The radius of fourth orbit in $\mathrm{He}^{+}$ion is ' $R_1{ }^{\prime} \mathrm{pm}$ and radius of third orbit in $\mathrm{Li}^{2+}$ ion is ' $R_2{ }^{\prime} \mathrm{pm}$. The value of ( $R_1-R_2$ ) in pm is
The de-Broglie wavelengths of two fast moving particles $X, Y$ are $1 \mathrm{~nm}, 3 \mathrm{~nm}$ respectively. Mass of $X$ is nine times the mass of $Y$. The ratio of kinetic energies of $X, Y$ is
Electronic configurations of four elements $A, B, C, D$ are given below
(A) $1 s^2 2 s^2 2 p^6 3 s^1$
(B) $1 s^2 2 s^2 2 p^6 3 s^2 3 p^1$
(C) $1 s^2 2 s^2 2 p^6 3 s^2$
(D) $1 s^2 2 s^2 2 p^6 3 s^2 3 p^2$
The correct order of first ionisation enthalpy of these elements is
A molecules has T -shape. The total number of electron pairs in the valence shell of central atom of it is
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