Chemguide: Support for CIE A level Chemistry

This statement introduces the concept of electronegativity.

Before you go on, you should find and read the statements in your copy of the syllabus.

Learning outcomes 3.1.1 to 3.1.3

Electronegativity is one of the key concepts in chemistry, and so you should spend some time making sure that you fully understand it - what it means, what causes electronegativity differences, and what effects it has.

To do that, first read the page about electronegativity in the bonding section of Chemguide. Read down as far as, but not including, the sections titled "Diagonal relationships in the Periodic Table" and "The polarising ability of positive ions".

When you are sure that you understand the bits that you have read, follow the link at the bottom of that page to look at it all again in an organic chemistry context. Read that second page down to, but not including, "Bond polarity and inductive effects". You will meet the rest of the page later on when it will have more meaning to you.

Learning outcome 3.1.4

From what you have read so far, you will know that when two atoms join together, the difference in their electronegativities affects what sort of bond is formed.

• If the electronegativities are the same, you will have a pure covalent bond.

• If one atom is slightly more electronegative than the other you will have a polar covalent bond, with the more electronegative atom having a slightly negative charge, and the other atom a slightly positive charge.

• If one atom is much more electronegative than the other you will have an ionic bond, with the more electronegative atom forming a negative ion and the less electronegative one a positive ion.

This statement seems to want rather more than this, suggesting that you should be able to predict what sort of bond will be formed given the electronegativities of the two atoms.

That isn't anything like clear-cut! I will give you the general rules, but they vary from source to source. It is very easy to think of exceptions.

You will notice that there is a glaring gap between 1.6 and 2.0. There is another rule which gets around than to some extent.

For electronegativity difference in the range 1.6 to 2.0:

• If a metal is involved, then the bond is considered ionic.

• If only non-metals are involved, then the bond is considered polar covalent.

In fact, this whole thing is riddled with inconsistencies. This is the first time that it has appeared in the CIE syllabus, and I would like to think that the examples they use in an exam will be clear-cut.

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