Draw the Lewis dot structure including its VSEPR shape for AsBr3. Show all calculations. Write down the hybridization, molecular geometry, bond angles, polarity.
Draw the Lewis dot structure including its VSEPR shape for AsBr3. Show all calculations. Write down the hybridization, molecular geometry, bond angles, polarity.
The Correct Answer and Explanation is:
AsBr₃ (Arsenic Tribromide) – Lewis Structure, VSEPR Shape, Hybridization, Geometry, Bond Angles, and Polarity
Step 1: Count Valence Electrons
- Arsenic (As): Group 15 → 5 valence electrons
- Bromine (Br): Group 17 → 7 valence electrons × 3 Br atoms = 21 valence electrons
Total = 5 (As) + 21 (Br) = 26 valence electrons
Step 2: Draw the Lewis Structure
- Place As in the center (less electronegative).
- Connect three Br atoms with single bonds to As (each bond = 2 electrons × 3 = 6 electrons).
- Distribute the remaining 20 electrons to the outer Br atoms (each Br gets 6 electrons to complete its octet → 18 electrons used).
- Place the remaining 2 electrons (1 lone pair) on As.
Lewis Structure:
..
:
Br–As–Br
|
Br
- Each Br has 3 lone pairs.
- As has 1 lone pair + 3 bonding pairs.
Step 3: VSEPR Shape and Electron Geometry
- Steric number (As) = 3 bonded atoms + 1 lone pair = 4
- Electron geometry = Tetrahedral
- Molecular geometry = Trigonal pyramidal (due to 1 lone pair)
Step 4: Hybridization
- Steric number = 4 → sp³ hybridization
Step 5: Bond Angles
- Ideal tetrahedral angle = 109.5°
- Lone pair repulsion reduces bond angles slightly → actual ~100–101°
Step 6: Polarity
- The molecule is polar:
- As–Br bonds are polar due to electronegativity difference.
- The trigonal pyramidal shape causes asymmetrical charge distribution → net dipole moment ≠ 0.
Summary
| Property | Value |
|---|---|
| Lewis Structure | As center with 3 Br bonds + 1 lone pair |
| Total Valence e⁻ | 26 electrons |
| Electron Geometry | Tetrahedral |
| Molecular Geometry | Trigonal Pyramidal |
| Hybridization | sp³ |
| Bond Angles | ~100–101° |
| Polarity | Polar |
Explanation
Arsenic tribromide (AsBr₃) is a covalent molecule composed of one arsenic atom bonded to three bromine atoms. The first step in determining its structure involves counting the total number of valence electrons. Arsenic contributes five electrons, while each of the three bromine atoms contributes seven, leading to a total of 26 valence electrons. These are then distributed to satisfy the octet rule, resulting in single bonds between As and each Br atom, with lone pairs completing the Br atoms’ octets. One lone pair remains on the arsenic.
Using VSEPR theory, we determine the electron domain geometry by considering both bonded atoms and lone pairs around arsenic. With three bonding domains and one lone pair, the electron geometry is tetrahedral. However, the presence of a lone pair alters the observed (molecular) shape, resulting in a trigonal pyramidal structure. The lone pair-bond pair repulsions slightly compress the bond angles from the ideal tetrahedral angle of 109.5° to about 100–101°.
The hybridization of the central arsenic atom is sp³, reflecting the four electron domains (three bonds + one lone pair). Because the shape is asymmetrical and the As–Br bonds are polar due to the electronegativity difference, the molecule has a net dipole moment, making it polar.
This polarity influences AsBr₃’s physical and chemical behavior, such as solubility and boiling point. Its molecular geometry and lone pair presence make it similar in structure to ammonia (NH₃), although heavier atoms like arsenic and bromine slightly alter bond angles and polar character. Understanding AsBr₃’s shape and electron distribution is essential for predicting reactivity and interactions with other molecules.
