HB, the hydrogen end is conespondingly more postive, This stung inated by wsing the symbols 8 and 6° when 6 (ea represen the ey deco charge separated between the two atoms (8°+ 6° =0) The covalent bonds in 0 and HF ae therefore polarbonds, These covaley bonds are not pure covalent but they have partial onie character. The differeacy intheelectronegativityofthetwo bonded atoms determines the extent of the ion character ofa covalent bond. For instance the H-F bond is 64% tonic and -C) is only 17% ionic, The patil ionic characer greatly affects the properties of molecule, For example, meting and boiling points of polar molecules are high td they are soluble inthe pola solvent suchas HO) Further more, the presence Of partial character shortens the covalent bond, that is, pulls the atoms closer togeher, For example, the expected H-F bond distance is 101A whereas the ail valeis 092A, Dueto shortened bond distances quite obvious hath 

in a molecute. Cl,, Q, and N, molecules are therefore non-polar, 

The molecules consisting of different atoms such as H,O, HF etc. presenta different situation. In these cases, the shared electrons are not equally attracted, In H,O, for instance, oxygen attracts the bonding electrons toa greater extent. The electron cloud of the molecule is shifted towards oxygen so that this end of molecule has a greater concentration of negative than positive charge. Similarly in HF molecule, floorine is more electronegative, so this end has greater portion of the negative charge (Fig. 4.2). Consequently in both the molecules, H,O and 205 

The negative si igni intheop vd sign signifies that actually the reaction at zinc electrode at posi »i.¢., itis the oxidation rather ste ia the direc than reduction whi ich oc 

Zn, — 2e op Je. Fy} Fo ouasicn = 0-76 V 

rease accordingly: For example the bond energy of H-F 

pond enersy must inc os eenle iS 268 KJ/mol more than the calculated value.