The periodic table

I start this post with a warning and a plea. It introduces a fundamental concept of science and it is absolutely fundamental to chemistry. My plea is that you don’t stop at this point but persevere as I promise you’ll be better informed than many scientific types at the end. I was inspired to write this after buying a book( yes a book ) which promised a guide to the periodic table. After reading it I ended up more confused than I started and I’m supposed to know about this stuff. I also felt I could do a lot better and this is my attempt.

First of all you need a couple of basic ideas. All matter is made up of 92 naturally occurring elements. An element is a type of matter which is unique. These elements are agglomerations of atoms. Crudely speaking an atom can be thought of as a miniature solar system. The nucleus ( or sun in our analogy ) consists of a fixed number of protons ( with a positive electric charge ) and a slightly variable number of neutrons( having no electric charge ). The number of neutrons is usually similar to, or more than, the number of protons. In orbit around the nucleus are electrons ( like planets in our analogy ). Electrons have a negative electric charge and are precisely the same number as protons. Elements start with one proton ( hydrogen ) and go up to 92. Elements with more than 92 protons can be made artificially.

Now electrons are precisely ordered in their orbits. There are 2 electrons in the outermost orbit, 8 in the next going inwards, then 16 then 32 proceeding inwards.

Now properties of elements depend heavily on the outermost electrons. In fact their chemistry is effectively decided by them. So we have a situation where the outermost repeat as the electron orbits progressively fill. This means that the chemical properties which heavily depend on the outermost electrons also repeat. This repetition isn’t precise as the inner electrons play a small part.as does the overall atomic size.

What this does mean is that the 92 natural elements fall fairly naturally into groups thus for example all inert gases are rather similar although they are very different in number of protons in the nucleus.

This means the most usual way of setting out the periodic table is to  set out the elements in rows of ascending number of protons with columns indicating the number of outermost electrons. Thus for example all the elements with one outermost electron are in a column and all have a similar chemistry. In the same way all who have the maximum number of electrons ( permitted under complex rules ) in the outermost layer are also similar.

Some elements known as the lanthanides and actinides fill their inner electrons  ( again under complex rules ) while keeping their outer electrons the same. These all have similar chemistry because it is the outermost that are important. Because their properties therefore don’t vary in any systematic way it is conventional to list them separately to the main periodic table.

There are several ways of arranging the periodic table. One possible is illustrated. The elements are identified by their chemical symbol but what matters is to see that this is a table of systematic changes. When I was at school the chemistry of elements seemed a unstructured mass of facts. The periodic table reduces them to a reasonably rational order.

I’ve deliberately taken the simple examples of columns with one and full outer electrons. When there are intermediate numbers the situation is a bit more complex and the variation within a column is greater.

Incidentally the element carbon occupies a rather unique and important position.  It has 4 outer electrons and can readily form 4 strong bonds.. Two or more carbon atoms can link in a chain. Carbon chains can combine with other elements ( mainly oxygen, nitrogen and sulphur ) to form very many molecules. I’m talking millions and this chemistry, known as organic chemistry, is the foundation of all life.

When I was a trainee scientist I vividly recall that a lecturer was appalled that we students hadn’t memorised the periodic table. We had exams approaching and he said one question will be to write out the table and woe betide anyone who couldn’t. Memorising the 92 is a bit of a strain and mnemonics were brought into play. The main bones are fairly easy but the lanthanides and actinides were difficult. I have a memory of students walking round campus muttering things like Ceasar Proudly Nods ( for Cerium, Praseodymium and Neodymium, early lanthanides) there was much more which I now forget. Strange looks were given. I can honestly say I have never ever needed to memorise anything other than the first couple of rows but even now I recall that strange Russian gentleman Helibebcnof ( look at the first full row but starting with Heliium from the top row )