For each of the following,
a. Draw the lewis structure
b. Indicate the molecular geometry
c. Indicate the bond angles
PCI3
IBr4-
SeF4
NH2-
SCN-
SnCl5-
The Correct Answer and Explanation is:
1. PCl₃
a. Lewis Structure:
P is the central atom with three Cl atoms bonded and one lone pair.
..
:Cl:
|
:Cl–P–Cl:
..
b. Molecular Geometry: Trigonal pyramidal
c. Bond Angles: ~107°
2. IBr₄⁻
a. Lewis Structure:
Iodine is central with four Br atoms and two lone pairs.
Br–I–Br
| |
Br Br
(with 2 lone pairs on I)
b. Molecular Geometry: Square planar
c. Bond Angles: 90°
3. SeF₄
a. Lewis Structure:
Se has four bonded F atoms and one lone pair.
F
|
F – Se – F
|
F
(one lone pair on Se)
b. Molecular Geometry: See-saw
c. Bond Angles: ~101° (equatorial), ~173° (axial)
4. NH₂⁻
a. Lewis Structure:
N is central with two H atoms and two lone pairs.
H–N⁻–H
..
..
b. Molecular Geometry: Bent
c. Bond Angles: ~104°
5. SCN⁻ (thiocyanate ion)
a. Lewis Structure:
Resonance structures exist:
S=C=N⁻ or ⁻S–C≡N
(Linear structure overall)
b. Molecular Geometry: Linear
c. Bond Angles: 180°
6. SnCl₅⁻
a. Lewis Structure:
Sn is central with five Cl atoms and one extra electron (negative charge).
Trigonal bipyramidal with 5 Cl atoms around Sn
b. Molecular Geometry: Trigonal bipyramidal
c. Bond Angles: 90°, 120°, 180°
Explanation:
The geometry and structure of a molecule depend on the Valence Shell Electron Pair Repulsion (VSEPR) theory, which states that electron pairs around a central atom repel each other and arrange themselves to minimize this repulsion.
PCl₃ has three bonded atoms and one lone pair on phosphorus, making it AX₃E and resulting in a trigonal pyramidal shape, similar to ammonia, with bond angles slightly less than the tetrahedral angle due to lone pair repulsion.
IBr₄⁻ has four bonded atoms and two lone pairs on iodine, giving an AX₄E₂ system. The lone pairs occupy opposite positions in an octahedral arrangement, resulting in a square planar geometry with 90° bond angles.
SeF₄ has four bonded atoms and one lone pair on selenium (AX₄E). This lone pair distorts the shape from trigonal bipyramidal to see-saw, affecting bond angles.
NH₂⁻ has two bonded atoms and two lone pairs on nitrogen. Like water, it’s bent, with angles slightly less than 109.5°, about 104°, due to increased repulsion from lone pairs.
SCN⁻ is a linear ion where resonance structures are possible, but all forms are AX₂ with a linear geometry and 180° bond angles due to no lone pairs on the central carbon.
SnCl₅⁻ forms a trigonal bipyramidal shape, common for five bonded pairs (AX₅), with a mix of 90°, 120°, and 180° angles. Negative charge increases electron density but doesn’t change geometry significantly.
This analysis relies on electron domain geometry and hybridization (sp³, sp³d, etc.), critical for predicting molecular behavior, reactivity, and physical properties.
