of topological Insulators, we first need to understand the Band Theory. Using a simple analogy with the particle-in-a-potential-well problem, let us explore how energy bands are formed in a solid. Consider a particle in a finite potential well from quantum mechanics - with certain approximations it can be seen as a model atom (hydrogen-like), where the potential well is created by the nucleus and the electron is a particle under that potential [@simon]. The one-dimensional version of this problem is solved in a standard undergraduate level quantum mechanics course [@rshankar]. The energy of such a system is discrete and can be computed to be $E_n = \frac{n^2\hbar^2 \pi^2}{2mL^2}$, where $L$ is the length of the box and $n$ is a level index (a non-zero +ve integer). Recall that $n$ = 1 corresponds the ground state energy. Now If we bring a second such finite well system close to the first, the full system can be treated as a model molecule with two electrons under the influence of the potential due to two nuclei. As per Pauli’s exclusion principle, these two electrons cannot be accommodated in a single ground state unless their spins are anti-aligned. In fact, for such a molecule, 