Writing Lewis Structures: More Obeying The Octet Rule Answers


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Example #1 - ammonium ion NH4+

1. This compound is covalent.

2. Determine the total number of valence electrons available:

One nitrogen has 5 valence electrons
Four hydrogen, each with one valence electron, totals 4
The positive charge means one electron has been removed
This means there are 9 minus one = 8 valence electrons, making 4 pairs, available.

3. Organize the atoms so there is a central atom (usually the least electronegative) surrounded by ligand (outer) atoms. Hydrogen is never the central atom.

4. Determine a provisional electron distribution by arranging the electron pairs (E.P.) until all available pairs have been distributed:

5. The formal charge (F) on the central atom is zero. The structure in step 4 is the correct answer. Note the use of brackets. This indicates that the positive charge is distributed over the molecule and is not part of any given atom.


Example #2 - hydronium ion H3O+

1. This compound is covalent.

2. Determine the total number of valence electrons available:

One oxygen has 6 valence electrons
Three hydrogen, each with one valence electron, totals 3
The positive charge means one electron has been removed
This means there are 9 minus one = 8 valence electrons, making 4 pairs, available.

3. Organize the atoms so there is a central atom (usually the least electronegative) surrounded by ligand (outer) atoms. Hydrogen is never the central atom.

It does not matter which of the three sides you use to put hydrogens on.

4. Determine a provisional electron distribution by arranging the electron pairs (E.P.) until all available pairs have been distributed:

5. The formal charge (F) on the central atom is zero. The structure in step 4 is the correct answer.

Note the use of brackets.


Example #3 - nitrogen trifluoride NF3

1. This compound is covalent.

2. Determine the total number of valence electrons available:

One nitrogen has 5 valence electrons
Three fluorine, each with 7 valence electron, totals 21
This means there are 26 valence electrons, making 13 pairs, available.

3. Organize the atoms so there is a central atom (usually the least electronegative) surrounded by ligand (outer) atoms. Hydrogen is never the central atom.

It does matter which of the two sides you use to put hydrogens on. Use sides that are next to each other. DO NOT put the hydrogens 180 degrees apart. There is a reason for this you'll learn later.

4. Determine a provisional electron distribution by arranging the electron pairs (E.P.) until all available pairs have been distributed:

5. The formal charge (F) on the central atom is zero. The structure on the right in step 4 is the correct answer.


Example #4 - aluminum tetrachloride ion AlCl4¯

1. This compound is covalent.

2. Determine the total number of valence electrons available:

One aluminum has 3 valence electrons
Four chlorine, each with 7 valence electrons, totals 28
One negative charge means one electron This means there are 3 + 28 + 1 = 32 valence electrons, making 16 pairs, available.

3. Organize the atoms so there is a central atom (usually the least electronegative) surrounded by ligand (outer) atoms. Hydrogen is never the central atom.

4. Determine a provisional electron distribution by arranging the electron pairs (E.P.) until all available pairs have been distributed:

5. The formal charge (F) on the central atom is zero. The right-hand structure in step 4 is the correct answer.


Example #5 - Carbon dioxide CO2

A tutorial on multiple bonds & Lewis structures is in the works.

Go to the tutorial on expanded and deficient octets.

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