## How to know where a double bond should go?

$FC=V-(L+\frac{S}{2})$

ZainAlrawi_1J
Posts: 71
Joined: Sat Aug 24, 2019 12:16 am

### How to know where a double bond should go?

As in part (c) of question 2C.5, the double bond is drawn between the O and N atoms, why is this the case? Why isn't it drawn between the Cl and O atoms?

sarahsalama2E
Posts: 164
Joined: Fri Aug 30, 2019 12:16 am

### Re: How to know where a double bond should go?

You have to check that the double bond wouldn’t violate a particular atoms octet rule, also you should draw the double bond so that it stabilizes the molecule the most and reduces the formal charge as much as possible.

Robert Cross 1A
Posts: 35
Joined: Wed Sep 11, 2019 12:15 am

### Re: How to know where a double bond should go?

Cl generally does not want to form double bonds with C because Cl has 7 valence electrons and just needs one additional bond to attain an octet

Brian J Cheng 1I
Posts: 115
Joined: Thu Jul 11, 2019 12:15 am

### Re: How to know where a double bond should go?

Check the atoms' octets as well as the formal charge. More electronegative atoms would prefer negative formal charges and thus form more stable structures if this is accomplished via bond rearrangement.

ZainAlrawi_1J
Posts: 71
Joined: Sat Aug 24, 2019 12:16 am

### Re: How to know where a double bond should go?

Just sorted it out by doing the formal charges for the atoms in both possible structures, unfortunately had to the formal charges for all the oxygen atoms to get a clear answer but its definitely obvious now. Thanks everyone!

VPatankar_2L
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### Re: How to know where a double bond should go?

If the central atom is in row one and row two, with the exception of the incomplete octet elements, you should calculate the number of valence electrons of the structure and then divide by two to find the number of electron pairs. When you add all of the atoms around the central atom and account for lone pairs, you will realize that the molecule has a greater number of pairs when the atoms are singly bonded, than you had accounted for when you did your valence electron calculation. Then, you can turn one of these single bonds into a double bond to reduce the number of pairs and match your previous calculation.

For expanded octet, you need to figure out if adding the double bond will lead to a lower formal charge on the atoms. The placement depends on which of the bound atoms has a higher electronegativity.

Aprice_1J
Posts: 55
Joined: Wed Sep 18, 2019 12:16 am

### Re: How to know where a double bond should go?

Something that helped me to figure out where different types of bonds should go besides just guessing was to always start by giving oxygen two lone pairs and a double bond. It might not always work out but knowing this made it a lot faster for me.

JChen_2I
Posts: 107
Joined: Fri Aug 09, 2019 12:17 am

### Re: How to know where a double bond should go?

Count all the total number of valence electrons and arrange them around the atoms. Then as you calculate formal charge, you can rearrange the lone pairs and see where there are double bonds.

305416361
Posts: 50
Joined: Fri Aug 09, 2019 12:17 am

### Re: How to know where a double bond should go?

It depends on the electron affinity of the atoms and how many electrons they need to complete their octet. in this case, for example, Cl already has 7 electrons and only needs one more to complete its octet, so adding a double and would violate the octet rule

405398682
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Joined: Wed Sep 18, 2019 12:21 am

### Re: How to know where a double bond should go?

Most time the double bond can go with any atom (mainly the central atom) in the structure as long as it meets the octant rule.

preyasikumar_2L
Posts: 101
Joined: Fri Aug 09, 2019 12:17 am

### Re: How to know where a double bond should go?

Cl only has one unpaired electron in its valence shell and would thus typically and ideally only form a single bond with other elements, so if possible, a double bond with Cl is avoided.