Electronic Stability & Periodic Table

Transcription

1 Electronic Stability & Periodic Table Things at higher energy are less stable!! All living things are dependent on their ability to acquire energy from unstable things! The compounds in the food you eat must be relatively unstable for you to get useful energy from the food. Generally speaking, calorically rich food is made up of reduced (hydrogen containing) carbon compounds. The higher the hydrogen content, the more energy the food contains. (Ex.: Fat is more caloric than carbohydrate or protein.) I. One way to consider this: If something exists for a relatively long time, we say it is stable. If something exists for a brief time & then changes, it was unstable. II. What makes atoms & ions be stable? It depends on e. 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. 1. Write e configurations for C, Ne, Mg, and I. 2. Circle the valence electrons in the e configuration of Ne. Is the Ne atom stable? 3. Is a C atom stable? Why or why not? 4. Is an I atom stable? Why or why not? 5. Let s look at the I - ion 1

2 How do I ions form? I o + e I (An I atom gains one e ) How many valence electrons does I - have? Is the I ion stable? Why? 6. Is the Mg atom stable? Why or why not? 7. Now consider the Mg 2+ ion? How do Mg 2+ ions form? Mg o Mg e (A Mg atom loses 2 e ) How many valence electrons does Mg 2+ have? Is the Mg 2+ ion stable? Why? B. For transition metals, lanthanides, & actinides it s trickier. Ex.: 1. Gold (Au) is quite stable as an atom Au 0, & unstable as Au 1+ or Au Iron (Fe) is moderately stable as Fe 0, Fe 2+, and Fe 3+. It can be shifted back and forth between these forms by altering the environment. 3. Manganese (Mn): relatively unstable as Mn 0 & fairly stable as Mn 2+. C. Most of our emphasis is on the representative elements. Groups 1A- 8A III. Development of the Periodic Table (Mendeleev) This discussion is important in helping you understand how science works and why humans have found it useful. A. Origin: Mendeleev organized the elements into a table based on chemical & physical properties & atomic weight (Note: Protons ( p + ) had not yet been found). 1. He arranged the table so that atoms in a column had similar chemical and physical properties: CH 4 NH 3 H 2 O HF SiH 4 PH 3 H 2 S HCl HBr 2

3 2. And so that atomic weight increased from left to right across the table. That is, F weighs more than O, O weighs more than N, etc. 3. And so that atomic weight increased from top to bottom. That is, F weighs more than Cl, Cl weighs more than Br. B. When he did this, he saw gaps in the table. 1. He thought the gaps were elements that 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. He called this undiscovered element, Ekasilicon. Periodic Table of the Elements Known in Mendeleev s Time H Li Be Na Mg B C N O F 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. Known since ancient times (C, S Fe, Cu, Ag, Sn, Au, Hg, Pb) Discovered in the Middle Ages (P) Discovered Date source: 3

4 C. About 15 years after his prediction, Eka-silicon (Ge) was discovered & found to have properties very close to those he predicted. Predicted for eka-silicon Measured for Ge Atomic wt Density (g/cm 3 ) Density Cl (g/cm 3 ) compound from Chemistry 3 rd ed., Atkins & Jones Scientists were impressed by the predictive power of Mendeleev s ideas!!! D. Why are elements in the same 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 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 and outer electrons F. Another way to look at the elements in the periodic table: 1. Metals Examples: Mg, Cr, Fe, Cu, Co, Au, Zn, Ag, Ni, Ti, Na, Li, Ca, Ba a) Most tend to lose e b) Conduct heat & electricity well c) Shiny, form into thin sheets & wires, etc. 2. Non-metals Examples: C, N, O, F, S, P, Br, Cl, He, Ne a) Most tend to gain e b) Often don t conduct heat & electricity well 3. Metalloids have intermediate properties. Examples: Si, As, Sb, Ge, Te 4

5 5 Modern Periodic Table of the Elements * 1A 8A 1 H A 3A 4A 5A 6A 7A 2 He Li Be Atomic number Elemental symbol Atomic weight 5 B C N O F Ne Na Mg B 4B 5B 6B 7B < B > 1B 2B 13 Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc (98) 44 Ru 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 (209) 85 At (210) 86 Rn (222) 87 Fr (223) 88 Ra Ac (227) 104 Rf (261) 105 Ha (263) 106 Sg (263) 107 Ns (265) 108 Hs (265) 109 Mt (266) 110 Ds (269) 111 Rg (272) 112 Cn (277) 58 Ce Pr Nd Pm (145) 62 Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Th Pa U Np Pu (244) 95 Am (243) 96 Cm (247) 97 Bk (247) 98 Cf (251) 99 Es (252) 100 Fm (257) 101 Md (258) 102 No (259) 103 Lr (260) *Organized by increasing number of protons rather than atomic weight.