Question 19.3: Interpreting Representations of Silicate Anions The followin......
Interpreting Representations of Silicate Anions
The following pictures represent silicate anions. What are the formula and charge of each anion?
STRATEGY
Each tetrahedron represents an SiO_4 structural unit, with an Si atom at the center of the tetrahedron and an O atom at each corner. Each terminal (unshared) O atom needs one electron to complete its octet and therefore has a charge of -1. Each bridging (shared) O atom completes its octet by forming bonds to two Si atoms and is therefore electrically neutral. To find the number of Si atoms in the formula, count the number of tetrahedra. To find the number of O atoms, count the number of corners (shared and unshared). To find the charge on the anion, count the number of unshared corners.
(a) The picture contains three tetrahedra, with three shared corners and six unshared corners (nine in all). Therefore, the anion is Si_3O_{9}^{6-}.
(b) The picture contains four tetrahedra, with three shared corners and 10 unshared corners (13 in all). Therefore, the anion is Si_4O_{13}^{10-}.
CHECK
We can check the charge on a silicate anion by assigning the usual oxidation states of +4 to silicon and -2 to oxygen. Thus, the charge on anion (a) must be (3)(4) + (9)(-2) = -6, and the charge on anion (b) must be (4)(4) + (13)(-2) = -10.