Monatomic vs Diatomic
Difference between monatomic and diatomic is primarily with regard to the atoms present in the species. As the names suggest, both these terms stand for different states of atomic associations where ‘mono’ means ‘one’ and ‘di’ means ‘two.’ Therefore, simply, monatomic means one ‘one atom’ and diatomic means ‘two atoms.’ However, there can be so many different variations to this basic definition.
What is Monatomic?
When a single atom exists on its own (which is rarely the case), we call it monatomic. This means the elements are in their pure singular form. The only practical example that may come under this category would be the noble gases that exist as atoms on their own as they have their outer shell completed with an octet of electrons. Hence, they do not look to accept or donate any more electrons in order to be more stable. Therefore, noble gases are stable in monatomic form. Some examples are; He – Helium, Ne – Neon, Ar – Argon, Xe – Xenon, Kr – Krypton, Rn – Radon.
There also exist single atoms in ionic forms especially in solutions, and some examples are; Na+, Ca2+, K+ etc. These ions have a fixed charge on them meaning that they have a constant valency. But, there are other types of ions that have many valencies and can exist in many ionic forms, still being monatomic. A good example is Iron; Fe2+ and Fe3+. Not only cations (positively charged) but anions (negatively charged) also exist in monatomic form; Cl-, F-, I- are few examples. These ionic species are not stable on their own and would naturally look for counterparts to form compounds. But, as mentioned above, they can be found in solutions upon the hydrolysis of their compounds. Ionic species form due to the lack of stability of the single atom in pure form that is unable to attain the noble gas electronic configuration. Therefore, these atoms either accept or donate electrons in order to gain stability.
What is Diatomic?
When two atoms are in association with each other, we call it diatomic. These atoms can be from the same type or different. When they are two similar atoms in association we call it ‘homonuclear diatoms’ and if they are made of different types we call it ‘heteronuclear diatoms’. Examples for some homonuclear diatoms would be O2, N2, H2, etc. whereas CO, NO, HCl, etc. can be given as examples for heteronuclear diatoms.
Diatoms can be considered as compounds as they form these associations in order to attain more stability by sharing electrons with each other so that both atoms attain noble gas electronic configuration. They can bond through covalent bonds by the overlapping of atomic orbitals or else they can form ionic bonds among them which is an attraction force between a positively charged species and a negatively charged species. Examples for covalent bonds among diatoms include CO, NO, etc. and HCl can be considered as a species with ionic attraction character. However, as the attraction force between H+ and Cl- is not very strong, it is not a very good example for ionic bonds that is another defined topic.
What is the difference between Monatomic and Diatomic?
• Monatomic species have one atom whereas diatomic species have two atoms.
• Monatomic species are generally not stable, but diatomic species are generally stable.
• Noble gases are only monatomic and are not found in diatomic form.
• Chemical bonds exist between diatomic species whereas there are no bonds in monatomic species.
- Helium electron configuration by Pumbaa (CC BY-SA 2.0 uk)
- Oxygen electron configuration by DePiep (CC BY-SA 3.0)