SiO32- Lewis structure, VSEPR, polarity
The Correct Answer and Explanation is:
Correct Answer:
1. Lewis Structure for SiO₃²⁻ (Silicate Ion):
- Silicon (Si) is the central atom.
- There are three oxygen (O) atoms bonded to the silicon.
- The ion has a -2 charge.
Steps:
- Count valence electrons:
Si has 4 valence electrons.
Each O has 6 valence electrons × 3 = 18.
Add 2 electrons for the 2− charge.
Total = 4 + 18 + 2 = 24 valence electrons. - Form bonds:
Draw Si in the center and connect each O with a single bond.
That uses 6 electrons (3 single bonds). - Distribute remaining electrons:
24 − 6 = 18 electrons left. Distribute 6 electrons (as 3 lone pairs) around each O. - Check octet and formal charges:
Each O has 6 electrons as lone pairs and 2 from the Si−O bond, which completes the octet.
Si has only 6 electrons, so we convert one Si−O bond into a double bond to satisfy the octet rule. - Final structure:
One double bond between Si and one O atom, and two single bonds with negative charges on the other two O atoms.
2. VSEPR (Electron Geometry and Molecular Shape):
- Central atom: Silicon (Si)
- 3 regions of electron density (3 bonding pairs)
- No lone pairs on Si
⇒ Electron geometry: trigonal planar
⇒ Molecular geometry: trigonal planar
3. Polarity:
- The molecule is polar overall.
Although the shape is symmetrical (trigonal planar), the presence of negative charges on two of the oxygens creates an uneven distribution of charge.
Explanation
The silicate ion, SiO₃²⁻, is a polyatomic anion composed of one silicon atom and three oxygen atoms, with an overall charge of negative two. To draw its Lewis structure, begin by counting all valence electrons. Silicon contributes 4, each oxygen contributes 6 (totaling 18), and the -2 charge adds 2 more, for a sum of 24 electrons.
We arrange the silicon atom at the center with three oxygen atoms bonded to it. Single bonds account for 6 electrons, leaving 18 to be distributed as lone pairs on the oxygen atoms. When we do this, each oxygen appears to have a complete octet, but silicon has only 6 electrons around it. To satisfy the octet rule for silicon, we convert one of the Si–O single bonds into a double bond. This structure gives one double bond and two single bonds, with negative charges on the two singly bonded oxygens.
Using VSEPR theory (Valence Shell Electron Pair Repulsion), the shape of the molecule is determined by the number of bonding regions around the central atom. With three bonding regions and no lone pairs, the molecule adopts a trigonal planar shape, which spreads out the atoms to minimize electron repulsion.
Even though the shape is symmetric, the presence of two negatively charged oxygens causes an unequal distribution of electron density. This asymmetry makes the SiO₃²⁻ ion polar. The polar nature affects how this ion interacts with other molecules, especially in aqueous solutions, where it tends to associate with cations like sodium or calcium.
In summary, SiO₃²⁻ has a Lewis structure with one double bond and two negatively charged single bonds, a trigonal planar geometry, and it is polar due to uneven charge distribution.
