I am reading an Introductory Semiconductor Physics by Parker. The textbook states the following:
The ionic crystal itself is formed by the ions coming together in an equilibrium configuration. Figure 1.2 shows how such an equilibrium configuration can come about. There is an atomic separation, $r_0$, at which there is balance between the repulsive and attractive electrostatic forces.
I have a few questions about this graph that the author does not clarify:
Why is it that the energy ($E_0$) at "balance" ($r_0$) is still negative? Naively, it seems like it should be $0$?
After a certain distance, "repulsion" goes to $0$ and, it seems, remains there, whilst "attraction" remains negative and, it seems, continues to remain negative, only equalling $0$ at a distance of infinity?
What is "total energy" supposed to be, and why does it equal "attraction" at some point slightly beyond $r_0$ and remain equal as far as can be seen?
What does negative "Energy" on the $y$-axis even mean? There exists the concept of "negative energy" in physics, but, according to the Wikipedia article, it is only used to describe a phenomena in certain (other) areas of physics.
I would greatly appreciate it if people could please take the time to clarify this.