Overview of Atomic Theory
Atomic Theory is the branch of chemistry concerned with the smallest form of an element that can exist chemically, the atom. Classical physics is helpful to understanding some properties of atoms. However, the range of behaviors of atoms exceeds the descriptive powers of classical physics. To explain the line spectrum of hydrogen, for example, Neils Bohr develped his early form of atomic theory. A more complete picture of the electronic structure of the atom is provided by modern quantum electrodynamics.

Atomic Theory on the MCAT
Questions directly concerned with Atomic Theory, or more generally, basic quantum mechanics, do appear with fair regularity on the MCAT, although they tend to be easier questions than they may seem at first glance. More important than the direct appearance of these concepts on the exam is that these initial chapters of Chemistry, dealing with the instrinsic structure of matter, i.e. Atomic Theory, Periodic Properties, and Chemical Bonding, are absolutely crucial for the scientific understanding of the physical and natural world. The rest of General Chemistry, Organic Chemistry, and Biology will make profoundly better sense, and be much more interesting besides, if you take special care to understand the structure of matter.