Dative Covalent Bonds: It donates an electron PAIR?

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Retsek
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#1
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#1
Hi, I don't get dative covalent bonds. I understand the concept of both the electrons in the bond coming from one atom, but often mark schemes are expecting you to say "Electron pair from ____ donated to ___". Which I don't get. Surely if they are sharing the electrons, it should only donate one electron, if it donated a pair it would be an ionic bond?
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MR1999
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(Original post by Retsek)
Hi, I don't get dative covalent bonds. I understand the concept of both the electrons in the bond coming from one atom, but often mark schemes are expecting you to say "Electron pair from ____ donated to ___". Which I don't get. Surely if they are sharing the electrons, it should only donate one electron, if it donated a pair it would be an ionic bond?
I'm not an expert but in ionic bonds, it's not always an electron pair that gets donated. It may be one electron or it could be three. But a dative covalent bond always involves an electron pair.

Also, in ions, when the electrons are donated, charge increases whilst in a dative covalent bond, when the electrons are donated, the charge becomes neutral.
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Daveboi115
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#3
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Ionic bonding involves the electrostatic attraction between the ions. Dative or coordinate covalent bonding involves the sharing of electron pairs from a single atom. An ammonium ion is a good example (NH4^+). One of the N-H bonds will be dative covalent. If your questions represent the ions by structure, a good rule of thumb is that dative covalent bonds are represented by an arrow which points to the atom NOT donating the electron pair.
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Retsek
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#4
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(Original post by MR1999)
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(Original post by Daveboi115)
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Okay so I understand this, but why does it donate a pair of electrons? If the covalent bond is a sharing of two electrons (usually one from each atom) why does it give a pair away?

Surely it makes more sense if it donates one, and then they share them?

Completely okay with the end result, a covalent bond where both electrons are provided by one atom, but mark schemes frequently use the phrase "electron pair is donated" and it's this that I don't get.
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cyanith
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#5
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Firstly, donating a pair of electron does not mean the bond has to be ionic. Ionic bonds can very well be formed with only one electron being donated. (HCl for example)
However, in a dative covalent bond an electron pair is being shared (not completely donated) but the pair comes from the same atom. It's easier to explain with an example.
Take, NH4+ Here, Nitrogen has five electrons in its outer shell. It uses three electrons to form bonds with 3 Hydrogen atoms. Now, it has two electrons left (one electron pair) So, it forms another bond with an H+ ion which does not have any electrons. So, the last electron pair is shared between Nitrogen and the H+ ion. Instead of giving a single atom like a regular covalent bond, both the electrons are from the N atom.
So, electron pair from the nitrogen is being donated/shared with hydrogen.
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Daveboi115
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#6
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Essentially, chemicals tend towards donating electrons in pairs. I would point out that the “Why?” Of this is a little beyond A-level standard so I wouldn’t worry about it too much (assuming of course you’re an A-level student). In a reasonably simple form, it comes down to the quantum spins of the electrons and their need to pair up which is energetically favourable to the compounds. The Pauli exclusion principle is in play here. It excludes the possibility of particles having the same quantum numbers.
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