What is the Lewis structure for IBr3, with the central atom of I. Is the molecular polar or nonpolar? Identify the intermolecular forces present?
The correct answer and explanation is:
Sure! Here’s a detailed explanation of the Lewis structure of IBr₃, its polarity, and the intermolecular forces involved:
Lewis Structure of IBr₃ (Iodine Tribromide)
- Central Atom: Iodine (I)
Iodine is less electronegative than bromine, so it will be the central atom. - Valence Electrons:
- Iodine (I): 7 valence electrons
- Each Bromine (Br): 7 valence electrons × 3 = 21 valence electrons
Total valence electrons: 7 + 21 = 28 electrons
- Bond Formation:
- Connect iodine to each bromine with a single bond: 3 bonds × 2 electrons = 6 electrons used
- Remaining electrons: 28 – 6 = 22 electrons
- Distribute Remaining Electrons:
- Place lone pairs on bromine atoms to complete their octets. Each bromine needs 3 lone pairs (6 electrons).
- 3 bromines × 6 electrons = 18 electrons assigned to Br atoms.
- Leftover Electrons:
- 22 – 18 = 4 electrons remain.
- Place leftover electrons on central iodine:
- Iodine gets 2 lone pairs (4 electrons).
- Formal Charges:
- All atoms have formal charges close to zero, structure is stable.
Shape and Polarity
- Electron geometry: Trigonal bipyramidal (based on 5 electron pairs: 3 bonding pairs + 2 lone pairs)
- Molecular geometry: T-shaped (because the two lone pairs occupy equatorial positions to minimize repulsion, leaving three bonded atoms in a T shape)
- Polarity:
The T-shaped molecular geometry is asymmetrical. The lone pairs cause an uneven distribution of electron density. Bromine atoms are more electronegative than iodine, pulling electron density toward themselves, but because of the shape, the dipole moments do not cancel out.
Therefore, IBr₃ is a polar molecule.
Intermolecular Forces in IBr₃
- London Dispersion Forces:
All molecules exhibit London dispersion forces (induced dipole-induced dipole) due to temporary fluctuations in electron distribution. Because IBr₃ is a relatively large molecule with many electrons, these forces are significant. - Dipole-Dipole Interactions:
Since IBr₃ is polar, molecules will have permanent dipoles. These permanent dipoles attract each other, creating dipole-dipole intermolecular forces. - Hydrogen Bonding:
Not present because IBr₃ does not contain hydrogen atoms bonded to highly electronegative atoms (N, O, or F).
Summary
| Aspect | Description |
|---|---|
| Lewis Structure | I central atom bonded to 3 Br atoms, 2 lone pairs on I |
| Molecular Shape | T-shaped |
| Polarity | Polar |
| Intermolecular Forces | London dispersion + Dipole-dipole |
300-Word Explanation
Iodine tribromide (IBr₃) consists of one iodine atom bonded to three bromine atoms. To draw its Lewis structure, iodine serves as the central atom due to its lower electronegativity compared to bromine. The total valence electrons available are 28, calculated by adding 7 from iodine and 21 from the three bromine atoms. Single bonds connect iodine to each bromine, using 6 electrons, leaving 22 electrons to distribute. Each bromine atom completes its octet by receiving three lone pairs (18 electrons), and the remaining 4 electrons form two lone pairs on iodine.
The molecular shape of IBr₃ is determined by the arrangement of electron pairs around iodine. With five electron pairs (three bonding pairs and two lone pairs), the electron geometry is trigonal bipyramidal. The two lone pairs occupy equatorial positions to minimize repulsions, leaving a T-shaped molecular geometry. This shape is asymmetrical, causing an uneven distribution of electron density.
Since bromine is more electronegative than iodine, the bonding electrons are pulled closer to bromine atoms. However, the lone pairs on iodine disrupt symmetry and prevent dipole cancellation, rendering the molecule polar. This polarity has implications for the types of intermolecular forces present.
IBr₃ exhibits London dispersion forces, the weakest intermolecular forces that arise from temporary fluctuations in electron density and are significant due to the large electron cloud of iodine and bromine. Additionally, because the molecule is polar, it also experiences dipole-dipole interactions—forces between permanent dipoles in neighboring molecules. However, IBr₃ does not engage in hydrogen bonding since it lacks hydrogen atoms bonded to electronegative atoms like nitrogen, oxygen, or fluorine.
In summary, the polar, T-shaped IBr₃ molecule features both London dispersion and dipole-dipole intermolecular forces, influencing its physical properties such as boiling point and solubility.