Why is there a negative charge on the oxygen atom with the single bond?

[Indranil]

My question is that why is the negative charge on the oxygen atom with the single bond, why not on the oxygen atom with the double bond? I am confused.
Source: https://chem-net.blogspot.in/2012/01/simple-procedure-for-writing-lewis.html

 

[BvU]

Count the electrons ... (use factor 0.5 for shared electrons)

 

[Indranil]

Count the electrons ... (use factor 0.5 for shared electrons)

I don't understand what you mean. Could you make your point easier, please? Could you tell me the method, please?

 

[Borek]

For each atom count electrons "belonging" to that atom, if an electron is shared by two atoms, count it as half.

 

[Indranil]

For each atom count electrons "belonging" to that atom, if an electron is shared by two atoms, count it as half.

Still, I don't understand how to count. Could you clarify it, please?

 

[Borek]

Each dot is a single electron, each bar is a bond - so two shared electrons.

How many dots on the left atom? How many shared electrons on the left atom? Just add.

 

[Indranil]

Each dot is a single electron, each bar is a bond - so two shared electrons.

How many dots on the left atom? How many shared electrons on the left atom? Just add.

If I count from the left atom with the double bond, 4 dots, and two shared electrons, total electrons are 8. now what to do next, sir?

 

[Borek]

4 dots, and two shared electrons, total electrons are 8

4+2 is not 8.

How many electrons should be present on the valence shell in a neutral, isolated oxygen atom?

 

[Indranil]

It should be 6 electrons. Now what to do next, sir?

 

[Borek]

Don't "sir" me.

As I wrote - your calculations are off, correct them. And use the same approach to calculate number of electrons on other oxygen atoms.

 

[Indranil]

Don't "sir" me.

As I wrote - your calculations are off, correct them. And use the same approach to calculate number of electrons on other oxygen atoms.

Still, I am confused. Could you calculate, please?

 

[Borek]

I am not going to spoon feed you, sorry. You will either try, or you will not learn anything.

 

[Indranil]

I am not going to spoon feed you, sorry. You will either try, or you will not learn anything.

How to try. could you give me some formula?.

 

[Borek]

What formula do you need to count the dots on the picture?

 

[Indranil]

What formula do you need to count the dots on the picture?

Formal charge.

 

[Borek]

I have no idea what you mean now.

You were told to count electrons, you even did it - you just made a trivial mistake when adding two numbers. Please reread the thread, you were told what to do several times. If you know what the dots and bars on the resonance structure are the task is trivial.