10 05 2012

Ammonia NH3 contains 3 electron pair bonds (N-H) and one lone pair, represented on the model I made below as the empty tip of the pyramid shape. The blue centre represents Nitrogen, and the 3 white bits are Hydrogen.

The pyramidal shape is a result of the electron pairs (bonded and lone) repelling each other. Remember where the most repulsion occurs:

Most electron repulsion between:       lone pairlone pair

Strong repulsion between:                      lone pairbonded pair

Least repulsion between:                         bonded pairbonded pair

These repulsion rules explain why the bonded pairs are pushed downwards to form the base of the pyramid, they don’t want to be anywhere near the lone pair at the tip of the pyramid. But they also don’t want to be near each other, so they go as far as they can away without breaking their bonds to the nitrogen centre.


What shape do you think the Ammonium ion NH4+ would have?

Ammonium is isoelectronic CH4 (Methane) so has the same shape, tetrahedral. Remember ‘iso’ in Greek means ‘equal’, so in chemistry it means ‘the same’ or ‘no change’, remember we studied Isotopes of elements in Lesson 2.

Methane (credit:

Here’s an ammonium ion I drew below:

NH3 + H+  = NH4+

Dative bond: Both electrons of a shared pair are donated by one of the bonded atoms.

In the example of ammonium this symmetrical ion has a dative bond that is hard to distinguish from its other covalent (electron sharing) bonds because of its symmetry. But as you can see there are two dots instead of dot cross, and the + sign atop the brackets surrounding the ion represents the electron the hydrogen atoms gives away. Electrons are negative so giving them away means you become more positive.

Ammonium is a positive ion so is called a cation. Here’s a model of amonium I made:






One response

12 05 2012
Water H2O « ruthlearns

[…] This oxonium ion is pyramidal as it is isoelectronic with ammonia, see previous post. […]

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