Intermolecular Forces: Review

Electronegativity

Electronegativity is a measure of an atom's ability to attract the shared electrons of a covalent bond to itself. If atoms bonded together have the same electronegativity, the shared electrons will be equally shared. If the electrons of a bond are more attracted to one of the atoms (because it is more electronegative), the electrons will be unequally shared. If the difference in electronegativity is large enough, the electrons will not be shared at all; the more electronegative atom will "take" them resulting in two ions and an ionic bond.

Imagine a game of tug-of-war. If the two teams are of equal strength, the rope stays centered. If one team is stronger, the rope is pulled in that team's direction. If one team is overwhelmingly stronger, the weaker team is no longer able to hold onto the rope and the entire rope ends up on the side of the stronger team. This is analogous to chemical bonds. If the two atoms of the bond are of equal electronegativity, the electrons are equally shared. If one atom is more electronegative, the electrons of the bond are more attracted to that atom. If one atom is overwhelmingly more electronegative than the other atom, the electrons will not be shared and an ionic bond will result.

The periodic table below shows the Pauling electronegativity scale. A value of 4.0 is assigned to fluorine, the most electronegative element. As you can see, electronegativities generally increase from left to right across a period and decrease down a group.

Pauling Electronegativity Scale

H 2.1

< 1.0     2.0 - 2.4   1.0 - 1.4   2.5 - 2.9   1.5 - 1.9   3.0 - 4.0

Li 1.0

Be 1.5

B 2.0

C 2.5

N 3.0

O 3.5

F 4.0

Na 1.0

Mg 1.2

Al 1.5

Si 1.8

P 2.1

S 2.5

Cl 3.0

K 0.9

Ca 1.0

Sc 1.3

Ti 1.4

V 1.5

Cr 1.6

Mn 1.6

Fe 1.7

Co 1.7

Ni 1.8

Cu 1.8

Zn 1.6

Ga 1.7

Ge 1.9

As 2.1

Se 2.4

Br 2.8

Rb 0.9

Sr 1.0

Y 1.2

Zr 1.3

Nb 1.5

Mo 1.6

Tc 1.7

Ru 1.8

Rh 1.8

Pd 1.8

Ag 1.6

Cd 1.6

In 1.6

Sn 1.8

Sb 1.9

Te 2.1

I 2.5

Cs 0.8

Ba 1.0

La 1.1

Hf 1.3

Ta 1.4

W 1.5

Re 1.7

Os 1.9

Ir 1.9

Pt 1.8

Au 1.9

Hg 1.7

Tl 1.6

Pb 1.7

Bi 1.8

Po 1.9

At 2.1

Fr 0.8

Ra 1.0

Ac 1.1

In the following two sets, rank the elements in order of increasing electronegativity (1 being the least electronegative).

Set 1:	Sb	1234		Set 2:	Se	1234
	P	1234			Br	1234
	N	1234			As	1234
	Bi	1234			Ga	1234

A polar bond is a bond between two atoms of varying electronegativity. The more electronegative element will attract electron density towards itself, resulting in uneven charge distribution. Because the negative charge has shifted to one side of the bond and the positive charge has remained stationary (in each nucleus), one end of the bond will have a partial negative charge and the other end will have a partial positive charge (represented by d ^- and d ⁺). The two figures below compare a bond between carbon and hydrogen (which have approximately the same electronegativity) and a bond between chlorine and hydrogen (chlorine is much more electronegative than hydrogen).