Final answer:
The duet rule in Chemistry refers to the stability achieved when hydrogen atoms have two electrons in their valence shell. This induces hydrogen to form one bond, seeking a duet of electrons, making it always a terminal, not a central atom in molecules.
Step-by-step explanation:
The duet rule is a principle in Chemistry that describes the filling of the outermost electron shell of certain small atoms in chemical compounds. This rule specifically applies to hydrogen, which is the simplest and lightest element in the periodic table. According to the duet rule, hydrogen is most stable when it has two electrons in its valence shell. These two electrons comprise a full 'duet', similar to the way eight electrons complete an 'octet' for many other elements, which is described by the octet rule. In compounds, hydrogen atoms generally form one single bond with other atoms to achieve this stable duet configuration.
For instance, in a molecule of water (H2O), each hydrogen atom shares an electron with the central oxygen atom, effectively giving each hydrogen a duet of electrons when considered with the electron it brought to the bond, resulting in stable chemical bonds. As a result of the duet rule, hydrogen is never found as a central atom in a molecule because it can form only one bond and does not have the capacity to form the multiple bonds typically associated with central atoms in more complex molecules.