The electron density surfaces below* reveal the location where the molecules are most reactive to nucleophilic attack
		On the extreme left, the back of the electrophilic carbon of CH3Cl can be seen inside the electron density surface. The concentric circles that form a "bullseye" around the back of the carbon clearly indicated that the back of the carbon is susceptible to nucleophilic attack.

Because the carbon of chloromethane bears a partial positive charge, and because a nucleophile can freely approach the carbon, chloromethane is capable of participating in a bimoleculer nucleophilic substitution reaction, an S_N2 reaction.

On the left below, the back of the tertiary carbon of t-BuCl can be seen between three methyl groups. The "bullseye" pattern can be seen deep in the crevice formed by the methyl groups. As the size and color of the bullseye patern would suggest, the electrophilic carbon is well protected by the surrounding methyl groups.

Since the reactive carbon of t-BuCl is shielded by the methyl groups, a nucleophile does not have unhindered access to the reactive carbon, and the reaction cannot proceed via an S_N2 mechanism.

*Calculated using the CAChe Molecular Modeling System's implementation of Extended Hückel calculations after MM2 minimizations.