Fight Number 6565

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  • Topic: Chemical bond, Atom, Electron
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  • Published : February 19, 2013
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Drawing the Lewis Structure: Here are some simple guidelines for drawing correct Lewis Structures of covalent molecules (and also polyatomic ions).

1. Determine the Number of valence electrons in the Lewis Structure: One of the easiest ways to start drawing a Lewis Structure is to make a list of each atom in the structure and next to each atom record how many valence electrons it has and how many it needs to fill its valence shell. In covalent bonding the valence electrons in the molecule come from the valence electrons in the atoms that formed the bond.

a. The number of valence electrons in any main group atom is equal to its group # (I,II,II,IV,V,VI,VII, VIII or 1,2,3,4,5,6,7,8) b. Compounds and molecules have an overall charge of zero. The number of electrons in the Lewis Structure is the same as the number of valence electrons in the atoms. c. Ions with a negative charge have gained valence electrons. Add one electron for each negative charge to the total. d. Ions with a positive charge have lost valence electrons. Subtract one electron for each positive charge.

2. Determine the number of bonds in the structure. Atoms form covalent bonds to complete their valence shells. It takes two electrons to make a single covalent bond, so the number of bonds can be found by dividing the # electrons needed by 2.

3. Draw simple and symmetrical structures. Nature likes simple and symmetrical arrangements of atoms and bonds. Connect all the atoms with the numbers of bonds you determined in step 2. Hydrogen will always be singly bonded on the outside of the molecule (also called a terminal atom).

4. Fill the valence shells. Covalent bonds are formed so that atoms can fill their valence shells. e. It takes 2 valence electrons to fill the H valence shell. f. Most other main group elements require 8 electrons to fill the valence shell. g. There are some notable main group exceptions to rules 2a and 2b, be careful of compounds containing P and S – these two atoms can often, but not always, “expand” their valence shells to either 10 or 12 electrons. It depends upon the specific molecule.

5. Check your structure
h. The number of valence electrons in the structure must match what you started with i. Each atom’s valence shell is filled.

Example #1, Water, H2O:

1.Determine the Number of valence electrons in the Lewis Structure

Atoms| Valence Electrons| # electrons needed|
H| 1| 1|
H| 1| 1|
O| 6| 2|
totals| 8| 4|

2.Determine the number of bonds in the structure.
# of bonds in H2O = 4 electrons needed / 2 electrons per bond = 2 bonds

3.Draw simple and symmetrical structures. The possible arrangements of 3 atoms and 2 bonds for H2O are

Hydrogens can only be terminal atoms not central atoms in a structure. Also, H-O-H is the most symmetrical, so use H-O-H.

4.Fill the valence shells. Of the 8 valence electrons we started with, 4 electrons were used in making the 2 bonds. That leaves 4 electrons to fill the valence shells. Each of the hydrogens has 2 electrons in their valence shell (1 bond = 2 electrons), and is filled. The 4 remaining electrons are placed around the oxygen atom as 2 pairs of non-bonding electrons (also known as lone pairs)

5.Check your structure
a.2 bonds + 2 lone pairs = 8 electrons used. Correct.
b.Hydrogen has two valence electrons around it. Oxygen has 8. Correct.

Example #2, Formaldehyde, CH2O:

1.Determine the Number of valence electrons in the Lewis Structure

Atoms| Valence Electrons| # electrons needed|
C| 4| 4|
O| 6| 2|
H| 1| 1|
H| 1| 1|
totals| 12| 8|

2.Determine the number of bonds in the structure.
# bonds in CH2O = 8 electrons needed / 2 electrons per bond = 4 bonds

3.Draw simple and symmetrical structures. Possible arrangements are

All the...
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