In a reaction

$A+B_2\to A{B}_2$

The limiting reagent, If 300 atoms of A and 200 molecules of B react with each other, is

                             $A + B_2\to A{B}_2$

Intial                  No atom   No molecules

After reaction   -------              -------                     No molecules

No molecules of $B_2$ react with No atoms of A 200 molecules of $B_2$ react with $\to$

Number of A atoms reacted $= \frac{200\times N_0}{N_0}$

i.e $B_2$ molecule are completely consumed

$\therefore B_2$ is limiting reasent