1. 1Chemistry 2C Lecture 16: May 5 th, 2010 Must know for this section of Class Lecture 16: Main Group Elements Atomic Radius decreases from left to right across a period increase as you go down a group Electronegativity increases from left to right across a period. decreases as you go down a group Ionization Energy Increases from left to right across a period. decreases as you go down a group Melting Points For metals it generally decreases as you go down a group For non-metals it generally increases as you go down a group Trends For each group Reaction with hydrogen Reaction with oxygen Reaction with nitrogen Reaction with halogens Reaction with water

2. 2Chemistry 2C Lecture 16: May 5 th, 2010 Most Abundant Elements a) Earths Crust b) In Human Body c) In the universe the most abundant element is hydrogen (75%), followed by helium (23%). Oxygen is about 1%, carbon 0.5%

3. 3Chemistry 2C Lecture 16: May 5 th, 2010 Atomic Radius decreases from left to right across a period increase as you go down a group

4. 4Chemistry 2C Lecture 16: May 5 th, 2010 Atomic Radii of Transition Metals

5. 5Chemistry 2C Lecture 16: May 5 th, 2010 Electronegativities Electronegativity increases from left to right across a period. decreases as you go down a group

6. 6Chemistry 2C Lecture 16: May 5 th, 2010 Ionization Energies Ionization Energy Increases from left to right across a period. decreases as you go down a group

7. 7Chemistry 2C Lecture 16: May 5 th, 2010 Melting Points For metals it generally decreases as you go down a group For non-metals it generally increases as you go down a group

8. 8Chemistry 2C Lecture 16: May 5 th, 2010 Ionization Energy and Cell Potentials Alkali metals like to give up electrons for M (g) -> M + (g) + e- First ionization energy (kJ/mole) Li : 520.2 Na : 495.8 K : 418.8 Rb : 403.8 Cs :375.7 Less energy is required to ionize the metal. Cs is easier to ionize than Li However, the reduction potential shows that Li is easier to oxidize than Cs. E 0 cell How to reconcile these facts? Li : -3.040V Na : -2.713V K : -2.924V Rb : -2.924V Cs :-2.913V

9. 9Chemistry 2C Lecture 16: May 5 th, 2010 This is not a contradiction because the cell potential is composed of multiple steps! for M (g) -> M + (g) + e- First ionization energy (kJ/mole) Li : 520.2 Na : 495.8 K : 418.8 Rb : 403.8 Cs :375.7 E 0 cell Ionization Energy and Cell Potentials M (s) -> M (g) sublimation M (g) -> M + (g) + e-ionization M + (g) -> M + (aq) hydration for M (s) -> M + (aq) + e- Li +, being small, has a higher charge density than Cs + and attracts the H 2 O molecules to itself more strongly. Thus the hydration energy is more favorable! Li : -3.040V Na : -2.713V K : -2.924V Rb : -2.924V Cs :-2.913V

10. 10Chemistry 2C Lecture 16: May 5 th, 2010 Li 2 CO 3 is an anti-depressant (referred to as “lithium” and is being phased out) Uses of Alkali Metals Initially only used in fancy camera batteries, lithium is now available in AA and 9V batteries for use in things like smoke detectors and digital cameras. Sodium vapor lamps – give nice yellow glow. Used in “low sodium salt” and as KI in normal table salt (iodized salt) to prevent iodine deficiency

11. 11Chemistry 2C Lecture 16: May 5 th, 2010 Potassium is also used in fertilizers, usually a K salt of Cl, SO 4 2-, CO 3 2-, NO 3 - Uses of Alkali Metals Rubidium is often used in special glass Cesium is used in atomic clocks, that are calibrated on the frequency of internal transition between atomic states in Cs. Since 1967, the International System of Units (SI) has defined the second as 9,192,631,770 cycles of the radiation which corresponds to the transition between two energy levels of the ground state of the Cesium-133 atom

