Introduction

There are hundreds of ways of presenting Periodic Tables. You can see some of them in this introduction to the periodic table.

The most familiar form is flat and rectangular with rows and columns, but this is only because this fits the pages of a book easily.

All modern Periodic Tables list the elements in order of increasing atomic number, often given the symbol Z. We know now that this is the number of protons in the nucleus of an atom and is sometimes called the proton number. However Dmitri Mendeleev, the Russian chemist who made the first Periodic Table in 1869, listed the elements known at that time in order of their relative atomic masses, A_r. This was for the simple reason that, at that time, the idea of atoms being made up of smaller sub-atomic particles, such as the proton, neutron and electron had not been developed.

Fortunately these two arrangements – in order of atomic number and of relative atomic mass - are almost identical. In the same way, you would probably get pretty much the same order if you listed all your classmates first in order of the width of their hands and then in order of the length of their feet.