Periodic table

Periodic Table

Introduction

The modern periodic table is a table showing known elements, their symbols, atomic numbers and relative atomic masses. The position of an element in the table shows whether it is a metal, non metal, or metalloid. It also indicates what properties it possesses. The table got its name from the frequent, or periodic, repeating of the properties of similar elements.

Today the table is taken for granted, but how did scientists first work it out? Throughout the 19th century, many scientist tried to create charts listing elements in order of the ‘size’ of their atoms. The idea was to try to find and show a relationship between the size of an atom and its properties, and to show a pattern of similarities between elements of a similar size.

The size of an atom really means its mass. It is possible to calculate the mass of an atom in relation to other atoms, using the mass of Carbon-12 as a base. This is called relative atomic mass. (Relative atomic mass is sometimes referred to as RAM.) The relative atomic mass, number of an element is the average of the masses of its isotopes. One way to imagine this is as an average mass of a sample of atoms in a particular element. The RAM of one particular element is always the same.

Early attempts - History

An early attempt to find a pattern in the properties of elements was made by the German chemist Johann Wolfgang Döbereiner (1780-1849).

He noticed that some elements could be arranged in groups of three. An element in a particular group had similar properties to the other two elements in that group. As there were three elements to a group, Döbereiner gave them the name of ‘triads’ (which means ‘groups of three’). He also discovered that if he put the elements of a triad in order of RAM, the RAM of the element in the middle was very close to the average RAM of the other two elements. Later, the English scientist John Newlands (1837-1898) arranged elements in groups of eight, but 'Newland's octaves', as they were called, were largely ignored by scientists.

Dmitri Mendeleev

The modern periodic table owes its existence to the Russian scientist Dmitri Mendeleev (1834-1907). He collected all the information he had about each element on separate cards. He then sorted the cards into order of increasing atomic mass, and laid them out in a horizontal row. When he came to an element with properties similar to one he had already laid down, he started a new row, directly beneath the last. This meant that elements in the same vertical column had similar properties to each other.

Mendeleev, found that there was a repetition of properties every eight or eighteen elements. When this didn't occur, he realised that there were probably some undiscovered elements. He solved this by leaving gaps, and by predicting what the properties of these missing elements would be. He was also confident enough to swap around a few elements when their mass did not match the patterns he had discovered. Mendeleev published the table in 1869.

Scientists later discovered several new elements as a direct result of trying to fill in the gaps that he had left. The properties of these elements were startlingly similar to those Mendeleev had predicted. His table was accepted as a very important guide to the properties of elements.

The modern periodic table

The major fault in Mendeleev’s table was that he believed that it was the mass of the elements that affected their properties. It is, in fact, their atomic numbers. Although the atomic number affects the mass, it is not the mass that is most important. It was the English physicist Henry Moseley (1887-1915) who discovered the significance of the atomic numbers. Once the elements were arranged by atomic number, the errors in Mendeleev’s table were corrected.

did you know?

periodic table n.

a table of the chemical elements arranged in order of atomic number, usu. in rows, so that elements with similar atomic structure (and hence similar chemical properties) appear in vertical columns.

related links

Periodic table of chemical elements