[yellowtail]

I'm a little confused and any help would be great.

This is what I understand: Nitrogen has a valency of 3, and can form covalent bonds with 3 hydrogen atoms to acheive a stable outer energy level (octet). The molecule formed is called ammonia (NH3) and is slightly polar.

Here's where I get confused: The nitrogen in ammonia is saturated and can form no additional bonds (in keeping with the covalent bonding rules). So how can an additional hydrogen ion join to form ammonium (NH4+)? Is it simply because the H+ ion is attracted to the polarity of the NH3?

Thanks in advance.

[AWK]

H+ "is atracted" by a nonbonding electron pair of  a nitrogen atom of ammonia.

[yellowtail]

Thanks but where does the non-bonding electron pair come from?

N, in the case of ammonia, is bonded to 3 hydrogen and so becomes satisfied (octet). No additinal bonding can take place.

[Dan]

N, in the case of ammonia, is bonded to 3 hydrogen and so becomes satisfied (octet). No additinal bonding can take place.

Careful. This is not necessarily true. Draw the lewis structure for the ammonium ion, the octet rule is not violated. In ammonia, you have three N-H bonds, in other words 6 bonding electrons (3 pairs), and two non-bonding electrons (lone pair) - so a full octet (8 electrons). Essentially what has happened is that H⁺, which has no electrons, it is just a proton, has stuck to the lone pair on N. No electrons have been added, there is still a full octet around N.

This is comparable to the formation of H₃O⁺
.

[yellowtail]

Great explanation Dan, thanks.

Just one thing, when you say the H+ has stuck to the lone pair what forces are involved: hydrogen bonding?

[Dan]

Great explanation Dan, thanks.

Just one thing, when you say the H+ has stuck to the lone pair what forces are involved: hydrogen bonding?

Glad I could help. This is electrostatic attraction between the positively charged hydrogen ion and the negatively charged electrons in the lone pair of N (do be aware however, that ammonia is not charged as a whole).
Electrostatic attraction is also responsible for hydrogen bonding (and actually underpins most of chemistry), so the concept is similar - positive attracts negative. This is not an example of hydrogen bonding though, this is a stronger interaction than hydrogen bonding. I would think that this would be classed as an ion-dipole interation - that is the attractive force between the H⁺ and the partially negative N of the polar ammonia molecule.

A hydrogen bond on the other hand would be the attraction between a partially  positive hydrogen (in a polar bond) and a partially negative atom (usually F, O or N).

[yellowtail]

Got it, much appreciated Dan.

[Yggdrasil]

This is not an example of hydrogen bonding though, this is a stronger interaction than hydrogen bonding. I would think that this would be classed as an ion-dipole interation - that is the attractive force between the H⁺ and the partially negative N of the polar ammonia molecule.

A hydrogen bond on the other hand would be the attraction between a partially  positive hydrogen (in a polar bond) and a partially negative atom (usually F, O or N).

I would say the bond between the proton and nitrogen in the ammonium ion is a covalent bond.  The non-bonding orbital of nitrogen that contains the lone pair of electrons overlaps with the empty s-orbital of the hydrogen to form a covalent bond.

A hydrogen bond is a dipole-dipole interaction that has a slight covalent character, making it stronger than a normal dipole-dipole or ion-dipole interaction.