BOND & polarity

Polar Bonds

Electronegativity + – H Cl 08/10/99 + – Electronegativity describes how electrons are shared in a compound Consider the compound HCl H Cl The electron clouds represent where the two electrons in the HCl bond spend their time (sizes of atoms are not being shown) The shared electrons spend more time around Cl than H. In other words Cl is more electronegative than H.

Electronegativity Electronegativity is a measure of the ability of an atom in a molecule to attract electrons to itself.

Electronegativity + – 0 0 H Cl H H

Electronegativities and Bond Type The type of bond or degree of polarity can usually be calculated by finding the difference in electronegativity of the two atoms that form the bond.

We know a COVALENT bond comes from sharing the bonding pair of electrons

F F Shared pair (bonding pair)

F F The nucleus of each atom pulls on the bonding pair.

F F Both atoms have equal pull, so the bonding pair is shared equally.

H Cl If two different atoms share a bond, one will pull more strongly on the bonding electrons.

H Cl

H Cl

H Cl

H Cl

H Cl

H Cl

H Cl

H Cl

The bonding electrons carry negative charge. Cl The bonding electrons carry negative charge.

H Cl The closer they get to the chlorine atom, the more negative it gets. The farther they get from the hydrogen, the more positive it gets.

H Cl _ + But the charge is only partial. Hydrogen has not lost the electrons as in the formation of an ion.

H Cl There is an unequal sharing of electrons.

H Cl d + d – The partial charge is denoted by a + or – and the Greek letter delta, d

d + d – The partial charge is denoted by a + or – and the Greek letter delta, d

d + d –

A polar bond is a bond in which the bonding electron pair is shared unequally. A polar molecule is a molecule with regions of partial negative (d –) and partial positive (d +) charge.

The degree of sharing (equal to unequal) is determined by the electronegativity difference between the two atoms.

F Two atoms of equal electronegativity will share the bond equally

resulting in partial charge. Cl 3.0 2.1 Two atoms with a small difference in electronegativity will share unequally, resulting in partial charge.

resulting in partial charge. d – d + H Cl Two atoms with a small difference in electronegativity will share unequally, resulting in partial charge.

The bonding pair is, on average, Cl This is a polar bond: The bonding pair is, on average, closer to one atom.

d – d + H Cl

resulting in a full charge. 4.0 0.8 F K Two atoms with a large difference in electronegativity will result in a loss of an electron, resulting in a full charge.

F K

F K

F K

K F

K F

K F

positive ion negative ion + _ K F positive ion negative ion

A polar bond in a molecule may make the entire molecule polar, with one end slightly positive and the other end slightly negative.

d + H Cl d – Hydrogen chloride is an example of a polar molecule.

The SHAPE of the molecule determines whether its polar bonds make the molecule polar

In CF4 the fluorines are symmetrically arranged around the carbon.

The fluorines all pull on the valence electrons in opposite directions, effectively cancelling out the polarity of the bonds

There is no partial charge on the molecule. CF4 has four polar bonds but it is a non-polar molecule: There is no partial charge on the molecule.

not symmetrically positioned The two hydrogens of water are not symmetrically positioned around the oxygen.

d + d – d + The O-H polarities do not cancel, and the molecule carries a partial charge.

Water is a liquid instead of a gas because the partial positives and negatives attract each other.

This attraction holds the molecules together, forming a liquid rather than the spread out molecules of a gas.