This paper is a review of the application of scanning tunneling microscopy (STM) to the study of the electronic properties of clean surfaces, the changes in the electronic properties upon adsorption and surface chemical reactions, and the role that the electronic properties of a surface play in the formation of chemical bonds during reactions and in the formation of surface structures. The effect of the local density of electron states on the STM image of atoms of various elements is shown. A description is given of the electronic properties and the atomic structure of the reconstructed Si(111)-(7$\times$7) surface. The example of the chemical reaction of NH$_3$ with the Si(111)-(7$\times$7) surface is used to show that the reactivity of various atoms is directly related to the presence of localized dangling bonds.