Concept: The shape of a molecule or ion is determined by the VSEPR (Valence Shell Electron Pair Repulsion) theory, which predicts the geometry based on minimizing repulsion between electron pairs (both bonding and non-bonding) around the central atom.
Why (C) is correct:
1. Determine the central atom: In the ion, chlorine (Cl) is the central atom.
2. Calculate the total number of valence electrons:
Valence electrons of Cl = 7
Valence electrons of O = 6 (for each of the 4 oxygen atoms) =
Negative charge = 1 (adds one electron)
Total valence electrons =
3. Determine the number of electron pairs: .
4. Draw the Lewis structure and determine bonding and lone pairs: In , the central Cl atom is bonded to four oxygen atoms. To satisfy the octet rule for oxygen and minimize formal charges, chlorine forms one single bond and three double bonds with oxygen atoms (or four single bonds with formal charges, but the overall electron domain geometry remains the same). Regardless of the bonding type (single or double), each bond counts as one electron domain. There are 4 bonding pairs and 0 lone pairs around the central Cl atom.
5. Apply VSEPR theory: With 4 bonding pairs and 0 lone pairs, the steric number is 4. According to VSEPR theory, a central atom with 4 electron domains and no lone pairs will have a tetrahedral electron geometry and a tetrahedral molecular geometry.
Option Analysis:
- A) Square planar: This shape typically occurs for a central atom with 4 bonding pairs and 2 lone pairs (steric number 6), or for certain d8 metal complexes. does not fit this.
- B) Square pyramidal: This shape typically occurs for a central atom with 5 bonding pairs and 1 lone pair (steric number 6). does not fit this.
- D) Trigonal bipyramidal: This shape occurs for a central atom with 5 electron domains (e.g., 5 bonding pairs and 0 lone pairs). has 4 electron domains.
Correct Answer: (C)