Covalent structures and bonding
Read our general notes on Risk Assessment
Please help me with question 7.17 on page 174 of the Nuffield Students’ Book. I can’t understand how dot-and-cross diagrams should be drawn.
Igloo writes
I am not going to answer this question for you anyway, because you must try and work it out for yourself. It’s probably been set as an assignment for you to carry out on your own. However, I will give you some ideas on dot-and-cross diagrams which I hope will be of some help to you.
A covalent bond between two atoms consists of two electrons, one derived from one of the atoms and the second (the cross) from the other atom.
If you have a look at page 141 of the Nuffield Students’ Book you will find dot-and-cross diagrams for methane, ammonia, water and hydrogen chloride (the big circles are usually omitted in dot-and-cross diagrams).
I usually get my students to construct the displayed formula of a molecule first, and then substitute each covalent bond with a dot and a cross to produce the dot-and-cross version. For example, for hydrogen chloride you would first draw the displayed formula, which is H-Cl, and then, by substituting a dot and cross for the bond and adding lone pairs of electrons, you arrive at the diagram given at the bottom of page 141. So draw displayed formulae for methane, ammonia and water, from these you can then easily convert to the dot-and cross versions on page 141.
Similarly, on page 143 of the Students’ Book, you will see dot and cross diagrams for ethane and ethene, together with a displayed formula below each diagram. Once again, it is helpful to see the link between the dot-and-cross diagrams and the appropriate displayed formulae.
In all dot-and-cross diagrams, you must remember to include the lone pairs of electrons, if any, on relevant atoms. Study the diagrams on pages 141 and 143 to see what I mean.
The general method I recommend is as follows:
[1] construct the displayed formula for the given substance
[2] exchange each covalent bond for a dot and a cross (arranged at right angles to the original bond)
[3] add lone pairs of electrons on each atom where these exist. To do this, calculate the number of electrons in the outer shell of the atom, subtract any electrons which have already been drawn in the covalent bond(s), then arrange the rest in pairs around each atom.
[4] Finally, check that there is a “full shell” of electrons around each atom - normally 8 electrons (when isn’t it 8?)
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
back to Covalent structures and bonding
Rate this page or react
Share your views on this page, 5 ratings so far
, rated at 
updated: 02 July 2003