12. 12Chemistry 2C Lecture 16: May 5 th, 2010 Reaction of Alkali Metals 1)With hydrogen 2)With oxygen 3)With nitrogen 4)With halogens 5)With water (already discussed) 1)Reaction with hydrogen H is about as electronegative as C ( a little less) Hydrogen can be found in one of three forms: H ·, H + or H - Proton Covalent or radical Hydride 2K (l) + H 2 (g) --> 2KH (s)  Li : 1.0 Na : 0.9 K : 0.8 Rb : 0.8 Cs :0.8 Electro-negativity Electro-negativity of H is 2.1 2 Na (s) + 2 H 2 O (l) → 2 Na + (aq) + 2 OH - (aq) + H 2 (g)

13. 13Chemistry 2C Lecture 16: May 5 th, 2010 Reaction of Alkali Metals 1)Reaction with hydrogen H is about as electronegative as C ( a little less) Hydrogen can be found in one of three forms: H ·, H + or H - Proton Covalent or radical Hydride 2K (l) + H 2 (g) --> 2KH (s)  Li : 1.0 Na : 0.9 K : 0.8 Rb : 0.8 Cs :0.8 Electro-negativity Electro-negativity of H is 2.1 This is a fairly ionic bond 1)With hydrogen 2)With oxygen 3)With nitrogen 4)With halogens 5)With water (already discussed) 2 Na (s) + 2 H 2 O (l) → 2 Na + (aq) + 2 OH - (aq) + H 2 (g)

14. 14Chemistry 2C Lecture 16: May 5 th, 2010 Reaction of Alkali Metals 2K (l) + H 2 (g) --> 2KH (s)  Same reaction with Sodium gives NaH, a useful reagent for removing H 2 O from organic solvents NaH (s) + H 2 O -> NaOH (s) + H 2 (g) In an organic solvent (not in water) Precipitates out

15. 15Chemistry 2C Lecture 16: May 5 th, 2010 Oxides Metals: 2 Mg (s) + O 2 (g) → 2 MgO (s) MgO (s) + H 2 O(l) → Mg(OH) 2 (aq) Mg(OH) 2 (s) → Mg 2+ (aq) + 2 OH - (aq) Non-metals: C (s) + O 2 (g) → CO 2 (g) CO 2 (g) + H 2 O (l) → H 2 CO 3 (aq) H 2 CO 3 (aq) + H 2 O (l) → HCO - 3 (aq) + H 3 O + (aq) Oxides of metals are basic Oxides of non-metals are acidic

16. 16Chemistry 2C Lecture 16: May 5 th, 2010 Reaction of Alkali Metals 1)Reactions with Oxygen Different behaviors 4Li (s) + O 2 (g) --> 2Li 2 O (s) O 2- : Oxide 2Na (s) + O 2 (g) --> Na 2 O 2 (s) O 2 2- : Peroxide K (s) + O 2 (g) --> KO 2 (s) O 2 - :Superoxide Very important feature: The oxides of the alkali metals dissolve in water to give a basic solution

17. 17Chemistry 2C Lecture 16: May 5 th, 2010 Reaction of Alkali Metals 2)Reaction with Oxygen Li 2 O (s) + H 2 0 --> 2Li + + 2OH - Acid Base Rx. Redox Rx. Very important feature: The oxides of the alkali metals dissolve in water to give a basic solution 2Na 2 O 2 (s) + H 2 0 --> 4Na + + 4OH - + O 2 (g) 4KO 2 (s) + 2H 2 0 --> 4K + + 4OH - + 3O 2 (g) The peroxides and superoxides are used in emergency breathing devices in submarines and spacecraft as they evolve O 2 when added to water These oxides also reaction with CO 2 to give O 2 Redox Rx.2Na 2 O 2 (s) + CO 2 (g) --> 4Na 2 CO 3 (s) + O 2 (g)

18. 18Chemistry 2C Lecture 16: May 5 th, 2010 Reaction of Alkali Metals The only element to react with N 2 (g) at Room temperature is Li 3)Reactions with Nitrogen 6Li (s) + N 2 (g) --> 2Li 3 N (s) Lithium Nitride Li 3 N (s) + 3H 2 O (l) -> 3Li + (aq) + OH- (aq) + NH 3 (g) which reacts violently with water to produce ammonia: All ionic nitrides display this pattern, due to the N 3- ion being an extremely strong Br Ø nsted base. It is, in fact, a stronger base than the hydride ion, so deprotonates hydrogen itself. Thus, it is used to store hydrogen by reacting with it in this way: Li 3 N (s) + 2H 2 (g) -> LiNH 2 (s) + 2LIH (s)

