Difference ionic and covalent bonds

[jostpuur]

According to chemistry books the ionic and covalent bonds are a different thing. In covalent bonds the electrons share the same space around the bound nuclei, while in ionic bonds two ions are bound by the electric force between the ions.

However, the truth is that the wave functions of the electrons are spread onto the surroundings of the both nuclei anyway, and I'm left unable to understand what the ionic bond is supposed to mean.

 

[Claude Bile]

My understanding is that you never get bonds that are perfectly ionic.

Claude.

 

[DrClaude]

It is a question of degree. Pure covalent bonds can only exist between identical atoms (such as a homonuclear diatomic molecule), while one could argue a long time whether pure ionic bonds can exist. [Edit: the other Claude was faster than me on this last point.]

For most molecules, a bond will be somewhere between covalent and ionic. When the sharing is more or less equal, then we call that a covalent bond; when it is very unequal, ionic. In QM (LCAO-MO), you would call a bond ionic if the corresponding bonding orbital leads to a much greater probability density of the electron around one nucleus than the other.

 

[dextercioby]

I've consulted several textbooks looking for 'quantum theory of the ionic bond' (let's say in CsF). Turned up nothing. So I'm assuming that the 'ionic bond' doesn't exist.

EDIT: Just saw post #3.

 

[DrDu]

You could have a look in Linus Pauling "The nature of the chemical bond"
Usually you can draw two potential energy curves, even for bonds between equal atoms.
One of the curves is predominantly ionic, the other one covalent.
At large distances, the ionic rises like 1/R and lies above the covalent one. In molecules which are
covalent at the equilibrium distance, the ionic curve lies always above the ionic one, in those molecules which are
predominantly ionic at equilibrium distances, the two curves nearly intersect at some larger distance.
There are pictures in Paulings book and good explanations.