# Which of the following is true ?

1. A

The outer electronic configuration of the ground state chromium atom is $3{\mathrm{d}}^{4}4{\mathrm{s}}^{2}.\mathrm{ }$

2. B

Gamma rays are electromagnetic radiations of wavelength of ${10}^{-6}\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{cm}\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{to}\text{\hspace{0.17em}\hspace{0.17em}}{10}^{-5}\mathrm{cm}.$

3. C

The energy of the electron in the 3d orbital is less than that in the 4s orbital of a hydrogen atom.

4. D

The electron density in the xy-plane in $3{\mathrm{d}}_{{\mathrm{x}}^{2}-{\mathrm{y}}^{2}}$ orbital is zero.

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### Solution:

(a) False. The configuration is $3{\mathrm{d}}^{5}4{\mathrm{s}}^{2}.$

(b) False. The wavelength of gamma rays is of the order of 10-11 m.

(c)True. In hydrogen atom, the energy of an electron depends only on the principal quantum number of the orbital which it occupies.

(d) False. In xz-plane, there is no electron density if an electron occupies $3{\mathrm{d}}_{{\mathrm{x}}^{2}-{\mathrm{y}}^{2}}$ orbital.

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