19. 19Chemistry 2C Lecture 16: May 5 th, 2010 Reaction of Alkali Metals Reacts directly in a redox reaction 4)Reactions with Halogens 2Na (s) + Cl 2 (g) --> 2NaCl (s) How to make salt the hard way Na (s) --> Na + + e - A solution of metal ions and solvated electrons in ammonia can be made by dissolving an alkali metal in liquid ammonia. Na NH 3 H3NH3N H3NH3N NH 3 (l) + e NH 3 H3NH3N H3NH3N - Sodium –Ammonium Complex Solvated Electron

20. 20Chemistry 2C Lecture 16: May 5 th, 2010 Reaction of Alkali Metals Na (s) --> Na + + e - The solvated electron in NH 3 absorbs strongly in the red and hence look blue to the eye. This is why it is often a joke that electrons are “blue” NH 3 (l) e NH 3 H3NH3N H3NH3N - Solvated Electron

21. 21Chemistry 2C Lecture 16: May 5 th, 2010 Group II Metals The name comes from traditional usage of the word “earth” which has to do with the fact that the oxides are more insoluble than those of Group I Increasing atomic radius Group I metals Be [He]2s 2 : 1.57 Mg [Ne]3s 2 : 1.31 Ca [Ar]4s 2 : 1.00 Sr [Kr]5s 2 : 0.95 Ba [Xe]6s 2 : 0.89 Ra [Rn]7s 2 : 0.90 Na 2 O (s) + H 2 0 --> 2Na + + 2OH - (can be up to 20M) Group II metals MgO (s) + H 2 0 --> 2Mg(OH) 2 (not as soluble, but does give a basic solution Ksp=1.8 x10 -11 ) Decreasing electro- negativity Radium is radioactive

22. 22Chemistry 2C Lecture 16: May 5 th, 2010 Group II Metals Alkali Earth Metals are common in the earth’s crust Ca #5 Mg #6 Na #7 K #8 Difference between Group I and Group II elements Ranking in mass in earth’s crust Group IGroup II More Metallic Harder Higher Melting Point Higher Density (+1) Ionic Radius (+2)

23. 23Chemistry 2C Lecture 16: May 5 th, 2010 Group II Metals Flame colors (typically used as salts) Be - None Mg - Bright White Ca - Orange-red Sr -Scarlet Ba -Green Used in fireworks Strontium

24. 24Chemistry 2C Lecture 16: May 5 th, 2010 Group II Metals Sources in nature: Like Group I, alkali earth metals are found in combination with O 2-, CO 3 - SO 4 2- etc. and never in the native (metallic) form http://www.theodoregray.com/PeriodicTable/Elements/ Garnierite (Ni,Mg) 3 Si 2 O 5 (OH) 4 Barite BaSO 4

25. 25Chemistry 2C Lecture 16: May 5 th, 2010 Group II Metals Sources in nature: Mg +2, Ca 2+, Sr 2+, and Ba 2+ are also found in seawater and can obtained as pure metals via reduction in an electrolytic cell http://www.theodoregray.com/PeriodicTable/Elements/ Limestone, Chalk, Marble Ca in CaCO 3 CaCO 3. MgCO 3

26. 26Chemistry 2C Lecture 16: May 5 th, 2010 Group II Metals Uses of these metals http://www.theodoregray.com/PeriodicTable/Elements/ Beryllium can be used to make X-ray windows (transparent to x-rays) Mg is a light weight metal, much lighter than Al Photographic ribbon (before flash lamps for cameras)

27. 27Chemistry 2C Lecture 16: May 5 th, 2010 Alkali Earth Metals Some differences in the chemistry of Group II and Group I metals can be traced back to the smaller ionic radii Group II Be 2+ [He] : 0.27 Å Mg 2+ [Ne] : 0.72 Å Ca 2+ [Ar] : 1.00 Å Sr 2+ [Kr] : 1.13 Å Ba 2+ [Xe] : 1.36 Å Li + : 0.59 Å Na + : 0.99 Å K + : 1.38 Å Rb + : 1.49 Å Group I As we discussed last lecture, the size of the ion has a significant effect on solubility properties of the ion. It is more difficult to separate the +2 cations from the anions due to the high charge and small size so Alkali Earth metals form less soluble salts Only the +2 oxidation state is available for group II metals Be has a very different chemistry than the other -- tends to form more covalent compounds

