Draw a proper Lewis Dot Structure and give the electron pair geometry for the following molecules: BF3, CCI4, HCI, SF4, XEF4
The Correct Answer and Explanation is :
Lewis Dot Structures and Electron Pair Geometries
1. BF₃ (Boron Trifluoride)
- Lewis Structure:
Boron (B) is the central atom, bonded to three fluorine (F) atoms. Boron has only 6 valence electrons, as it does not complete an octet in this case. Each F atom is surrounded by 6 electrons as lone pairs. - Structure:
F
|
B
/ \
F F - Electron Pair Geometry: Trigonal Planar
The central atom (B) has 3 regions of electron density, all bonded pairs. Bond angles are 120°.
2. CCl₄ (Carbon Tetrachloride)
- Lewis Structure:
Carbon (C) is the central atom, bonded to four chlorine (Cl) atoms. Each Cl atom has 3 lone pairs, completing the octet rule for all atoms. - Structure:
Cl
|
Cl – C – Cl
|
Cl - Electron Pair Geometry: Tetrahedral
The central atom (C) has 4 regions of electron density, all bonded pairs. Bond angles are 109.5°.
3. HCl (Hydrogen Chloride)
- Lewis Structure:
H is bonded to Cl. Cl has 3 lone pairs, and H completes its duet with a single bond. - Structure: H—Cl
- Electron Pair Geometry: Linear
The central atom (Cl) has 4 regions of electron density (1 bond and 3 lone pairs). Bond angle is 180°.
4. SF₄ (Sulfur Tetrafluoride)
- Lewis Structure:
Sulfur (S) is the central atom, bonded to 4 fluorine atoms and has one lone pair. - Structure:
F
|
F – S – F
|
F - Electron Pair Geometry: Trigonal Bipyramidal
The lone pair occupies an equatorial position, causing bond angles to deviate. Axial F-S-F angles ~173°, equatorial angles ~102°.
5. XeF₄ (Xenon Tetrafluoride)
- Lewis Structure:
Xenon (Xe) is the central atom, bonded to 4 fluorine atoms, with 2 lone pairs. - Structure:
F
|
F – Xe – F
|
F - Electron Pair Geometry: Octahedral
The lone pairs are opposite each other, creating a square planar molecular geometry. Bond angles are 90°.
Explanation (~300 words)
The Lewis structures of molecules provide a way to represent the distribution of electrons in a molecule. Using valence shell electron pair repulsion (VSEPR) theory, the electron pair geometry considers regions of electron density (bonds and lone pairs) around the central atom. The arrangement minimizes repulsion, giving rise to specific geometries.
- In BF₃, boron does not complete its octet and adopts a trigonal planar geometry with 120° bond angles.
- In CCl₄, carbon forms a tetrahedral geometry as it has 4 bond pairs and no lone pairs.
- HCl is linear as it has only one bond pair and no lone pairs on hydrogen.
- In SF₄, the lone pair on sulfur distorts the ideal trigonal bipyramidal geometry, resulting in a “see-saw” shape with uneven angles.
- XeF₄ demonstrates the use of expanded octets, where xenon’s lone pairs result in an octahedral electron geometry but a square planar molecular shape.
Understanding these structures helps predict molecular shapes, bond angles, and reactivity.