Numbering the Groups in the Periodic Table

This page looks at the way that groups of elements in the Periodic Table are numbered - for example, Group 1 containing the metals from lithium to caesium.

There is a problem here because the numbering system I am using doesn't correspond to the latest IUPAC version, and I need to explain why that is, and why I am not about to change to the IUPAC system.

The IUPAC system

The IUPAC system numbers the groups from 1 to 18. The diagram below shows how this works for the first part of the Periodic Table. This represents a good proportion of what we deal with at this level, at least in exams used in the UK.

Note:  The numbers would normally go over the top of the groups, of course, but it was easier to make it look tidy in this case by writing them underneath!

At this level, all we do in addition to the elements in this bit of the Periodic Table is to look in more detail at the whole of the groups 1, 2, 14, 17 and 18. But we virtually ignore the other elements in the centre of the table.

Why am I not using the IUPAC system?

There are two reasons for this:

  • The UK syllabuses that I track aren't using it at the time of writing (October 2012). Although Chemguide is used world-wide, my target audience consists of students doing A level (and equivalent) UK-based exams.

  • Even if they were using the IUPAC system, I would be unhappy about using it myself. I think it causes a significant problem in teaching chemistry at an introductory level. Much more about that later.

What I am using is a simplified version of an older system. The notation I'm using for the bit of Periodic Table shown above is:

This is also the system used by current UK examiners.

In this system, the main groups (shown in red and green) are numbered from 1 to 8 (or from 1 to 7 and then 0 - the noble gases are sometimes described as Group 8, sometimes as Group 0).

The d-block elements (shown in blue) don't need to be given group numbers at this level, because we virtually never look at any of these apart from those in the first row.

Traditionally, trying to give the d-block elements group numbers led to a totally confusing system of numbering involving A and B sub-groups, with names such as group VIA and IVB (that's 6A and 4B) and so on.

This was made even worse because the use of the A and B letters in the US was completely the reverse of that in Europe. And so IUPAC sorted it out by numbering everything from 1 to 18.

The main down-side of using the IUPAC system

So why, if the IUPAC system is so much tidier, doesn't everybody use it?

The IUPAC system messes up the way that the Periodic Table is first introduced as an important tool in learning chemistry.

Teaching the electronic structures of the first 20 elements (up to Ca) relates directly to the Periodic Table. Students can see that elements in the same group have the same number of outer electrons (with the one exception of helium).

And, more than this, the number of outer electrons is the same as the group number - as long as you aren't using the current IUPAC system.

So, if an element is in Group 2, it has 2 outer electrons. So even if you can't work out the full structure, you know that barium has 2 outer electrons. Similarly, if you find iodine is in Group 7, you know it has 7 outer electrons.

Apart from the elements in Groups 1 and 2, that obviously doesn't work if you use the IUPAC version.

Having an easy way of working out the number of outer electrons is directly related to being able to easily deduce the charges on many simple ions, and that leads logically on to writing chemical formulae.

So the simple fact of having the group numbers equal to the number of outer electrons makes the otherwise tedious and mysterious process of working out formulae relatively simple and clear.

There is probably no point in the whole of chemistry teaching where it is easier to turn students off the subject completely than the introduction to atomic structure and to formulae. The IUPAC numbering system simply makes this more difficult than it need be.

Is there any disadvantage of using the non-IUPAC numbering method at this level?

". . . at this level"? Basically, that's anything under degree level.

No, there isn't any disadvantage. The IUPAC numbering allows you to attach numbers to the d-block groups in a clear way. But we never need to do that.

In a life-time of teaching chemistry to students up to the age of 18, I have never explored group trends in the d-block elements. In fact, there are very few elements that even get a mention outside the first row.

Actually, without looking it up, I don't even know which elements lie underneath iron, for example, in the Periodic Table! And that is equally true for all but two of the d-block groups (I can do the copper and zinc groups!). Does that bother me? Not in the least - it is irrelevant to the students that I am aiming to help.

And although I have looked at the old numbering system using an example from chemistry for 14 - 16 year olds, it is still relevant at the next stage. It is still useful to be able to look up, say, phosphorus in the Periodic Table, and know (without any thought) that because it is in Group 5, it has 5 outer electrons, and then go on from there.

The IUPAC numbering may well make things tidier for university people, but it makes a mess of an essential teaching tool for the rest of us. That doesn't seem to me to be a good bargain.

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© Jim Clark 2012