Difference Between Octet and Duplet | Octet vs Duplet

Key Difference – Octet vs Duplet

There are chemically active and inactive atoms or compounds present in nature. This characteristic is mainly dependent on the number of electrons present in the outermost shells of the atoms. Atoms having incomplete shells may become active in order to complete their electron configuration to become stable. Atoms that are inactive have a complete electron configuration; thus, these atoms do not react with any other atom unless in special conditions. Noble gases are always inactive in nature. Hence, they are known as inert gases. Inert gases are in the eighth column in the periodic table. Other elements in the same period (row) tend to obtain the electron configuration of the inert gas in the end of that period, which is the most stable form in nature. Active atoms tend to complete the number of electrons according to octet rule or duplet rule. The key difference between octet and duplet is that octet is an atom or an ion having a maximum of eight electrons in the outermost shell while duplet is an atom having the maximum of two electrons in the outermost shell.

1. Overview and Key Difference
2. What is an Octet
3. What is a Duplet
4. Side by Side Comparison – Octet vs Duplet
5. Summary

What is an Octet?

An octet is an atom or an ion having eight electrons in the outermost shell of that atom. All noble gases except helium have eight electrons and are inert in nature. The electron configuration of a noble gas will always end as follows.

ns2 np6

For example, the electron configuration of Neon is 1s22s22p6 . Neon is an inert gas.

Other elements having seven, six, etc. electrons in the outermost orbital tends to obey octet rule by obtaining electrons from outside; some other elements having one, two, etc. electrons tend to lose electrons and obtain the electron configuration of the nearest inert gas. But other elements in the middle of the periodic table will form new bonds with elements having excess electrons to share those electrons and become an octet.

Difference Between Octet and Duplet | Octet vs Duplet

Figure 01: Neon, an octet

What is a Duplet?

Hydrogen atom and helium atom are the smallest elements in nature and have only one orbital around their nucleus. This orbital is called 1s orbital. This orbital can contain a maximum of two electrons. Hydrogen atom has one electron and helium has two electrons. Hence, helium is called a duplet. Helium has the maximum number of electrons that it can have; thus it is a stable element in nature. Therefore, Helium is also an inert gas. But hydrogen has only one electron and the only orbital it has is incomplete. Thus, hydrogen atom alone is very reactive and it tends to form a covalent bond with another hydrogen atom by sharing the only electrons they have. Then these hydrogen atoms become duplets as they now have two electrons in their outermost orbitals. But Lithium can also behave as a duplet by removing the electron from its outermost orbital. The electron configuration of Lithium is 1s22s1. By removing the 2s1 electron, it can become a duplet. In that case, He, H and Li+ are duplets that can exist as stable duplets.

All duplets have electron configuration ending as follows.


Difference Between Octet and Duplet | Octet vs Duplet

Figure 02: Helium, a duplet

What is the difference between Octet and Duplet?

Octet vs Duplet

Octet has eight electrons in the outermost shell. Duplet has two electrons in the outermost shell.
Common Electron Configuration
Octet has ns2 np6 type electron configuration at the end. Duplet has ns2 type configuration at the end.
Number of Orbitals
Octet may have a minimum of two orbitals. A duplet has only one orbital.
Type of Orbitals
An octet can have all types of orbitals such as s, p, d, f, etc. Duplet has only s orbital.

Summary – Octet vs Duplet

All elements tend to become stable. But with incomplete electron configurations, atoms cannot be stable; thus, they become very reactive in order to fill the shells with electrons by gaining, losing or sharing electrons. The atoms or molecules which obey the octet or duplet rule are stable. The key difference between octet and duplet is that octet has eight electrons in its outermost orbital whereas duplet has two electrons in its orbital.

1. Nema, P., 2011. Topperlearning. [Online] Available at: http://www.topperlearning.com/forums/home-work-help-19/what-is-octet-duplet-valency-and-ions-chemistry-structure-of-the-atom-46317/reply [Accessed 29 05 2017].
2.Chang, R., 2010. Chemistry. 10th ed. New York: McGraw-Hill.
3.Anon., 2015. Grade stacker. [Online] Available at: http://gradestack.com/NTSE-Complete-Course/Chemical-Bonding/Chemical-Activity-of-An/19176-3853-37737-study-wtw [Accessed 29 05 2017].

Image Courtesy:
1. “Electron shell 002 Helium – no label” By Pumbaa (original work by Greg Robson) – Electron shell diagrams (corresponding labeled version) (CC BY-SA 2.0 uk) via Commons Wikimedia
2. “Electron shell 010 Neon” By Pumbaa (original work by Greg Robson)  (CC BY-SA 2.0 uk) via Commons Wikimedia

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