Why does the periodic table of elements look like it does?

At this time of year, when I have few work meetings, I like to attend short courses at the City Literary Institute in central London. So I recently attended a course on the periodic table.

What does the periodic table look like? Check it out here.

How many elements are there? When the Russian scientist Dmitri Mendeleev invented the periodic table in 1869, there were only 83 known elements. For a long time, the table consisted of 92 elements and, as I recall, that was the situation when I studied chemistry at school in the 1960s. Today we have identified 118 elements (four of them announced this year), although only 92 of them are naturally occurring on earth.

How are the elements grouped? They are organised organised according to their atomic number (the number of protons in an atom of the element) and then placed in 7 rows and 18 columns which are determined by the characteristics of the electrons in each of these atoms.

Now, when I was school (which admittedly was a long time go), we were taught that each atom consisted of a nucleus containing neutrons (no charge) and protons (positive charge) surrounded by electrons (negative charge) and the visual model of the atom showed the electrons as essentially the same size as neutrons and protons and moving in close, circular orbits around the nucleus, a bit like the planets orbiting the sun.

However, on this course our lecturer Gary Retallick painted a more sophisticated picture.

  • First, there is a common analogy about the structure of an atom in which the nucleus is a fly in the centre of a sports stadium and the electrons are tiny, tiny gnats circling the stadium, so most of the atom is ’empty’.
  • Second, electrons are so much smaller than a proton – about 1/1800 of the size.
  • Third, electrons do not go round in neat circular orbits  but, because of probability theory, more in the form of ‘clouds’.
  • Fourth, not all electrons in any given atom are in the same orbit or ‘cloud’ – there are four energy levels (electrons can jump or fall between these) and each energy level can host a different maximum number of electrons – 2 in level one, 8 in level 2, 18 in level 3, 32 in level 4.

The combination of the atomic number and the deployment of elections in a given atom determines its physical and chemical characteristics and where it is located in the periodic table.

So, now you know.



XHTML: You can use these tags: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>