Question

Which element among the following does NOT form diatomic molecules?
A. Argon
B. Oxygen
C. Nitrogen
D. Bromine

Answer 1

Hint: The electron structure in atomic physics and in quantum chemistry is the arrangement of electrons in an atom or molecule in atomic or molecular orbitals. A stable structure of electrons refers to an atom in which the outermost shell (valence) is complete. An unstable electron configuration corresponds to an atom that incomplete the outermost (valence) casing.

Complete answer:
Diatomic molecules are molecules composed of only two atoms, of the similar or different chemical elements. The prefix di- is of Greek origin, meaning “two”. If a diatomic molecule consists of two atoms of the similar element, like hydrogen (${{H}_{2}}$) or oxygen (${{O}_{2}}$), then it is said to be homonuclear. Otherwise, if a diatomic molecule consists of two different atoms, such as carbon monoxide (CO) or nitric oxide (NO), the molecule is claimed to be heteronuclear. The bond in a homonuclear diatomic molecule is nonpolar.
A. Argon
Argon is an inert gas (noble gas). It is stable thus chemically unreactive. Hence, it does not form diatomic molecules.
B. Oxygen
Electronic configuration of oxygen $1{{s}^{2}}2{{s}^{2}}2{{p}^{4}}$. It requires two electrons to complete its octet. Thus, it combines with another oxygen atom or with another atom to gain its stability or to become stable. Hence, it forms diatomic molecules.
C. Nitrogen
Electronic configuration of nitrogen $1{{s}^{2}}2{{s}^{2}}2{{p}^{3}}$. It requires three electrons to complete its octet. Thus, it combines with another nitrogen atom or with another atom to gain its stability or to become stable. Hence, it forms diatomic molecules.
D. Bromine
Electronic configuration of bromine $[Ar]3{{d}^{10}}4{{s}^{2}}4{{p}^{5}}$. It requires one electron to complete its octet. Thus, it combines with another bromine atom or with another atom to gain its stability or to become stable. Hence, it forms diatomic molecules.

Therefore, the correct answer for this question is Option no.(A) Argon.

Note:
Always remember, noble gas or inert gas are stable thus they are chemically unreactive. Hence, they do not form diatomic molecules.
Noble gas or inert gas consist of element Ne, Ar, He, Xe, Kr and Rn.
Elements whose octet is not complete are chemically unstable and reactive thus to gain stability or to become stable they combine with another atom. Thus, they form diatomic molecules.