Where duplet is formed?
A hydrogen atom has only one electron in its valence shell. It needs one more electron to fill its valence shell. The completed shell has the electronic arrangement of the noble gas helium, yet an octet is not completed. However we still get a stable molecule.
Where the octet remains incomplete?
The elements of group 1, 2 and 13 contain less than four electrons in their valence shell. An octet cannot be achieved by electron sharing for these elements. As a result, elements of these groups should not ordinarily form covalent compounds, but some elements do. Boron halides (BF3 and BCl3) have incomplete octets (where only six electrons surround the boron atom), yet they form covalent compounds. These compounds are thus electron-deficient compounds.
Where the octet is expanded?
The elements belonging to groups 15, 16 and 17 have more than four electrons in their valence (outermost) shell. The elements of these groups form stable compounds in which there are more than eight electrons around the central atom. For example PF5, PCl5,IF6 and SF6 are some typical compounds of this type. Here, Phosphorus atom has five electrons, and sulphur atom has six electrons in their valence shells. The Cl and F atoms have 7 electrons in their valence shells. Therefore, they need one more electron each to attain noble gas configuration.In doing so,Phosphorous has 10 electrons,Iodine has 14 electrons and Sulphur has 12 electrons in the outermost shell inPF5 ,PCl5 ,IF7 and SF6 respectively.