The Periodic Table

The Periodic Table of the Chemical Elements is also known as the Periodic Table of the Elements, and as (simply) the Periodic Table. It is a tabular presentation of the chemical elements, as shown below.

The Russian chemist Dmitri Ivanovich Mendeleev (1834 - 1907) devised the modern Periodic Table of the elements in 1869 by arranging the elements known at that time in order of increasing atomic weight, while also leaving gaps for undiscovered elements. More information about the history of the periodic table will be added at a later date.

Download this version of the Periodic Table as a .jpg (image file).

There are many slightly different versions of the periodic table.

Reasons for the differences between versions include:

• Scientific knowledge at the time of publication:
  More elements have been added to the periodic table as they have been discovered. More more information about known elements, e.g. their atomic number and atomic mass, has also become available over time.
• Level of detail needed by users of different versions of the periodic table.
  For example, versions of the periodic table intended for younger students may not include all of the elements, some of which are very rare and some of which only exist for a very short length of time. Some simple versions of the periodic table include only the Atomic Number of each element, omitting the Atomic Mass (or 'Mass Number').
• Colour (or other) Coding of categories of elements:
  Some versions of the periodic table may appear to be more complicated because they include colours or shading to indicate categories of elements, many of which share similar properties in at least some respects - though there may also be trends within members of a category. There are two main types of categories of elements within the periodic table. They are called 'Groups' and 'Periods' and are explained later.
• Labelling:
  Different versions of the periodic table may be presented and labelled in different ways. For example, some label blocks of elements according to the layers of electrons surrounding the atom of each element, hence you may see labels such as 's-block', 'd-block', 'p-block' and 'f-block'. This is explained later.

What do all versions of the Periodic Table have in common ?

• Symbols of the Elements:
  Chemical elements are always referred to using the same symbols - irrespective of the language of the publication in which the periodic table (or any use of the symbols) appears. However, many Periodic Tables used in schools, colleges, laboratories and other work-places also include the name of the element in the local language or (languages).
  The names of the chemical elements are not included in the periodic table shown above because this page is too narrow to fit them all in. See the List of Chemical Elements and their Symbols for information about chemical symbols and their corresponding names in English.
• Numbers:
  Even simple versions of the periodic table include the atomic number of each element shown in the table.
  The atomic number of each element is a fixed value. These define the element and so do not vary between sources such as textbooks, websites, or expert opinion. For example, the atomic number of Nitrogen is always 7.
  The atomic mass (or 'mass number') of each element is also the same on different versions of the periodic table. However, it is not quite as 'fixed' a value as the atomic number because some elements also exist in (less common) forms that have a higher than usual atomic mass, though those forms tend to be less stable. This is explained further in the section about radioactivity.
  
• Layout:
  Although the level of detail of labelling many vary (as mentioned above), the overall layout of the Periodic Table is a key aspect of it and remains the same across different publications, posters etc.. This is because the layout in terms of the main rows and columns and the numbers of rows and columns of the Periodic Table is it itself an important summary of how the elements in the table relate to each other and hence of similarities and trends in their structures and (physical and chemical) properties. This is explained further by information about the Groups and Periods of the Periodic Table.