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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

what are they for
Yakubb Reply
hydrogen is a monovalent why aluminum is a divalent
Naldo Reply
Did you mean trivalent? This is because Aluminium has three electroms orbiting in the valence (furthest shell) which are involved in covalent bonding where each electron becomes a pair with another one in the other non metal and make a single bond. N. B: each single covalent bond contains 2 electr
Abdelkarim
Electrons, one from each atom. Did you know that god said '' And, [O Muhammad], you are not [engaged] in any matter or recite any of the Qur'an and you [people] do not do any deed except that We are witness over you when you are involved in it. And not absent from your Lord is any [part] of an atom
Abdelkarim
... atom's weight within the earth or within the heaven or [anything] smaller than that or greater but that it is in a clear register. ''
Abdelkarim
wat gives perfume it's sweet smell
Olaobaju Reply
These are organic compounds, which have plentiful functional groups which react with certain substances through a pathway in the cells lining nostrils which sends impulses that make you sense its sweet. Did you prophet Muhammad (peace upon him) encourages the use of perfume.
Abdelkarim
what is it use for
amagai
Prophet Muhammad (peace be upon him) used to love cleanliness and good scent. ... It was highlighted, in many hadiths, his love for fragrance and good scent like musk, 'ud and ambergris.
Abdelkarim
the types of giant covalent structure
Nobert Reply
define the following terms. electrovalent bonding
Nobert
 An unknown noble gas was allowed to flow into a 300.0 mL glass bulb until the P = 685 torr. Initially, the glass bulb weighed 32.50 g, but now it weighs 33.94 g. If the temperature is 27.0 °C, what’s the identity of the gas?
Roseline Reply
* Use PV=nRT with correct units to find n (number of moles) * Use n = mass/ Ar (Ar is relative atomic mass) Ar = 131.15 -> Xenon N. B: P is in pascals, V in m3, n in mol, R in J/ k. Mol, T in kelvin *Thank God
Abdelkarim
N. B: As it is a noble gas it is mono-atomic so the Ar does not need to be divided by two (not diatomic).
Abdelkarim
@Abdelkarim... good!
Jag
Lovely, please pray to god and read about islam which will help you and anyone to stop alcoholic drinks gradually. LOVE
Abdelkarim
what is molecule
Olom Reply
molecules are produced by double atom from example this is hydrogen atom and this is hydrogen2 are call that hydrogen molecules or gass
Naldo
What is the generic name for the compound
Orisanmi Reply
what is the formular for methane
Tamaranimiweremi Reply
CH4 , it is the simplest alkane
Abdelkarim
what is the formula for alkaline
Olom
hi
Yusuf
I wish to learn to more of chemistry, can someone please teach me.
Yusuf
@Olom Alkaline doesn't have a general formula.
Jag
what is zero gravity
Blessing Reply
every object is that zero gravity
Rabiu
Probably when an object is in space and there are no nearby masses that pull her, and exert gravity
Abdelkarim
Alright. .good job
Rabiu
And all majesty to God, (وَهُوَ ٱلَّذِی خَلَقَ ٱلَّیۡلَ وَٱلنَّهَارَ وَٱلشَّمۡسَ وَٱلۡقَمَرَۖ كُلࣱّ فِی فَلَكࣲ یَسۡبَحُونَ) [سورة الأنبياء 33 And it is He who created the night and the day and the sun and the moon; all [heavenly bodies] in an orbit are swimming. General theory of relativity in Qur
Abdelkarim
what is molecule?
Olom
what is lattice energy
Getrude Reply
why is CO a neutral oxide and CO2 an acidic oxide
Emmanuel Reply
Because when CO2 dissolves in water forming a weak acid. CO does not dissolve in water as it has strong triple bond.
Abdelkarim
What is acid
Progress Reply
which donate H+ or accept lone pair of electron
Kajal
kinetic theory of matter and gas law
Victoria Reply
hi
Victoria
pls explain
Victoria
what is clay
Thankgod Reply
material containing clay minerals. Clays develop plasticity when wet, due to a molecular film of water surrounding the clay particles, but become hard, brittle and non–plastic upon drying or firing. Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours
Abdelkarim
due iron oxide. The four types of clay are Earthenware clay, Stoneware clay, Ball clay, and Porcelain. All of them can be used to make pottery, but the end result would differ a lot thanks to their different textures, colors, and flexibilities.
Abdelkarim
And do you know that god has created human from clay (وَلَقَدۡ خَلَقۡنَا ٱلۡإِنسَـٰنَ مِن صَلۡصَـٰلࣲ مِّنۡ حَمَإࣲ مَّسۡنُونࣲ) [سورة الحجر 26] And We did certainly create man out of clay from an altered black mud. You can install Quran from paly store for free with translations.
Abdelkarim
darw a periodic table
Hazard Reply
draw a periodic table
Hazard
You will arrange the elements into row and coloumns according to increasing proton number. You may want to use symbols or their names. Hydrogen, Helium, etc. God has created all these elements from nothing, in Islam we know God is the creator.
Abdelkarim
why are you drawing a periodic table? why not just print one from the internet and use as a reference
Jakhari
Great thought
Bright
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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