Ch 201 Problem 8.42

The iodine monobromide molecule, IBr, has a bond length of 2.49 Angstrom and a dipole

moment of 1.21 D.

(a) Which atom of the molecule is expected to have a negative charge? Explain.

Br is more electronegative than I, so Br would be expected to have a negative charge.

(b) Calculate the effective charges on the I and the Br atoms in IBr, in units of the electronic charge e.

The dipole moment is given by μ = qr, where q is the charge separated (in C) and r is the distance separating the charge. Since we're given the dipole moment (1.21 D) and the bond length (2.49 Angstrom), we solve for the charge q. Dipole moments are given in D units - recall that 1 D = 3.34 x 10-30 C-m. We convert the bond length into meters:

tell mathcad what a debye is

here's the dipole moment:

solve the definition of the dipole moment for the charge

so the amount of charge separated by the difference in EN is 1.623x10-20 Coulombs. We want the answer in units of electronic charge e:


so the Br has a charge of -0.101 and the I has a charge of +0.101.

DAC 6/24/08