Book excerpt: This textbook is based on a mixture of simplified institutional theory and solved problems. The choice has been to limit the attention to key concepts and to the most typical aspects of atoms, molecules and solids, looking at the basic "structural" aspects without dealing in detail with the properties originating from them. The problems are entangled to the formal presentation of the arguments, being designed as an intrinsic part of the pathway the student should move by in order to grasp the key concepts.