Draw the Lewis structure for AsBr5 and state the parent geometry, molecular geometry, and hybridization.
The Correct Answer and Explanation is:
Lewis Structure of AsBr₅:
- Count valence electrons:
- Arsenic (As) has 5 valence electrons.
- Each bromine (Br) has 7 valence electrons, and there are 5 Br atoms.
- Total valence electrons = 5 (As) + 5 × 7 (Br) = 40 electrons.
- Sketch structure:
- Place As in the center.
- Connect each Br to As with a single bond (5 bonds × 2 electrons = 10 electrons).
- Distribute remaining 30 electrons as lone pairs on Br atoms (6 per Br to satisfy octet).
- Final Structure:
Br | Br - As - Br | Br (All Br atoms have 3 lone pairs each, As has no lone pairs)- As has 5 bonding pairs and no lone pairs → total of 10 bonding electrons (expanded octet).
Answers:
- Parent Geometry: Trigonal bipyramidal
- Molecular Geometry: Trigonal bipyramidal
- Hybridization: sp³d
Explanation
Arsenic pentabromide (AsBr₅) consists of a central arsenic atom bonded to five bromine atoms. To determine its Lewis structure, we start by calculating the total number of valence electrons. Arsenic contributes 5, while each of the five bromines contributes 7, giving a total of 40 electrons. In the Lewis structure, five single bonds are formed between arsenic and each bromine atom, using 10 electrons. The remaining 30 electrons are distributed as lone pairs on the five Br atoms (6 electrons per Br), ensuring each Br achieves a stable octet.
Arsenic, being in period 4 of the periodic table, can expand its octet and accommodate more than 8 electrons. In AsBr₅, arsenic is surrounded by 10 electrons (5 bonding pairs), which is allowed due to available 4d orbitals. Since there are 5 regions of electron density around the central atom and no lone pairs, the electron geometry and the molecular geometry are both trigonal bipyramidal. This geometry places three Br atoms in the equatorial positions (120° apart) and two in the axial positions (180° apart from each other and 90° from equatorials) to minimize electron repulsion according to VSEPR theory.
The hybridization of arsenic in this structure is sp³d, which corresponds to five hybrid orbitals formed by mixing one s, three p, and one d orbital. These orbitals are used to form sigma bonds with the five bromine atoms. There are no lone pairs on the arsenic atom, so all hybrid orbitals are involved in bonding. This molecular arrangement explains the stable and symmetrical trigonal bipyramidal shape observed for AsBr₅.
