On the basis of VSEPR theory explain why Berrylium chloride (BeCl2) molecule is linear whereas water (H2O) molecule is angular?

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To understand the differences you need to look at the electron configurations of each of these elements.  Be, atomic number 4, has a  total of 4 electrons.  its configuration is 1s2 2s2.  It has two valence electrons, and no electron pairs.  Each of these electrons can take in forming bonds,...

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To understand the differences you need to look at the electron configurations of each of these elements.  Be, atomic number 4, has a  total of 4 electrons.  its configuration is 1s2 2s2.  It has two valence electrons, and no electron pairs.  Each of these electrons can take in forming bonds, in this case with chlorine atoms.  Chemical bonds involve electrons and because all electrons have a negative charge, the two bonds between the Be and Cl will arrange themselves around the central atom (Be) so that the electrons in the bonds are as far apart as possible. Thus, the molecule is linear with the chlorines on opposite sides of the Be.

In the case of water, oxygen now has a total of 8 electrons, with a configuration is 1s2 2s2 2p4.  Oxygen has two electron pairs in the outer energy level, as well as two valence electrons available for bonding.  The two single electrons can form bonds with two hydrogen atoms and would like to again form a linear shape. However, the two electron pairs exert a strong repulsive force on the O-H bonds, distorting them into the bent shape of the water molecule. This shape maximizes the distance between all the electrons surrounding the central oxygen atom.

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