Consider PBr3.
A. How many valence electrons does PBr3 have?
B. Draw the Lewis structure of PBr3.
C. What are the electron group arrangement and molecular shape of PBr3?
D. Are the P-Br bonds polar or nonpolar? (EN of P = 2.1; EN of Br = 2.8)
E. Is the PBr3 molecule polar or nonpolar?
The correct answer and explanation is :
A. How many valence electrons does PBr3 have?
To determine the total number of valence electrons in PBr3, we need to consider the valence electrons of each atom in the molecule:
- Phosphorus (P) is in group 15 of the periodic table, so it has 5 valence electrons.
- Bromine (Br) is in group 17, so each bromine atom has 7 valence electrons.
Since there are three bromine atoms in PBr3, the total number of valence electrons is:
[
5 \, (\text{from P}) + 3 \times 7 \, (\text{from Br}) = 5 + 21 = 26 \, \text{valence electrons}.
]
Thus, PBr3 has 26 valence electrons.
B. Draw the Lewis structure of PBr3:
- Step 1: Determine the central atom. Phosphorus is the least electronegative element and is typically the central atom in its compounds.
- Step 2: Connect the atoms with single bonds. Each phosphorus atom forms single bonds with three bromine atoms.
- Step 3: Distribute the remaining electrons. After forming three P-Br single bonds, there are 26 total valence electrons. Each P-Br bond uses 2 electrons, so that accounts for 6 electrons, leaving 20 electrons.
- Place the remaining electrons as lone pairs on the bromine atoms.
- Each bromine atom will have 6 electrons remaining to be placed as lone pairs, giving each bromine atom a full octet.
- Step 4: Check the octet rule. Phosphorus can expand its valence shell, so it does not need a full octet. Each bromine atom has 8 electrons around it, and phosphorus has 3 bonding pairs and one lone pair, totaling 8 electrons.
The Lewis structure is as follows:
Br
|
Br-P-Br
|
lone pair on PC. What are the electron group arrangement and molecular shape of PBr3?
- Electron Group Arrangement: PBr3 has 3 bonding pairs and 1 lone pair of electrons around phosphorus. This results in a tetrahedral electron group arrangement.
- Molecular Shape: Since the lone pair of electrons on phosphorus affects the shape, the molecular shape is trigonal pyramidal. This is because the lone pair repels the bonding pairs, causing the Br atoms to be pushed down into a pyramidal arrangement.
D. Are the P-Br bonds polar or nonpolar?
The electronegativity of phosphorus (P) is 2.1, and the electronegativity of bromine (Br) is 2.8. The difference in electronegativity is:
[
2.8 – 2.1 = 0.7
]
An electronegativity difference between 0.4 and 1.7 indicates that the bond is polar covalent. Therefore, the P-Br bonds are polar.
E. Is the PBr3 molecule polar or nonpolar?
Despite the fact that the P-Br bonds are polar, the overall polarity of the PBr3 molecule depends on its shape. As mentioned earlier, the molecular shape of PBr3 is trigonal pyramidal due to the lone pair of electrons on phosphorus. This shape causes the dipoles from each P-Br bond to not cancel out. In a trigonal pyramidal structure, the molecule has a net dipole moment because the three polar bonds create a resultant dipole that points in the direction of the lone pair on phosphorus.
Thus, PBr3 is a polar molecule.
Explanation (300 words):
Phosphorus trihalide (PBr3) is a polar molecule due to its molecular shape and the polarity of the individual P-Br bonds. Each P-Br bond is polar because of the difference in electronegativity between phosphorus (2.1) and bromine (2.8). The electronegativity difference of 0.7 causes a dipole moment with bromine being partially negative (δ-) and phosphorus being partially positive (δ+). However, whether the molecule as a whole is polar or nonpolar depends on the molecular shape and the arrangement of these dipoles.
PBr3 has a trigonal pyramidal molecular shape because the central phosphorus atom has three bonding pairs and one lone pair of electrons. This geometry is a result of the tetrahedral electron group arrangement around phosphorus. The lone pair on phosphorus causes a distortion in the shape, pushing the three bromine atoms into a pyramidal structure, which prevents the dipoles from canceling out. In a tetrahedral structure with symmetrical bonds, the dipoles would cancel, resulting in a nonpolar molecule. However, because of the asymmetry created by the lone pair, the individual bond dipoles do not cancel, and there is a net dipole moment pointing in the direction of the lone pair on phosphorus.
Thus, despite having polar P-Br bonds, the overall PBr3 molecule is polar due to its trigonal pyramidal shape, which results in a net dipole moment.