$KCl$ has the same structure as $NaCl$. If ${r_{Na^+}}/{r_{Cl^-}} = 0.55$ and ${r_{Na^+}}/{r_{K^+}} = 0.74$,then the ratio of the edge lengths of $KCl$ and $NaCl$ is:

In the $x$-ray reflection $(n=1)$, the distance between two parallel planes of $NaCl$ is $280 \ pm$ and the diffraction angle is $5.2^{\circ}$. What is the wavelength of its light radiation? (Given: $\sin 5.2^{\circ} = 0.09$)

The formula for the determination of the density of a unit cell is:

Lithium metal crystallises in a body-centred cubic crystal. If the length of the side of the unit cell of lithium is $351 \ pm,$ the atomic radius of lithium will be $............$ $pm$.

At $T \ K$,copper (atomic mass $= 63.5 \ u$) has $fcc$ unit cell structure with edge length of $x \ \mathring{A}$. What is the approximate density of $Cu$ in $g \ cm^{-3}$ at that temperature? $(N_A = 6.0 \times 10^{23} \ mol^{-1})$

An element occurs in the body-centred cubic $(BCC)$ structure with an edge length of $288 \ pm$. The density of the element is $7.2 \ g \ cm^{-3}$. The number of atoms present in $208 \ g$ of the element is nearly: