Beryllium VSEPR model

Consider the triatomic molecule beryllium chloride (BeCl2). Experimental evidence shows that the BeCl2 molecule in the gas phase is linear with two equivalent Be—Cl bonds, consistent with the VSEPR model prediction. The construction of a valence bond picture of bonding in linear BeCl2 requires the existence of unpaired electrons in Be atomic orbitals that overlap with the half-filled 3p orbital on each Cl to form two equivalent bonds that are 180° apart. However, the ground state electron configuration for Be is ls 2s, which contains no unpaired electrons. 

There are two electron pairs in the valence shell for beryllium, and the VSEPR model predicts that they will have a linear arrangement (see Figure 10.2). Fluorine atoms are bonded in the same direction as the electron pairs. Hence, the geometry of the Bep2 molecule is linear—that is, the atoms are arranged in a straight line (see Figure 10.4). 

Beryllium hydride, BeH2, has four valence electrons, two from beryllium and one each from the two hydrogen atoms, all of which appear in its Lewis diagram. In VSEPR theory, the steric number is 2, so the molecule is predicted to be linear, and this prediction is verified by experiment. The electron configuration of the central atom is Be (ls) (2s). There are no unpaired electrons to overlap with H(ls) orbitals, so the VB model fails to predict the formation of BeHi. 

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