The key difference between monatomic and diatomic is that the monatomic species have one atom whereas the diatomic species have two atoms.
Hence, the 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.’
What is Monatomic?
When a single atom exists on its own (which is rarely the case), we call it monatomic. That means, the elements are in their pure singular form. However, the only practical example that may come under this category is the noble gases that exist as atoms on their own as they have their outer shell with a complete 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.
Furthermore, 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+. Thus, not only cations (positively charged) but anions (negatively charged) also exist in monatomic form; Cl–, F–, I– are few examples that exist in monatomic form. These ionic species are not stable on their own and would naturally look for counterparts to form compounds.
But, as mentioned above, we can find them in solutions upon the hydrolysis of their compounds. Ionic species form due to the lack of stability of the single atom in a 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 may occur as same type or different. When they are two similar atoms in association we call it ‘homonuclear diatoms’ and if they consist of different types we call it ‘heteronuclear diatoms’. Examples of some homonuclear diatoms would be O2, N2, H2, etc. whereas CO, NO, HCl, etc. can be given as examples for heteronuclear diatoms.
We can consider diatoms as compounds because 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 bind 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 cation (positively charged) species and an anion (negatively charged) species. Examples of covalent bonds among diatoms include CO, NO, etc. and we consider HCl 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?
The term monatomic refers to the presence of one atom while the term diatomic refers to the presence of two atoms in association with each other. Therefore, the key difference between monatomic and diatomic is that monatomic species have one atom whereas diatomic species have two atoms. Moreover, another difference between monatomic and diatomic is that the monatomic species are generally unstable with the exception of noble gases while the diatomic species are generally stable because there is a chemical bond between the two atoms that form in order to complete the electron octet around each atom.
The below infographic shows the difference between monatomic and diatomic in tabular form.
Summary – Monatomic vs Diatomic
The two terms monatomic and diatomic describes the number of atoms present in a chemical species. Hence, the key difference between monatomic and diatomic is that monatomic species have one atom whereas diatomic species have two atoms.
1. “Monatomic Gas.” Wikipedia, Wikimedia Foundation, 17 Sept. 2018. Available here