Question

Use formal charge to choose the best Lewis structure for CH₃SeOCH₃.

Answer 1

To write the Lewis structures for molecular compounds one must follow the following steps.

(1) Correct skeleton structure of the compound should be written first.

(2) After writing the skeleton structure, total number of electrons should be calculated by adding the valence electrons of each atom in compound.

(3) Electrons must be distributed in such a way that all the atoms must have complete octets.

(4) If any atom lacks an octet, form double or triple bonds to give them octets.

And,

To determine the best Lewis structure using formal charges, following rules must be followed:

(1) In a neutral molecule, sum of all formal charges must be zero.

(2) In an ion, sum of all formal charges must be equal the charge of the ion.

Formal charge on an atom is determined by following formula:

…… (1)

(a)

For the given structure, calculate the formal charge on each atom using equation (1), and according to rule for correct Lewis structure sum of all the formal charges must be zero.

For all Hydrogen atoms, substitute 1 for number of valence electrons, zero for number of non-bonding electrons and 2 for number of bonding electrons.

Thus,

For first Carbon atom, substitute 4 for number of valence electrons, zero for number of non-bonding electrons and 10 for number of bonding electrons.

Thus,

For second Carbon atom, substitute 4 for number of valence electrons, zero for number of non-bonding electrons and 8 for number of bonding electrons.

Thus,

For Oxygen atom, substitute 6 for number of valence electrons, 6 for number of non-bonding electrons and 4 for number of bonding electrons.

Thus,

And, for Sulphur atom, substitute 6 for number of valence electrons, zero for number of non-bonding electrons and 10 for number of bonding electrons.

Thus,

Now, calculate sum of all formal charges as follows:

Therefore, this is not correct Lewis structure for .

(b)

For the given structure, calculate the formal charge on each atom suing equation (1), and according to rule for correct Lewis structure sum of all the formal charges must be zero.

For all Hydrogen atoms, substitute 1 for number of valence electrons, zero for number of non-bonding electrons and 2 for number of bonding electrons.

Thus,

For both Carbon atoms, substitute 4 for number of valence electrons, zero for number of non-bonding electrons and 8 for number of bonding electrons.

Thus,

For Oxygen atom, substitute 6 for number of valence electrons, 4 for number of non-bonding electrons and 4 for number of bonding electrons.

Thus,

And, for Sulphur atom, substitute 6 for number of valence electrons, 2 for number of non-bonding electrons and 8 for number of bonding electrons.

Thus,

Thus, total formal charge is also zero.

Therefore, this is correct Lewis structure for .

(c)

This given structure is not correct because oxygen atom must be attached to two other atoms by single bond and if it is attached to only one atom then the bond must be a double bond.

Here, oxygen is attached to Sulphur by a single bond.

Therefore, this is not correct Lewis structure for .

(d)

This given structure is not correct because oxygen atom must be attached to two other atoms by single bond and if it is attached to only one atom then the bond must be a double bond.

Here, oxygen is attached to Sulphur by a single bond. Also, one of the carbon atom has 5 bonds but carbon can only form 4 bonds.

Therefore, this is not correct Lewis structure for .

Therefore, correct option is .

Answer 2