr Nb Mo Rh Pd Ag Cd Sn Sb Te I Ba Ta W Os Ir Pt Au Hg Pb Bi Note: Some of the lanthanides & actinides were known by 1839, but these groups have been omitted for clarity. Mendeleev was able to use the elements shown above to make some striking predictions based on periodicity. X Known since ancient times P Discovered in the Middle Ages X Discovered 1735-1839 Date source: http://education.jlab.org/qa/discover_ele.html 18
C. About 15 years after his prediction, Ekasilicon (Ge) was discovered & found to have properties very close to those he predicted. predicted for eka-silicon measured for Ge Atomic wt. 72 72.59 Density (g/cm 3 ) 5.5 5.32 Density Cl (g/cm 3 ) 1.9 1.84 compound from Chemistry 3 rd ed., Atkins & Jones Scientists were impressed by the predictive power of Mendeleev s ideas!!! 19
D. Why are elements in the column in the Periodic Table similar in their chemical reactivity? 1. Representative elements in the same column have the same valence e! numbers. 2. Because of this they tend: a) to form the same types of ions b) to have similar (not identical) chemical reactivities 20
E. Why aren t elements in the same column [ex.:oxygen (O) & selenium (Se)] identical in chemical reactivity? 1. nuclei are different, which influences outer e! 2. different amounts of inner e! (which cause electrostatic shielding) between nuclei & outer e! Draw pictures to illustrate? 21
G. Other ways to classify elements: 1. Metals a) Most tend to lose e! b) Conduct heat & electricity well c) Shiny, form into thin sheets & wires, etc. 2. Non-metals a) Most tend to gain e! b) Often don t conduct heat & electricity well 3. Metalloids have intermediate properties. Nice Periodic Tables on the www: http://www.ptable.com/ 22