The present paper studies MX crystals in rock-salt structure (M: Li, Na, K; X: F, Cl, Br, I). They are often described as being formed by ions. Pictures based on quantum mechanical calculations sustain and quantify it. The tools used are (i) the Quantum Theory of Atoms in Molecules, (ii) the Electron Localization Function, and (iii) the maximization of the probability to find in a spatial domain a number of electrons equal to that of the ion under consideration. The present paper shows that the images provided by these three different tools to analyze the quantum mechanical calculations yield, for these systems, very similar results, in the sense that the spatial domains and probability distributions are close. While results for the first two methods are already present in the literature, the last of the methods is applied for the first time to these systems, and details about the method of calculation and program are also given.