What is the Lewis structure for SiO32- ?
The Correct Answer and Explanation is:
Lewis Structure for SiO₃²⁻ (Silicate ion):
To draw the correct Lewis structure for SiO₃²⁻, follow these steps:
- Count the total valence electrons:
- Silicon (Si) has 4 valence electrons.
- Oxygen (O) has 6 valence electrons × 3 = 18.
- The ion has a 2− charge, so add 2 more electrons.
- Total = 4 + 18 + 2 = 24 valence electrons.
- Determine the skeleton structure:
- Silicon is the central atom.
- Attach the three oxygen atoms to silicon with single bonds.
- Distribute the remaining electrons:
- Each Si–O bond uses 2 electrons × 3 = 6 electrons.
- 24 − 6 = 18 electrons left.
- Place these 18 electrons around the oxygen atoms (6 per oxygen) to complete their octets.
- Check for octet rule:
- Each oxygen has 8 electrons (2 in the bond and 6 as lone pairs).
- Silicon has only 6 electrons (3 single bonds). To give it a full octet, one oxygen forms a double bond with silicon.
- Adjust for resonance and charge:
- Two of the oxygens remain as single bonds with full negative charges.
- One oxygen forms a double bond with silicon.
- The overall charge is 2−, shared across the two single-bonded oxygens.
Final Lewis Structure:
- Silicon (Si) in the center.
- One double bond to an oxygen (O), two single bonds to two other oxygens.
- Each single-bonded oxygen has 3 lone pairs and carries a negative charge.
- The double-bonded oxygen has 2 lone pairs.
- The entire structure is enclosed in brackets with a 2− charge.
Explanation
The silicate ion, SiO₃²⁻, is composed of one silicon atom and three oxygen atoms. In drawing its Lewis structure, the first step is counting the total valence electrons. Silicon contributes 4, each oxygen contributes 6 (totaling 18), and the extra 2 electrons from the negative charge give a sum of 24 electrons.
Silicon, being less electronegative, serves as the central atom. Each oxygen is connected to silicon by a single bond, using 6 electrons in total. The remaining 18 electrons are distributed around the oxygen atoms, giving each a full octet. However, this arrangement leaves silicon with only 6 electrons, which does not fulfill the octet rule.
To correct this, one oxygen forms a double bond with silicon. This change does not affect the octet of oxygen but allows silicon to reach an octet (4 shared pairs). The result is a structure with one double-bonded oxygen and two single-bonded oxygens. The single-bonded oxygens each hold a negative charge due to having one more electron than needed for bonding.
This Lewis structure illustrates resonance because the double bond can shift among the three oxygen atoms. However, a single structure often shows one double bond with the understanding that resonance exists. The formal charges show that the total charge of the ion is 2−, aligning with the chemical formula.
This configuration ensures that all atoms follow the octet rule, formal charges are minimized, and the total charge is correct. Understanding the Lewis structure of SiO₃²⁻ is important in predicting molecular geometry and reactivity in both chemical and geological contexts.
