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By the end of this section, you will be able to:
  • Compute formal charges for atoms in any Lewis structure
  • Use formal charges to identify the most reasonable Lewis structure for a given molecule
  • Explain the concept of resonance and draw Lewis structures representing resonance forms for a given molecule

In the previous section, we discussed how to write Lewis structures for molecules and polyatomic ions. As we have seen, however, in some cases, there is seemingly more than one valid structure for a molecule. We can use the concept of formal charges to help us predict the most appropriate Lewis structure when more than one is reasonable.

Calculating formal charge

The formal charge    of an atom in a molecule is the hypothetical charge the atom would have if we could redistribute the electrons in the bonds evenly between the atoms. Another way of saying this is that formal charge results when we take the number of valence electrons of a neutral atom, subtract the nonbonding electrons, and then subtract the number of bonds connected to that atom in the Lewis structure.

Thus, we calculate formal charge as follows:

formal charge = # valence shell electrons (free atom) # lone pair electrons 1 2 # bonding electrons

We can double-check formal charge calculations by determining the sum of the formal charges for the whole structure. The sum of the formal charges of all atoms in a molecule must be zero; the sum of the formal charges in an ion should equal the charge of the ion.

We must remember that the formal charge calculated for an atom is not the actual charge of the atom in the molecule. Formal charge is only a useful bookkeeping procedure; it does not indicate the presence of actual charges.

Calculating formal charge from lewis structures

Assign formal charges to each atom in the interhalogen ion ICl 4 .

Solution

  1. We divide the bonding electron pairs equally for all I–Cl bonds:
    A Lewis structure is shown. An iodine atom with two lone pairs of electrons is single bonded to four chlorine atoms, each of which has three lone pairs of electrons. Brackets surround the structure and there is a superscripted negative sign.
  2. We assign lone pairs of electrons to their atoms . Each Cl atom now has seven electrons assigned to it, and the I atom has eight.
  3. Subtract this number from the number of valence electrons for the neutral atom:
    I: 7 – 8 = –1
    Cl: 7 – 7 = 0
    The sum of the formal charges of all the atoms equals –1, which is identical to the charge of the ion (–1).

Check your learning

Calculate the formal charge for each atom in the carbon monoxide molecule:

A Lewis structure is shown. A carbon atom with one lone pair of electrons is triple bonded to an oxygen with one lone pair of electrons.

Answer:

C −1, O +1

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Calculating formal charge from lewis structures

Assign formal charges to each atom in the interhalogen molecule BrCl 3 .

Solution

  1. Assign one of the electrons in each Br–Cl bond to the Br atom and one to the Cl atom in that bond:
    A Lewis structure is shown. A bromine atom with two lone pairs of electrons is single bonded to three chlorine atoms, each of which has three lone pairs of electrons.
  2. Assign the lone pairs to their atom. Now each Cl atom has seven electrons and the Br atom has seven electrons.
  3. Subtract this number from the number of valence electrons for the neutral atom. This gives the formal charge:
    Br: 7 – 7 = 0
    Cl: 7 – 7 = 0
    All atoms in BrCl 3 have a formal charge of zero, and the sum of the formal charges totals zero, as it must in a neutral molecule.

Check your learning

Determine the formal charge for each atom in NCl 3 .

Answer:

N: 0; all three Cl atoms: 0

A Lewis structure is shown. A nitrogen atom with one lone pair of electrons is single bonded to three chlorine atoms, each of which has three lone pairs of electrons.
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Questions & Answers

How does an element differ from a compound? How are they similar?
Adeola Reply
an element is an indivisible particles that can take part in a reaction and consist of smaller or tiny particles i.e proton, neutrons and electron while a compound is when two or more element chemically combine together. They are similar when they are homogeneous compound. they take the same rxn.
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How to get the Lewis formula of SeCl+3
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Starch is a mixture (of chemicals) of amylose and amylopectin. Both are macromolecules and polymers. You can search on wikipedia.
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Roles can be classified or correlate it to different areas: For example: Chlorine can be used in reactions (in industry) to manufacture HCl, which then can be used for other things. Or in swimming pools to kill bacteria. Or as a component in compounds with pharmaceutical roles (drugs). For Al:
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Lewis structure for no3
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It is an acid which partially ionises in water.
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occurrence and preparation of the representatives metals
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list the 20, periodic table and their symbols
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hydrogen:h helium;he lithium:l beryllium:be Boron:b Carbon;C Nitrogen:n Oxygen:O FLUORINE:f Neon:n Sodium:s Magnesium:mg Aluminum:a Silicon:s Phosphorus:p Sulphur:s Chlorine:c Argon;a Potassium:p Calcium:c
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Hydrogen, helium, lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine, neon, sodium, magnesium, aluminium, silicon, phosphorus, sulphur, chlorine, argon, potassium, calcium
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is the substance that dissolves in the solvent
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No, covalent compound ➡️ molecule. As both H and Cl are non-metals and and form covalent bind by sharing valence e-. But can fully ionice in water forming H+ (a proton, a reason for acidity) and Cl- (anion =Chloride) Hydrogen Chloride is a gas at room; Hydrochloric acid = HCl (aq), dissolved in w
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The question marks are an emoji in the first sentence is an unread emoji. HCl Covalent compund -> molecule
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is the study of composition of substances and the way they behave under different conditions
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Intermolecular forces exist between molecules of different units like van der waal force, hydrogen bonds
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scientific study of structure of substances and of the way that they react with other substances
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Practice Key Terms 5

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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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