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Chapter 8: Problem 50

(a) Write a Lewis structure for the phosphorus trifluoride molecule, \(\mathrm{PF}_{3}\). Is the octet rule satisfied for all the atoms in your structure? (b) Determine the oxidation numbers of the \(\mathrm{P}\) and \(\mathrm{F}\) atoms. (c) Determine the formal charges of the \(\mathrm{P}\) and \(\mathrm{F}\) atoms. (d) Is the oxidation number for the \(\mathrm{P}\) atom the same as its formal charge? Explain.

Short Answer

Expert verified
The Lewis structure of PF3 is as follows: ``` F | P - F - F || ``` All atoms satisfy the octet rule. The oxidation numbers of P and F atoms are +3 and -1, respectively. The formal charges of P and F atoms are both 0. The oxidation number of P (+3) is not the same as its formal charge (0) due to different considerations regarding electron distribution and electronegativity.

Step by step solution

01

Draw the Lewis Structure of PF3

To draw the Lewis structure, first, note the number of valence electrons for each atom. Phosphorus (P) has 5 valence electrons, and Fluorine (F) has 7 valence electrons. The molecule has 3 fluorine atoms, so the total number of valence electrons is 5 + 3*(7) = 26. Now, place the P atom in the center and connect it to each of the F atoms with a single bond. By doing this, we have used 6 valence electrons. Next, distribute the remaining 20 valence electrons around the F atoms, so each F atom has 8 electrons (including the shared electrons in the bond). The final Lewis Structure is: ``` F | P - F - F || ```
02

Check Octet Rule

All the F atoms have 8 electrons (one bond and 6 lone pairs), satisfying the octet rule. The P atom has 8 electrons (three bonds and one lone pair), also satisfying the octet rule. Thus, the octet rule is satisfied for all the atoms in the PF3 molecule. #b) Finding Oxidation Numbers of P and F atoms#
03

Oxidation numbers

To determine the oxidation numbers of P and F atoms, assign electrons to the atoms according to a set of rules: 1. A fluorine atom has an oxidation number of -1. 2. Covalent bonds' shared electrons are assigned to the more electronegative atom. Phosphorus is less electronegative than fluorine, so all electrons in the P-F bonds are assigned to fluorine. Thus, each F atom has an oxidation number of -1, and the P atom has an oxidation number of +3 (as there are three F atoms). #c) Finding Formal Charges of P and F atoms#
04

Formal Charges

To determine the formal charges of P and F atoms, use the formula: Formal charge = (# of valence electrons) - (# of lone pairs) - 0.5*(# of bonding electrons) For P atom: Formal Charge(P) = 5 - 2 - 0.5*(6) = 0 For F atoms: Formal Charge(F) = 7 - 6 - 0.5*(2) = 0 Both P and F have a formal charge of 0. #d) Comparing Oxidation Number and Formal Charges of P Atom#
05

Oxidation Number vs Formal Charge

The oxidation number of P is +3, whereas its formal charge is 0. These two values are not the same. The oxidation number is related to the electronegativity differences between atoms in a compound and helps to identify electron transfer in redox reactions. On the other hand, the formal charge is related to the electron distribution within a molecule and helps to predict the most stable Lewis structure. In this case, the formal charge shows that the drawn Lewis structure follows the octet rule and is stable, but the oxidation number indicates that the phosphorus atom loses 3 electrons to the three fluorine atoms.

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