The $K_{\alpha}$ $X$-ray emission line of tungsten occurs at $\lambda = 0.021 \, nm$. The energy difference between $K$ and $L$ levels in this atom is about:

In $\Delta ABC$,the expression $a(\cos^2 B + \cos^2 C) + \cos A(c \cos C + b \cos B)$ is equal to:

Which one of the following is most acidic?

Which of the following can show optical isomerism?

The wavelengths of $K_{\alpha}$ $X$-rays of two metals $A$ and $B$ are $\frac{4}{1875R}$ and $\frac{1}{675R}$ respectively,where $R$ is the Rydberg constant. The number of elements lying between $A$ and $B$ according to their atomic numbers is:

The bonds between $P$ atoms and $Cl$ atoms in $PCl_5$ are likely to be