Formal Charge and Lewis Formulas

EXERCISE 9.14 Beryllium chloride, BeCl2, is a solid substance consisting of long (essentially infinite) chains of atoms with Cl atoms in bridge positions. 

However, if the solid is heated, it forms a vapor of BeCl2 molecules. Write the electron-dot formnla of the BeCl2 molecule. 

The rules for computing oxidation number are similar, except that both bonding electrons are assigned to the more electronegative atom. 

Earlier we looked at writing the Lewis formula for carbonyl chloride, COCI2 (Example 9.7). In the solntion to that example, we wrote the answer as follows  

You may recall that we had to decide whether to draw the double bond between C and O or between C and Cl. We decided in favor of a carbon—oxygen double bond by applying the simple idea that multiple bonds most likely involve C, N, O, and S atoms. In this section, we will describe how you can use the concept of formal charge to determine the best Lewis formula. Formal charge is also a help in writing the skeleton structure of a molecule. 

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Lewis structures and any resonance structures for the ion with formula CH2NO2. Show all formal charges and lone pairs of electrons. 

Formal charge and oxidation number, (a) Write a Lewis formula for hydiazoic acid, HN3, whose skeleton is H—N—N—N. Make certain the structure obeys the octet rule, (b) Locate all formal charges, (c) Calculate the oxidation number for each atom, (d) Account for the ct that, for the middle N atom, the oxidation number and formal charge are identical. 

Thus, (4.9a) gives the general condition for an idealized covalently bonded closed-shell Lewis-like duodectet structure (4.9b) with no formal charge on the central metal atom. (The more general conditions for coordinative [dative] ligands and other departures from the idealized Lewis-like formula will be discussed below.)... 

The curious reversal from the C+0 polarity expected on electronegativity grounds may be attributed to the formal charges (C and 0+) that are associated with the usual triple-bonded Lewis-structure formula. 

Now we will apply this formal-charge concept to the cyanate ion OCN We chose this example because many students incorrectly write the formula as CNO , and then try to use this as the atomic arrangement in the Lewis structure. Based on the number of electrons needed, the carbon should be the central atom. We will work this example using both the incorrect atom arrangement and the correct atom arrangement. Notice that in both structures all atoms have a complete octet. 

Q Draw and interpret Lewis, condensed, and line-angle structural formulas. Calculate formal charges. 

To calculate formal charges, use the completed Lewis structure and the following formula ... 

Given a molecular formula, draw a Lewis diagram for the molecule and assign formal charge (Section 3.8, Problems 39-50). 

Let us now write some Lewis formulas for, and assign formal charges to, the atoms in nitrosyl chloride, NOCl, a compound often used in organic synthesis. The Cl atom and the O atom are both bonded to the N atom. Two Lewis formulas that satisfy the octet rule are... 

Write Lewis formulas for three different atomic arrangements with the molecular formula HCNO. Indicate aU formal charges. Predict which arrangement is likely to be the least stable and justify your selection. 

A stepwise process is used to convert a molecular formula into a Lewis structure, a two-dimensional representation of a molecule (or ion) that shows the relative placement of atoms and distribution of valence electrons among bonding and lone pairs. When two or more Lewis structures can be drawn for the same relative placement of atoms, the actual structure is a hybrid of those resonance forms. Formal charges are often useful for determining the most important contributor to the hybrid. Electron-deficient molecules (central Be or B) and odd-electron species (free radicals) have less than an octet around the central atom but often attain an octet in reactions. In a molecule (or ion) with a central atom from Period 3 or higher, the atom can hold more than eight electrons by using d orbitals to expand its valence shell. 

Complete the following Lewis structures. Indicate any unshared pairs of electrons and formal charges. The formulas represent neutral molecules. 

Lewis dot formulas are the major means by which structural information is communicated in organic chemistry. These structural formulas show the atoms, bonds, location of unshared pairs, and formal charges. 

A new formula should be drawn to show the result of the electron shift(s). All formulas should be proper Lewis structures and should include formal charges as appropriate. The maximum number of valence electrons should not be exceeded for any atom in a formula. 

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