ملتقى طلاب صيدلة الاسكندرية

[Dr.ahmed91]

معلش عشان ابقى واثق انه كله وصل

ياريت تشوفوا دي

2.3 Polar Covalent Bonds
1. Electronegativity (EN) is the ability of an element to attract electrons that it is sharing in a covalent bond.
1) When two atoms of different EN forms a covalent bond, the electrons are not shared equally between them.
2) The chlorine atom pulls the bonding electrons closer to it and becomes somewhat electron rich ق bears a partial negative charge (d–).
3) The hydrogen atom becomes somewhat electron deficient ق bears a partial positive charge (d+).

2. Dipole:

A dipole

Dipole moment = charge (in esu) x distance (in cm)
m = e x d (debye, 1 x 10–18 esu cm)

1) The charges are typically on the order of 10–10 esu; the distance 10–8 cm.

Figure 2.1 a) A ball-and-stick model for hydrogen chloride. A calculated electrostatic potential map for hydrogen chloride showing regions of relatively more negative charge in red and more positive charge in blue. Negative charge is clearly localized near the chlorine, resulting in a strong dipole moment for the molecule.

2) The direction of polarity of a polar bond is symbolized by a vector quantity:
(positive end) (negative end) ق

3) The length of the arrow can be used to indicate the magnitude of the dipole moment.

2.4 Polar and Nonpolar Molecules
1. The polarity (dipole moment) of a molecule is the vector sum of the dipole moment of each individual polar bond.

Table 2.1 Dipole Moments of Some Simple Molecules

Formula

m

(D)

Formula
m

(D)

H2
0

CH4
0
Cl2
0

CH3Cl
1.87
HF
1.91

CH2Cl2
1.55
HCl
1.08

CHCl3
1.02
HBr
0.80

CCl4
0
HI
0.42

NH3
1.47
BF3
0

NF3
0.24
CO2
0

H2O
1.85

Figure 2.2 Charge distribution in carbon tetrachloride.

Figure 2.3 A tetrahedral orientation Figure 2.4 The dipole moment of
of equal bond moments causes their chloromethane arises mainly from the
effects to cancel. highly polar carbon-chlorine bond.

2. Unshared pairs (lone pairs) of electrons make large contributions to the dipole moment. (The O–H and N–H moments are also appreciable.)

Figure 2.5

Bond moments and the resulting dipole moments of water and ammonia.