Polarity of Molecules Revisited

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Presentation transcript:

Polarity of Molecules Revisited SCH4U1 February 25th 2011 Mr. Dvorsky

Polar Molecules Polar molecules are those that contain an uneven distribution of charge The degree of polarity is measured by a dipole moment [=the magnitude of separated charges times the distance between them].

The bond dipole The difference in electronegativity between elements is used to predict the polarity of each bond, known as the bond dipole For diatomic molecules like HF and HBr, the bond dipole and dipole moment are the same Polarity is represented by an arrow pointing toward the more electronegative atom

Polar Molecules The arrows help emphasize that bond dipoles are vector quantities that have direction and magnitude. The arrows point towards the negative end The prediction of dipole moment of more complicated molecules require knowledge of the molecular shape in addition to the bond dipoles.

Polarity Consider the molecule BeH2 From VSEPR, we know the molecule will be linear. 2 valence electrons +1 from each hydrogen = 4 Therefore number of electron pairs = 4/2 = 2 # of bonding pairs = 2 # of lone pairs = 0 AX2 = linear

Polarity Even though BeH2 has polar bonds, this molecule is nonpolar (has no dipole moment) The sum of the molecule is the sum of the individual vectors generated by the bonds In this molecule the vectors point in exactly opposite direction ..so they cancel

Polarity Consider OF2 – 4 electron pairs, 2 bp & 2 lp = AX2E2, which is Bent or V-shaped -the vectors do not cancel out, since they are not pointing in opposite directions -therefore OF2 is polar -differs from BH2 in shape, due to lone pairs

Molecules with lone pairs of electrons on a central atom are generally polar

Polarity Two other examples of nonpolar molecules with polar bonds are BCl3 and CH4 The cancellation of bond dipoles is not as clear as in BeH2, but in the two molecules below they sum to zero.

Polarity If we consider molecules in which an atom of another element replaces one of the hydrogen atoms in CH4, the new molecule is polar

Polarity You can add the bond dipole vectors to determine if the final result is zero (nonpolar molecule) or non-zero (polar molecule) See pg 255, Vectors and vector addition

Molecules are nonpolar when there are no lone pairs on the central atom, and all of the atoms bonded to the central atom are identical.

Intermolecular Forces SCH4U1 February 25th 2011 Mr. Dvorsky