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ELECTRONEGATIVITY: One way to estimate the degree of ionic or covalent character in a chemical bond is to compare electronegativities of atoms involved. Electronegativity is a measure of the force of an atom’s attraction for electrons that it shares in a chemical bond with other atoms. In the 1930’s, Linus Pauling assigned electronegativity values to all elements relative to F (the most electronegative element), which he gave a value of 4.0 . Linus Pauling's Table of Electronegativities H 2.1 Li 1.0 Na 1.0 K 0.9 Rb 0.9 Cs 0.8 Fr 0.8 Be 1.5 Mg 1.2 Ca Sc Ti V Cr Mn Fe 1.0 1.3 1.4 1.5 1.6 1.6 1.7 Sr Y Zr Nb Mo Tc Ru 1.0 1.2 1.3 1.5 1.6 1.7 1.8 Ba La Hf Ta W Re Os 1.0 1.1 1.3 1.4 1.5 1.7 1.9 Ra Ac 1.0 1.1 Co Ni Cu Zn 1.7 1.8 1.8 1.6 Rh Pd Ag Cd 1.8 1.8 1.6 1.6 Ir Pt Au Hg 1.9 1.8 1.9 1.7 B 2.0 Al 1.5 Ga 1.7 In 1.6 Tl 1.6 C 2.5 Si 1.8 Ge 1.9 Sn 1.8 Pb 1.7 N 3.0 P 2.1 As 2.1 Sb 1.9 Bi 1.8 O 3.5 S 2.5 Se 2.4 Te 2.1 Po 1.9 F 4.0 Cl 3.0 Br 2.8 I 2.5 At 2.1 Bear in mind that these values can vary slightly depending upon the chemical environment and so the values are average values. Note that EN increases across any period and decreases down any group (in most cases). Pure covalent bonds involve equal sharing of the bonding electron pairs. Pure covalent bonds occur when both atoms involved have equal EN (i.e., EN = 0) ... For example, H2, N2, O2, F2, Cl2, Br2, I2, Cx, S8, PH3, and CS2 are all pure covalent. Nonpolar covalent bonds are those in which EN 0.4. Examples include all the pure covalent compounds listed above as well as compounds, e.g., CH4 (EN = 0.4) and BH3 (calculate EN) Polar covalent bonds are those in which the bonding pair of electrons is unequally shared (0.5 EN 1.7). Examples include HBr, HCl, etc. In polar covalent compounds, the more electronegative atom has a partial negative charge ( -) and the less electronegative atom has a partial positive charge ( +). Ionic bonds are those in which EN 1.8 and are generally considered to have complete charge separation, i.e., considered to be made up of cations and anions. Examples include ORGANIC CHEMISTRY INTRO 1 P ..shell O N M L K Nucleus (n (n ......ns .. (n Compare .shell ...shell (n. . . .this :O F : O S S P N Cl Cl Si C . P B Al N Si C Al B = 6) 5) 4) 3) 2) 1) ...... ..table .. 2)f 6s .. 1 5s 4s 3s 2s 1s 1, 6p 5p 4p 3p with a 2p .. H O .. periodic 33,C:N1s 6d 5 5d 4d 3d Cl S Si P ....The table. 5, 6f..C2s 5f 7, 7 7, Li 4f filling 6g N9 5g 9, 6h Na Clis the order C 3s 11S.. same as K P4s the layout Cl of the Rb 5ss-, p-, d- and Cs 6s on 4f f-blocks the Fr 7s 5f periodic table, i.e., this is the filling order. Note the patterns. The orbital filling order is: ns, (n-2)f, (n-1)d, np. The porbitals begin filling after the 2s orbital. The dorbitals begin filling after the 4s orbital. The forbitals begin filling after the 6s orbital. (n- np 1)d He NaCl, Li N, and CaO. Exceptions are HF and alkali metal iodides. EN in HF is 1.9 but 2p Ne3 this compound behaves as a polar covalent compound. EN in LiI is only 1.5 but LiI 3p Arionically. BF3 is also anomalous. It is covalent although EN = 2.0. behaves 3d 4p Kr A scale of bond type versus EN follows... 4d 5p Xe 5d 6p RnE 6d 7p Uuo 0 1.8 0.4 n o n p o la r 0.5 covalent p o la r c o va le n t 3.2 io n ic 1.7 p u re c o va le n t In polar covalent bonds, the electron distribution is said to be polarized, i.e., not equally distributed but closer to the more electronegative atom. For example, in HCl.... EN = (3.0 – 2.1) = 0.9 i.e., polar covalent A separation of '+' and '-' charge is called a dipole. Dipoles are sometimes illustrated with an arrow pointing toward the more electronegative atom. + The tail of the arrow is crossed to look like a + sign. The head of the arrow points in the direction of electron shift. H Cl The shifting of electron density through sigma bonds due to EN differences between atoms is called an 'inductive effect'. Electropositive elements (metals) such as Zn and Hg, inductively donate electrons through sigma (single) bonds with carbon. Electronegative elements (nonmetals) such as oxygen and chlorine inductively withdraw electrons from carbon through their sigma bonds with carbon. Problem: Calculate EN values and show bond dipoles and dipole arrows. H H C H :O: C H .. O .. H H H C C H H .. H N H acetic acid ethyl amine ORGANIC CHEMISTRY INTRO 2