Alternative strategies to the one used in this chapter have been proposed for applying the VSEPR theory to molecules or ions with a single central atom. In general, these strategies do not require writing Lewis structures. In one strategy, we write (1) the total number of electron pairs \(=[\) (number of valence electrons) \(\pm\) (electrons required for ionic charge) \(] / 2\) (2) the number of bonding electron pairs \(=\) (number of atoms) -1 (3) the number of electron pairs around central atom \(=\) total number of electron pairs \(-3 \times[\) number of terminal atoms (excluding \(\mathrm{H}\) )] (4) the number of lone-pair electrons = number of central atom pairs - number of bonding pairs After evaluating items \(2,3,\) and \(4,\) establish the VSEPR notation and determine the molecular shape. Use this method to predict the geometrical shapes of the following: (a) \(\mathrm{PCl}_{5} ;\) (b) \(\mathrm{NH}_{3} ;\) (c) \(\mathrm{ClF}_{3} ;\) (d) \(\mathrm{SO}_{2} ;\) (e) \(\mathrm{ClF}_{4}^{-}\); (f) \(\mathrm{PCl}_{4}^{+}\). Justify each of the steps in the strategy, and explain why it yields the same results as the VSEPR method based on Lewis structures. How does the strategy deal with multiple bonds?

Step by step solution

01

Calculation for PCl5

Use the given formulas to calculate the number of total electron pairs, bonding electron pairs, electron pairs around the central atom, and lone-pair electrons. For \(PCl_5\), the number of valence electrons on P is 5 and Cl contributes 5 electrons, so total electron pairs = (5 + (5*7))/2 = 20. The number of atoms is 6, so bonding pairs = 6-1 = 5. Therefore, there are no lone pair electrons. The shape is determined by 5 regions of electron density which implies in a trigonal bipyramidal shape.

02

Calculation for NH3

Repeat the process with \(NH_3\). Total pairs = (5 + (3*1))/2 = 4, bonding pairs = 4 - 1 = 3. Thus, there is one lone pair electron. This molecule has 4 electron density regions, yielding a tetrahedral electronic geometry, however due to the presence of lone pair electrons, molecular geometry is trigonal pyramidal.

03

Calculation for the Remaining Molecules/Ions

Repeat the process for the remaining molecules/ions \(\mathrm{ClF}_{3}\), \(\mathrm{SO}_{2}\), \(\mathrm{ClF}_{4}^{-}\), and \(\mathrm{PCl}_{4}^{+}\) by substituting the appropriate number of valence electrons and atoms into the formulas.

04

Comparison with Lewis structures

Compare the result obtained through this method with that obtained from Lewis Structure to confirm its validity. For example, both NH3 and SO2, with calculated lone pair electrons, will have trigonal pyramidal and V-shape (or bent) geometry, respectively, which corresponds to the Lewis structures.

05

Dealing with multiple bonds

In a molecule with a multiple bond, both the actual bond and implied bond pairs are considered to be a single region by both this strategy and VSEPR. Hence, the molecular geometry of a molecule such as SO2 with multiple bonds (double bonds in this case), is correctly predicted to be V-shaped.

Unlock Step-by-Step Solutions & Ace Your Exams!

• Full Textbook Solutions

  Get detailed explanations and key concepts

• Unlimited Al creation

  Al flashcards, explanations, exams and more...

• Ads-free access

  To over 500 millions flashcards

• Money-back guarantee

  We refund you if you fail your exam.

Start your free trial

Over 30 million students worldwide already upgrade their learning with Vaia!