Question

The atomicity of He is:
(A) 1
(B) 2
(C) 3
(D) 4

Answer 1

Hint: To determine the answer we should know what atomicity is. Atomicity of molecules is the number of atoms present in the molecule. In the case of a homo-atomic molecule, we can determine the atomicity of the molecule by dividing the molecular mass of the molecule by the atomic mass.

Formula used: $Atomicity=\dfrac{Molecular mass}{Atomic mass}$

Complete answer:
Atomicity represents the total number of atoms present in a molecule.
We can calculate the atomicity by taking the sum of all numbers of atoms present in the molecule.
 $Atomicity=\dfrac{Molecular mass}{Atomic mass}$
Let’s consider an example, in the case of an oxygen molecule.
Molecular mass of oxygen $ = 32$
Atomic mass of oxygen $ = 16$
Atomicity $ = \dfrac{{32}}{{16}}$
$ = 2$
Atomicity of oxygen is 2.
Similarly, we can calculate the atomicity of the Helium molecule.
Helium is monoatomic. Therefore, its molecular mass and atomic mass are the same.
Molecular mass $ = \,4$
Atomic mass $ = \,4$
$Atomicity=\dfrac{Molecular mass}{Atomic mass}$
$ = \,\dfrac{4}{4}$
$ = \,1$
So, the atomicity of Helium is $1$. The element with atomicity one is known as monatomic.

Therefore, from the above explanation, the correct option is (A) $1$.

Note:In the case of a hetero-atomic molecule or the above formula cannot be used because every atom has a different atomic mass. In the case of hetero-atomic, the atomicity is determined by adding the number of atoms present in a molecule. Helium is an inert gas. All inert gases are monatomic. Because they have their orbitals filled. They do not lose or accept or share electrons and forms bond. Therefore, they exist as monatomic forms. On the basis of atomicity, molecules are classified as monatomic, diatomic, triatomic, and so on. Generally, molecules are termed as polyatomic molecules if the molecule has more than $2$atoms.

Some examples of atomicity is are as follows:

Molecule	Atomicity	Example
Monoatomic	$1$	${\text{He,Na}}$
Diatomic	$2$	${{\text{O}}_{\text{2}}}{\text{,}}{{\text{N}}_{\text{2}}}$
Triatomic	$3$	${{\text{O}}_{\text{3}}}{\text{,C}}{{\text{O}}_{\text{2}}}$
Tetratomic	$4$	${{\text{P}}_{\text{4}}}$
Pentatonic	$5$	${\text{C}}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{3}}}$
Octatonic	$8$	${{\text{S}}_{\text{8}}}$