Which of the following has maximum number of atoms?

The correct option is D

18g of CH₄

Formula to derive the number of atoms:

$=\frac{\left(Massofthesubs\tan ce\times Numberofatomsinthemolecule\right)}{MolarMass}\times N_A$

For the given options,

Options	Mass of the substance	Number of atoms in the molecule	Molar Mass
18g of H2O	18	3	18
18g of O2	18	2	32
18g of CO2	18	3	44
18g of CH4	18	5	16

Applying the above values in the formula:

Option-A: 18g of H₂O

$$\begin{gathered} =\frac{18\times 3}{18}\times N_A \\ =3N_A \end{gathered}$$

Option-B: 18g of O₂

$$\begin{gathered} =\frac{18\times 2}{32}\times N_A \\ =1.125N_A \end{gathered}$$

Option-C: 18g of CO₂

$$\begin{gathered} =\frac{18\times 3}{44}\times N_A \\ =1.23N_A \end{gathered}$$

Option-D: 18g of CH₄

$$\begin{gathered} =\frac{18\times 5}{16}\times N_A \\ =5.625N_A \end{gathered}$$

From the above, it's clear that Option-D: 18g of CH₄ has the maximum number of atoms.

Hence, Option D is correct.