A Binary solid has rocksalt structure The edge length is $400 \mathrm{pm}$ and the radius of cation is $75 \mathrm{pm}$, the radius of anion is

We know that in rocksalt structure the relationship between edge length and radius of cation and anion can be mathematically represented as:

$a=2\left(r_{+}+r_{-}\right)$

It is provided in the question that edge length $‘a’$ is $400 \mathrm{pm}$, the radius of cation is $75 \mathrm{pm}$.

Substituting these values in the equation and solving we get:

$400=2\left(75+{\mathrm{r}}_{-}\right)$

$\Rightarrow r_{-}=\frac{400-150}{2}$

$\Rightarrow r_{-}=\frac{250}{2}=125 pm$
Hence, the required answer is $125 \mathrm{pm}$.