Chemists have arranged the chemical elements into a table called the periodic table. This helps us to make sense of the different properties of the elements and their compounds. It also helps us to predict how they will behave in different situations.
There are several different versions of the periodic table, but all have a similar appearance. Each element is shown by its symbol, and sometimes also by its name.
In the periodic table, the elements are arranged in order of proton number, also called atomic number. This is the number of positive protons in each atom. The proton number is shown is shown, written underneath each element in the table below.
Putting elements in this order gives a repeating pattern of their properties. In the periodic table each element is placed beneath those with similar properties.
Development of the periodic table.
When many of the elements had been discovered, attempts were made to put them into a sensible arrangement. During, the period of the 18th century, actual masses could not be measured so they were compared against the mass of hydrogen (the lightest). This is called the relative atomic mass.
In 1817 Johann Döbereiner proposed his 'law of triads'. He realised the relative atomic mass of the middle element in a group of three elements (that had similar properties) was close to the average of the other two elements.
The next significant stage in the development of the periodic table came from John Newlands. He arranged the known elements in order of their atomic masses. He proposed a 'law of octaves', meaning every eighth element had similar properties. This did not work for all the known elements, so was dismissed…