Although ionic solids, such as NaCl, can be appropriately modeled as stacking spheres because the Na⁺ and Cl^– ions largely do not share electrons, the Si and O of silicates are covalently bonded, which is to say that shared electrons make the molecular unit hold together. Furthermore, the molecular orbits are hybridized to make certain geometrical arrangements favorable; in this case, the Si binding electrons undergo sp³ hybridization to make a tetrahedral arrangement.

Using LCAO (linear combination of atomic orbitals) ‘cartoons’, here is a sequence of transitions that shows:

• the tetrahedral arrangement of sp³ bonds of Si, then
• Si with four O as spheres at the apices of the tetrahedron, then
• one of the apical O’s hybridizes as well (also sp³, but with two lone pair electrons at two of the apices, just as in H₂O), permitting the bonding of another silicate unit through that O.

Replay as necessary by pressing the button above. Use the polyhedral mode to verify the utility of that representation.

Next: Silicate polymerization to form chains, rings, and sheets
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