Posted by: Mark Foreman | May 20, 2013

Dot and cross model of sodium borohydride (or lithium aluminium hydride)

Dear Reader,

It seems that my readers love sodium borohydride and they want to know why it is a more selective reducing agent than lithium aluminium hydride. This page is devoted to the Lewis structures of the borohydride and related anions. Some time ago someone asked me to draw a Lewis structure of the anion in sodium borohydride, to give it its correct name sodium tetrahydroborate. Now rather than just do it, I am going to show you how to draw it.

Start by drawing the central atom

Just a little boron atom on its own.

Just a little boron atom on its own.

Next we add the four hydrogen atoms around it.

Now it has the hydrogen atoms

Now it has the hydrogen atoms

We are not done yet, we need to start to add the electrons. I would always start by thinking how many electrons we get from the central atom. In this case as it is a boron (group III) we get three electrons. We will mark these electrons as X symbols. We should stick them between hydrogen atoms and the boron atoms to start to make some of the bonds.

Now with three electrons drawn in

Now with three electrons drawn in

We should understand very quickly that the boron has a weedy three valence electrons around it while the hydrogens have either one or none. These atoms clearly do not have the same number of electrons as noble (not nobel) gases. So lets add the electrons from the hydrogens to see if we can make things better. We will use z symbols for the hydrogen electrons. Again we put the electrons into bonds to form some covalent bonds which make up our ion (charged molecule or atom).

Now with the boron and hydrogen electrons added

Now with the boron and hydrogen electrons added

Now at this stage we should count the electrons around the atoms. The boron has seven electrons which is one short of the ideal eight electrons in the outer shell of neon. Three of the hydrogen atoms have two electrons which is perfect as helium has two electrons in it. One of the hydrogens only has one electron around it. Now we have a choice, we could make a radical (high energy) or we could add an additional electron (symbol #) to finish off the molecule. This electron will occupy the last space in the sigma molecular orbitials which make the covalent bonds in the molecule.

A finished borohydride anion

A finished borohydride anion

Now we have finished our borohydride anion, now we can move onto something more fun.

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Responses

  1. […] agents, if you want to know how to make dot and cross diagrams of the anions then please click here otherwise keep on […]


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