Water H2O

10 05 2012

Water is something we can’t live without; roughly 70% of an adult human body is composed of water. I will be writing a followup post about the properties, but for now lets just focus on the molecular structure.

I made the model below to show its shape. The red centre is oxygen, the two whites are hydrogen, and the two empty red prongs represent two lone pairs.

The presence of two lone pairs means lots of repulsion between them, which pushes the two sets of bonded pairs closer together, so the two atoms of hydrogen are forced closer together to form a V shape.

Based on this we can predict the shape for hydrogen sulfide (H2S), bent for the same reasons H2O is bent, see below:

and also the oxonium ion H3O+

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

I highly recommend visiting Wolfram Alpha, simply type in a formula, name or equation and a treasure of information comes up.

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Ammonia

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.

credit: wikimedia.org

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: wikimedia.org)

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:

 

Methane:





Tetrachloromethane

9 05 2012

Molecular shape: the basic principles

Positioning of the bonds, and the bond angles, that form an atom are determined by electron repulsion between pairs of electrons (lone or bonded pairs).

Tetrahedron shaped molecules

Tetrachloromethane (CCl4), otherwise known as carbon tetrachloride, is an example of this, and it has four electron pair bonds. The 4 chlorine atoms are at the corners of a tetrahedron with the carbon atom at its centre.

 

The angle between adjacent bonds in this molecule is 109° 28’, and there is no other shape in which it could exist with bond angles greater than this.

Why?

The maximum angle has already been reached due to repulsion between the 4 pairs of bonded electrons. They want to be as far apart as possible without breaking the structure, think of it like magnets, electrons are negative and don’t want to be together or near each other particularly, they are attracted to positive things like protons in the nucleus of an atom.

Here’s a rough dot cross diagram I did on a post-it, the dots represent Carbon’s 4 electrons in its outer shell/energy level, therefore valence of 4, the crosses obviously represent Chlorine’s outer electrons (we know this because of its electronic configuration, see Lesson 2: Atomic Structure, Isotopes and Electronic Configuration)