Final answer:
The order of a regular molecular orbital begins with sigma 1s and follows with orbitals of increasing energy levels and principal quantum numbers, so it is sigma 1s, followed by sigma 2s, and then sigma 2p, going forward with sigma orbitals of higher principal quantum numbers.
Step-by-step explanation:
The order of a regular molecular orbital, specifically a sigma molecular orbital, begins with the lowest principal quantum number which is followed by the type of orbital (s, p, d, or f). Therefore, we would expect the order to start with sigma 1s, followed by sigma 2s, then potentially moving onto p and d orbitals at higher energy levels and principal quantum numbers if they are involved in bonding. The lower the principal quantum number and the lower the energy level, the lower the energy of the molecular orbital.
In Molecular Orbital Theory, when two s orbitals overlap, it forms a sigma 1s molecular orbital. If we consider higher energy orbitals, a sigma 2s molecular orbital would follow after sigma 1s as the next in order of increasing energy. As for p orbitals, they would come into play after the s orbitals of the same principal quantum number have been filled. Therefore, the sigma 2p would follow the sigma 2s orbital. In summary, the ordering is as follows: sigma 1s, sigma 2s, sigma 2p, and further would continue to sigma 3s, sigma 3p, and so on as the principal quantum number increases.