I'm graduate student in physical & theoretical chemistry and I have the following to say;
1.) The first time I took physical chemistry, and used Atkin's book I thought the book and subject matter where far too difficult. Organic, Inorganic, Biochemistry, ect.. where a cake walk. You studied and passed the course with a passing grade. P-chem was a different story altogether. I heard horror story after horror story about the difficultly of the subject matter. Then, I took P-chem and was horrified for the first time by any subject matter... the rumors were true. However, I now understand it's primarly not the book but the training most chemistry students don't receive prior to taking physical chemistry that makes physical chemistry so difficult to understand for most chemistry students.
Physical Chemistry is the bastard child of chemistry because it overlapps so much with physics. At times, I wonder wether I'm a chemist or physicist. I study equations all day long. My undergraduate and graduate studies deal greatly with courses in physics and mathematics than with coursework in chemistry. Matter of fact, the majority of my graduate course work is in physics than chemistry. Seldom do I ever work in a lab working with chemical reactions.
In other words, it was only when I had sufficient training in physics and mathematics did Atkins book become easy to understand. I now look at Atkins as simple in understanding. If your a chemist (non-physical) chances are you will find it very difficult to take p-chem as a comfortable subject matter. Not because your not a good chemist but because it takes a lot of training in the p-chem specialty to understand the material easily. Physics students in general can understand p-chem more easily than most chemistry students bc they have undertaken the neccesary physics and mathematics.
To understand Atkin's books the following math is required. All which was required for the P-chem specialty (undergraduate):
3-semesters of single variable calculus
1-semester of multi-varialble calculus
differential equations
partial differential equations
vector analysis
linear algebra
In addition, our program highly reccomends the following (graduate level):
Advanced Calculus(Real and Complex analysis)
2 semesters of Mathematical Methods in Physics (tensor analysis, special functions, group theory, integral transforms, Fourier analysis, ect... ).
Even with the above mathematical background most chemistry students don't have a solid foundation in the physics to understand p-chem.
2.) P-chem requires a solid foundation in physics (more than most chemist are required to take at most universities).
Quantum Mechanics-2 semesters (a solid foundation in quantum mechnics is of vast importance bc all advanced molecular theory is based on wavefunctions and perturbation theory).
Thermodynamics (Classical)
Statistical Mechanics-2 semesters
Classical Mechanics-(which is needed in understanding what the Hamiltonian and Lagragian operators defined in Quantum Theory)
Once all of the above was taken did Atkins book finally-became readible.
You have to realize that p-chem is the application of physics to chemical systems in an attempt to elucidate chemical behaviour. No other subject in chemistry attempts this process. This is the primary reason why most P-chem students have heavy course work in physics and mathematics.
In addition, again I still wonder wether I'm a chemist or physicist. Currently, I'm studying Density Functional Theory and Electron Density Theory both from the physics department in undertanding molecular structure. Soon my studies will depart into Quantum Field Theory to better understand chemical behaviour using physics again as my primary tool.
P-chem is a facinating field indeed. It's a beautifull union of physics and chemistry. Without physics, P-chem would not simply exist. Thus, P-chem heavily overlaps with physics.