28. 28Chemistry 2C Lecture 16: May 5 th, 2010 Reaction of Alkali Earth Metals 1)With hydrogen 2)With oxygen 3)With nitrogen 4)With halogens 5)With water 1)Reaction with hydrogen Ca (s) + H 2 (g) --> CaH (s) These reactions are for all Group II metals except Be All react with hydrogen to form metal hydrides 2)Reaction with Oxygen 2Mg (s) + O 2 (g) --> 2MgO (s) All react with oxygen to form metal oxides

29. 29Chemistry 2C Lecture 16: May 5 th, 2010 Reaction of Alkali Earth Metals 1)With hydrogen 2)With oxygen 3)With nitrogen 4)With halogens 5)With water 3) Reaction with Nitrogen 3Ca (s) + N 2 (g) --> Ca 3 N 2 (s) These reactions are for all Group II metals except Be Only react with extreme difficulty 4)Reaction with Halogens Ca (s) + Cl 2 (g) --> CaCl 2 (s) All react with halogens to form metal halides  Really need to crank up the heat!

30. 30Chemistry 2C Lecture 16: May 5 th, 2010 Reaction of Alkali Earth Metals 1)With hydrogen 2)With oxygen 3)With nitrogen 4)With halogens 5)With water 3) Reaction with Water These reactions are for all Group II metals except Be Magnesium does not react with water to any significant extent. Magnesium metal does however react with steam Mg(s) + 2H 2 O(g) → Mg(OH) 2 (aq) + H 2 (g) Beryllium metal does not react with water or steam, even if the metal is heated to red heat. Calcium reacts slowly with water. The calcium metal sinks in water and after an hour or so bubbles of hydrogen are evident, stuck to the surface of the metal. Ca(s) + 2H 2 O(g) → Ca(OH) 2 (aq) + H 2 (g)

31. 31Chemistry 2C Lecture 16: May 5 th, 2010 Use of alkali Earth Metal compounds Mg(OH) 2 Milk of Magnesia MgSO 4 Epsom salts CaCO 3 Treatment of bone loss Ca(OH) 2 Slaked lime –cheap base Ca 3 (PO 4 ) 2 found in bones and teeth CaSO 4 · 2H 2 OGypsum(used in drywall) CaSO 4 · ½ H 2 OPlaster of Paris(from dehydration of gypsium) BaSO 4 Barium “milkshake” for GI imaging CaOQuicklime4 th ranked industrial chemical in US CaCO 3 (s) --> CaO (s) + CO 2 (g)in a lime kiln  limestonequicklime CaO is easy to transport and you mix with water to get slaked lime CaO (s) + H 2 O --> Ca(OH) 2 (s) Details about CaO

32. 32Chemistry 2C Lecture 16: May 5 th, 2010 Use of alkali Earth Metal compounds Quicklime4 th ranked industrial chemical in US Details about CaO Slackedlime it used to make cement (along with sand) Ca(OH) 2 (s) + CO 2 (g) -> CaCO 3 (s) + H 2 O (l) Sets the morter Details about CaCO 3 Found in Davis water as “hardness” and is responsible for scale on faucets, stalagmites, stalactites and tufa towers CO 2 (g) + H 2 O (l) “H 2 CO 3 ” (aq) H + (aq) + HCO 3- (aq) HCO 3- (aq) H + (aq) + CO 3 2- (aq)

33. 33Chemistry 2C Lecture 16: May 5 th, 2010 Use of alkali Earth Metal compounds Details about CaCO 3 So to dissolve off hard water scale, use a weak acid (like vinegar) to drive reaction to the left HCO 3- (aq) H + (aq) + CO 3 2- (aq) H + (aq) + CO 3 2- (aq) HCO 3- (aq) CaCO 3 (s) + H + (aq) Ca 2+ (aq) + HCO 3- (aq) For scale which is solid CaCO 3 and acid: more soluble in water