Working out Dative Covalent Bonds in Lewis Structures - help!

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stardust101
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Would greatly appreciate any help going. I'm self-taught.

Thought I had Lewis structures nailed but came across some holes in my understanding.

1. If there are formal charges on certain atoms in a Lewis structure and they can't be cancelled out by equal and opposite formal charges on other atoms (eg. -1 on an oxygen atom and +1 on a sulphur atom), does this mean that the molecule has a positive/negative charge overall and is an ion? It seems to work that way for the ammonium ion and sulphate ion (N has formal charge of +1 in the ammonium ion and two oxygen atoms have formal charge of -1 in the sulphate anion).

2. I'm working out formal charges using the formula - is it correct?
Formal charge of atom = No. of valence electrons in the atom - (No. of lone electrons + No. of bond pairs)

3. This is the real sticking point. I was doing well until meeting this question George Facer's exam questions:
Which does not contain a dative covalent bond?
PCl4+
NH4+
PCl6-
Al2Cl6

I already know that NH4+ and Al2Cl6 contain covalent bonds but I realised that my Lewis structure technique does not allow for working out which electrons come from which atom. The way I learnt, you simply share electrons out equally and don't worry about what electron originally belonged to each atom. After sharing them out equally, I work out formal charges using the equation above and try to get the numbers as low as possible - this usually works.

Now I understand, having looked at the answer, that it makes sense for PCl6- to have a dative covalent bond because P only has five electrons in its outer shell so can only form five normal covalent bonds. But I could also think of 'explanations' for why PCl4+ might have a covalent bond. I don't know any method to just work it out for sure, or a way to share out electrons that factors in the electrons in each atom's outer shell. So it seems like my system is seriously flawed.

If someone could help I would be incredibly grateful.
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stardust101
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'But I could also think of 'explanations' for why PCl4+ might have a covalent bond.'

I meant a dative covalent bond, sorry.
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charco
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(Original post by stardust101)
Would greatly appreciate any help going. I'm self-taught.

Thought I had Lewis structures nailed but came across some holes in my understanding.

1. If there are formal charges on certain atoms in a Lewis structure and they can't be cancelled out by equal and opposite formal charges on other atoms (eg. -1 on an oxygen atom and +1 on a sulphur atom), does this mean that the molecule has a positive/negative charge overall and is an ion? It seems to work that way for the ammonium ion and sulphate ion (N has formal charge of +1 in the ammonium ion and two oxygen atoms have formal charge of -1 in the sulphate anion).

2. I'm working out formal charges using the formula - is it correct?
Formal charge of atom = No. of valence electrons in the atom - (No. of lone electrons + No. of bond pairs)

3. This is the real sticking point. I was doing well until meeting this question George Facer's exam questions:
Which does not contain a dative covalent bond?
PCl4+
NH4+
PCl6-
Al2Cl6

I already know that NH4+ and Al2Cl6 contain covalent bonds but I realised that my Lewis structure technique does not allow for working out which electrons come from which atom. The way I learnt, you simply share electrons out equally and don't worry about what electron originally belonged to each atom. After sharing them out equally, I work out formal charges using the equation above and try to get the numbers as low as possible - this usually works.

Now I understand, having looked at the answer, that it makes sense for PCl6- to have a dative covalent bond because P only has five electrons in its outer shell so can only form five normal covalent bonds. But I could also think of 'explanations' for why PCl4+ might have a covalent bond. I don't know any method to just work it out for sure, or a way to share out electrons that factors in the electrons in each atom's outer shell. So it seems like my system is seriously flawed.

If someone could help I would be incredibly grateful.
To decide whether a bond is dative or not you need to consider the number of electrons in the outer shell of each of the atoms in the structure (or at least the central atom)

In carbon monoxide, for example:

carbon starts off with 4 outer electrons and oxygen 6
They share three pairs
Both end up with 8 outer electrons.
It should be apparent that one of the three covalent bonds must be dative, with both electrons being donated by the oxygen.

It might help to look at some videos showing development of Lewis structures. I've done a few here:

https://www.youtube.com/watch?v=Oh4d...70531B822BC838
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stardust101
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Thank you very much, so kind of you to explain, Charco. I'll watch your videos and try again
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