What is the molecular geometry of the sulfate ion, SO4²-?
A. trigonal planar
B. tetrahedral
C. trigonal pyramidal
D. bent
The Correct Answer and Explanation is:
The correct answer is B. tetrahedral.
The sulfate ion (SO₄²-) consists of one sulfur atom centrally located and surrounded by four oxygen atoms. To determine the molecular geometry of the sulfate ion, we can use the Valence Shell Electron Pair Repulsion (VSEPR) theory. This theory posits that the electron pairs around a central atom will arrange themselves as far apart as possible to minimize repulsion.
Steps to Determine Geometry:
- Count Valence Electrons:
- Sulfur (S) has 6 valence electrons, and each oxygen (O) has 6 valence electrons. For the sulfate ion, which has a -2 charge, we add 2 additional electrons.
- Total = 6 (from S) + 4 × 6 (from O) + 2 (for the charge) = 32 valence electrons.
- Draw the Lewis Structure:
- Place the sulfur atom in the center and surround it with four oxygen atoms. Each sulfur-oxygen bond will use 2 electrons, resulting in 8 electrons used for the bonds.
- After forming single bonds, we distribute the remaining electrons to satisfy the octet rule. Each oxygen can complete its octet by forming a double bond with sulfur, but in the case of sulfate, sulfur can expand its octet.
- Identify Electron Regions:
- Sulfur is surrounded by four regions of electron density (the four S-O bonds). Since all are bonded pairs, there are no lone pairs on the sulfur atom.
- Determine Molecular Geometry:
- With four regions of electron density and no lone pairs, the arrangement is tetrahedral according to VSEPR theory.
Summary:
The tetrahedral geometry of SO₄²- leads to bond angles of approximately 109.5 degrees between the S-O bonds. This arrangement minimizes the repulsion between the bonding electron pairs, resulting in a stable structure. Thus, the molecular geometry of the sulfate ion is tetrahedral, reflecting its symmetry and the equal distribution of bonding pairs around the central sulfur atom.