Final answer:
The electron density in the sigma 1s molecular orbital is greatest between the two positively charged nuclei because it represents a bonding molecular orbital. On the other hand, the antibonding orbital shows decreased electron probability between the nuclei.
Step-by-step explanation:
The probability diagram shows the electron density in the sigma 1s molecular orbital is greatest between the two positively charged nuclei because this region represents a bonding molecular orbital. The sigma 1s orbital is a result of constructive reinforcement of the wave functions from atomic orbitals with the same sign, leading to increased electron probability between the nuclei.
On the other hand, the antibonding orbital (sigma 1s-star) is formed from the interaction of atomic orbitals with opposite sign, resulting in decreased electron probability between the nuclei.