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Name: Philippa
Status: student
Grade: n/a
Country: United Kingdom
Date: Spring 2009


Question:
Hi, I am not quite sure how to explain this, but I do not properly understand why the ammonium ion (NH4+) has a positive charge? When ammonia (NH3) reacts with for example, HCl, to gain the hydrogen ion it forms a co-ordinate bond with the H+, I understand that. But because ammonia is a neutral molecule, and its gaining a proton, the lone pair of electrons on the nitrogen is donated to the H+, shouldn't this make the ammonium ion a neutral molecule? I am confused.



Replies:
Philippa,

There are a couple of ways to think about this problem.

Firstly, if ammonia (NH3) is a neutral molecule, it has the same number of protons as electrons. If you add H+, a single proton, however you work out the bonding the NH4(+) molecule must have one more proton than electrons so it must be singly positively charged.

You are right about how we consider the bonding to work: N has a lone pair in NH3 and it donates this pair as a dative covalent bond with H. We generally draw the bonding (Lewis) diagram with the positive charge on the N (although the best representation is with the big square brackets around NH4 and a + outside the brackets) in NH4(+)and this can be explained in the following way:

A normal single covalent bond exists when two atoms donate an unpaired electron each to a central area, they pair up and form a covalent bond. The dative bond in NH4(+) can be thought of as the N losing one of the electrons in its lone pair to the H+, making it neutral and the N positively charged, then the remaining electron from the N lone pair and the electron now on the H atom forming a normal covalent bond. The overall effect is that the positive charge is transferred to the atom donating the lone pair to make the dative bond. I'm not saying that this initial electron transfer to H actually happens, but it does help to explain why we generally draw NH4(+) with the charge on the N.

Best wishes,

Tom Collins


Philippa,

It might help you to think of this from an accounting perspective. In NH3, N contributes 7p, 7e, the 3 H contribute 1p, 1e each. The number of protons and electrons match. In HCl, H 1p, 1e and Cl 17p, 17e. As you suggested, when NH4(+) forms from the reaction of NH3 and HCl, the H from the HCl is transfered to the NH3 without its electron. So the Cl now has the electron which used to belong to the H. Thus, NH4(+) has a total of 11p an and 10e giving it the 1+ charge, and the Cl has 17p and 18e, giving it a 1- charge.

Now whether the ammonium ion is formed from the reaction of ammonia and hydrochloric acid, it will always have 11p, 10e, and thus a positive charge.

Greg (Roberto Gregorius)
Canisius College


Part of the confusion is caused by a practice that has imbedded itself in chemistry texts -- specifically, trying to put chemical bonds into various "boxes" such as "co-ordinate" bonds etc.

Chemical bonds come in all "flavors" from purely transferring an electron to newly discovered Rydberg bonds that have a bond length of hundreds of Angstroms. So the artificial "pigeon holing" of chemical bonds is more confusing than helpful. That is my opinion, so I will stop preaching.

To make things simpler, consider the reaction of gaseous NH3 and gaseous HCl that react to form solid ammonium chloride: NH4Cl, which is neutral as are the reactants. Ammonia consists of 3 identical N--H bonds and a lone pair. When the lone pair (two electrons) combines with HCl, the lone pair bonds with the H atom forming 4 identical N--H bonds and a Cl atom. It becomes a matter of interpretation whether you want to consider this (NH4)[+1] -- (Cl)[-1] or NH4Cl [0].

Part of your confusion also arises because you did not mention the Cl[-1]. Since that is "clearly" a negative ion the other product must be positively charged -- electrical charge is always "conserved". What is on the reactant side must be the same as what is on the product side.

If NH4 were neutral, then charge would not be conserved, because (Cl)[-1] would be left.

To further make the point of artificial classification of bonding, consider CaF2. The traditional model of this molecule's bonding is Ca[+2] and two F[-1] "obviously", and the repulsion of the two F[-1] "ions" would strongly suggest that the molecule should be linear. However, in the gas phase at high temperature the structure of CaF2 is a bent triatomic molecule -- apex angle of about 120 degrees. This data is in conflict with the simple ionic bonding model.

Vince Calder


I will explain this in two ways. I hope one will make sense to you.

The most direct way to determine the charge of NH4+ is to count the electrons and protons. There is one more proton than there are electrons, so the net charge is +1. However, you must first accept the count of each of these particles. For me just to tell you is probably not very convincing.

You concede that NH3 is neutral. To turn NH3 to NH4+, you are adding a single proton, but not changing the number of electrons. The unbounded pair of electrons on the NH3 nitrogen is donated to the H+, but the electrons do not leave the molecule. Basically,

NH3 + H+ --> NH4+

When NH3 is reacted with HCl, the proton from the HCl transfers to the NH3, and the Cl nucleus and all the electrons of the HCl are removed as Cl-. So electric charge is conserved in the overall reaction

NH3 + HCl --> NH4+ + Cl-

Richard Barrans, Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming


Hi Philippa,

When the NH3 molecule accepts a proton to form ammonium, here is the reaction:

NH3 + H+ ---> NH4+

Certainly all reactions must be balanced with respect to (1) each kind of atom and (2) total electrical charge, so if NH3 (charge = 0 ) combines with the proton (charge = +1), the product molecule must have a charge of 0+1 = +1 . Also, since NH3 has eight electrons and H+ has none, the NH4+ molecule must have 8 electrons as well. Since the proton in NH4+ has a charge of +1, N has +5 and each electron is -1, the total charge of NH4+ must be 5+4(1)-8 = +1.

In the Lewis dot structure formalism, when we say that NH3 "donates" a lone pair to H+ to form the coordinate covalent bond, all we are really saying is that both of the electrons involved in forming the new N...H bond must have come from the N atom of the NH3 molecule, since there are no electrons at all in a bare proton. However, those electrons haven't gone anywhere - they are still part of the N atom's octet, before and after the bond is formed.

Hope this is helpful to you.

Best, Dr. Topper



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