The factors affecting the electron availability of a compound might reasonably be thought to have far-reaching consequences upon its reactivity with various compounds.  For example, an area of high electron density is unlikely to be attacked by OH^-, but an area of low electron density is likely to be far more susceptible to attack by the same reagent.

Inductive Effects

So far, when considering covalent single bonds, we have though of the electrons as being in between the two nuclei of the atoms involved in the bond.  However, what was not stated is that the electron density is shifted towards the more electronegative of the pair, i.e. this can be thought of as the electrons spending more time nearer one of the atoms.

In the alkyl halide above, the C-F bond is polarized towards fluorine (meaning that there is more electron density towards that end of the bond).  This imbalance of charge can be represented as above by the use of δ+ and δ-. This effect is due to the greater electronegativity of the fluorine over the carbon.

It can also be visualised as a contribution to the overall structure from this resonance form:(Note that though it does involve charge separation, usually a high energy process, the negative charge is stabilised by being on the highly electronegative fluorine atom.)

The inductive effect diminishes through a greater number of bonds. i.e.:

In the alkyl halide above, the greatest inductive effect experienced is on carbon 1, followed by carbon 2, 3, and 4 in order.  This can be thought of as the fact that carbon 1 will be left slightly electron deficient, so in order to rectify the loss, it pulls some electron density over from carbon 2.  However, the effect is very slight beyond carbon 2.

Inductive effects work through sigma bonds, and can push electrons in either direction with respect to carbon.  i.e. metals (e.g. magnesium, lithium) inductively donate electrons (because they are electropositive), and electronegative elements such as chlorine, fluorine, and oxygen, inductively withdraw electrons.

Field Effects

An effect which is similar in nature to the inductive effect operates through the space surrounding the molecule, or (if in solution) through the solvent molecules that surround it.  This is known as a field effect.