7.12 – NOTES Bond and Molecular Polarity Using Models

V. Electronegativity and Polarity A. Electronegativity difference and bond character Electronegativity difference can determine the type of bond formed: attraction for bonded e-; F, O, N, Cl (4.0, 3.5, 3.0, 3.0); metals – low EN; F – O = 0.5 *note that He, Ne and Ar have never made a bond. In the 1960’s Neil Bartlett made first compound with Xe and F; Kr and Rn have also been made into compounds with F

Percentage ionic character: how much of the time is this compound ionic? How much of the time is the e- transferred, even the most ionic bonds have some covalent tendencies; 1.7 and up is usually ionic;

Case #1 Electrons equally shared – Nonpolar covalent bond (npc) Equal sharing of electrons, no poles; Atom A and B share electrons equally, 50-50; EN difference near 0; (0 to .4)

Case #2 Electrons shared, but not equally – Polar covalent bond Atom A and B share, but one is more “greedy” than the other b/c has higher EN; δ is symbol used to show partial charge; If “B” has more e- than A, then B is somewhat (-), A must be somewhat (+); has a dipole .4 to 1.7

Case #3 Electrons transferred completely (almost) – Ionic bond A gives the e- to B; complete transfer of e-; whole # charges, NO δ;

B. Molecular polarity Is the entire molecule polar?   If a molecule has nonpolar bonds, the molecule must be nonpolar overall. If a molecule has polar bonds, the entire molecule can be either polar or nonpolar, depending on the geometry.

Examples: consider water and carbon dioxide. The C – O bond is polar as is the O – H bond Line Test – can you draw a single straight line separating the (+) and (-) charges? yes – the molecule is polar no – the molecule is nonpolar;

O O = C = O H H (nonpolar) (polar)