請問Covalent Bond 和 Polar Covalent Bond 的特性有甚麼不同??

The purest covalent bonds exist between two atoms with absolutely no difference in electronic structure. For simplicity, we choose diatomic molecules with atoms of the same element because they are considered perfectly the same in electronegativity and hence would get an averagely symmetric shape of electron cloud.

But this electron cloud could change in shape automatically even at 0K, this is what we call the zero point energy. And any difference of the environment on the two sides would polarise that bond. But we still consider these bonds as averagely non-polar.

On the other side, we define caesium fluoride, composinf of two stable elements of highest possible difference of electronegativity in the periodic table, as purely ionic compound. As they involved most thorough electron transfer and least polarization and crystal lattice distortion among all ionic compound. That is the formal negative charge on the fluoride ion is perfectly -1 and the charge on the caesium ion is perfectl +1.

In between pure covalent bonds and pure ionic bonds, there are a range of variation of covalent and ionic character. As the difference in electronegativity decreases, the covalent character of a pair of interaction increases. And if the electronegativity difference widen up, the ionic character increases.

Polar covalent bonds often can leave a molecule polar if the dipole moments cannot be cancelled.

Ionic compounds usually exist in the form of crystals. They can dissolve in some solvents such as water, to lower their own polarity and increase the solvent's polarity.

2007-05-08 00:38:33 補充：
Bonds of properties in between covalent and ionic are polar covalent bonds. Polar covalent bonds mean development of partial charges on the two sides (the more electronegative side partially negative and the less electronegative side partially positive).

Covalent bonds are forces that hold atoms together. The forces are formed when the atoms of a molecule share electrons.

Covalent Bond 的特性: covalent bonds are strong bonds. they cannot be broken under biological conditions unless by enzymic catalysis.

而Polar Covalent Bond 係 Covalent Bond 的其中一種, 佢有上面Covalent Bond 的特性, 仲有, 因為佢ge molecule 有一邊係partial positive, 有一邊係partial negative, 令到其他有charge ge野(如 ions)可以溶係入面。

水就係其中一種Polar Covalent Bond。